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{{StatPsy}}
[[Image:Prison.jpg|right|frame|Will the two prisoners cooperate to minimize total loss of liberty or will one of them, trusting the other to cooperate, betray him so as to go free?]]
 
:''Many points in this article may be difficult to understand without a background in the elementary concepts of [[game theory]].''<!--Replace with suitable template?-->
 
   
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{{dablink|This article contains mathematical terminology from [[game theory]], which should not be confused with the common usage.}}<!--Replace with suitable template?-->
In [[game theory]], the '''prisoner's dilemma game''' ('''PDG''') is a type of non-[[zero-sum]] game in which two players try to get rewards from a banker by cooperating with or betraying the other player. In this game, as in many others, it is assumed that the primary concern of each individual player ("prisoner") is self-regarding; i.e., trying to maximise his own advantage, with less concern for the well-being of the other players.
 
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[[Image:Prison.jpg|right|frame|Will the two prisoners cooperate, or will both of them betray to lessen their own terms, ending up with longer ones?]]
   
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The '''Prisoner's Dilemma''' constitutes a problem in [[game theory]]. It was originally framed by [[Merrill Flood]] and [[Melvin Dresher]] working at [[RAND]] in 1950. [[Albert W. Tucker]] formalized the game with prison sentence payoffs and gave it the "Prisoner's Dilemma" name (Poundstone, 1992).
In the prisoner's dilemma, cooperating is [[dominance (game theory)|strictly dominated]] by defecting (i.e., betraying one's partner), so that the only possible [[Nash equilibrium|equilibrium]] for the game is for all players to defect. In simpler terms, no matter what the other player does, one player will always gain a greater payoff by playing defect. Since in any situation playing defect is more beneficial than cooperating, all [[perfect rationality|rational]] players will play defect.
 
   
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In its "classical" form, the prisoner's dilemma (''PD'') is presented as follows:
The unique equilibrium for this game does not lead to a [[Pareto efficiency|Pareto-]]optimal solution—that is, two "rational" players will both play defect even though the total reward (the sum of the reward received by the two players) would be greater if they both played cooperate. In equilibrium, each prisoner chooses to defect even though both would be better off by cooperating, hence the dilemma. One resolution of the dilemma proposed by Douglas Hofstadter in his [[Metamagical Themas]] is to reject the definition of "rational" that led to the "rational" decision to defect. Truly rational (or "[[superrationality|superrational]]") players take into account that the other person is superrational, like them, and thus they cooperate. Many authors and researchers seem unaware of this resolution of the dilemma, however.
 
   
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:Two suspects are arrested by the police. The police have insufficient evidence for a conviction, and, having separated both prisoners, visit each of them to offer the same deal. If one testifies ("defects") for the prosecution against the other and the other remains silent, the betrayer goes free and the silent accomplice receives the full 10-year sentence. If both remain silent, both prisoners are sentenced to only six months in jail for a minor charge. If each betrays the other, each receives a five-year sentence. Each prisoner must choose to betray the other or to remain silent. Each one is assured that the other would not know about the betrayal before the end of the investigation. How should the prisoners act?
In the '''iterated prisoner's dilemma''' the game is [[repeated game|played repeatedly]]. Thus each player has an opportunity to "punish" the other player for previous non-cooperative play. Cooperation may then arise as an equilibrium outcome. The incentive to defect may then be overcome by the threat of punishment, leading to the possibility of a cooperative outcome. As the number of iterations approaches infinity, the [[Nash equilibrium]] tends to the [[Pareto optimum]].
 
   
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If we assume that each player prefers shorter sentences to longer ones, and that each gets no utility out of lowering the other player's sentence, and that there are no reputation effects from a player's decision, then the prisoner's dilemma forms a non-[[zero-sum]] game in which two players may each "cooperate" with or "defect" from (i.e., betray) the other player. In this game, as in all game theory, the only concern of each individual player ("prisoner") is maximizing his/her own payoff, without any concern for the other player's payoff. The unique equilibrium for this game is a [[Pareto efficiency|Pareto-]]suboptimal solution—that is, rational choice leads the two players to both play ''defect'' even though each player's individual reward would be greater if they both played ''cooperatively''.
==The classical prisoner's dilemma==
 
The Prisoner's dilemma was originally framed by Merrill Flood and Melvin Dresher working at [[RAND]] in 1950. [[Albert W. Tucker]] formalized the game with prison sentence payoffs and gave it the "Prisoner's Dilemma" name. <!-- refs to follow later on today-->
 
   
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In the classic form of this game, cooperating is [[dominance (game theory)|strictly dominated]] by defecting, so that the only possible [[Nash equilibrium|equilibrium]] for the game is for all players to defect. In simpler terms, no matter what the other player does, one player will always gain a greater payoff by playing defect. Since in any situation playing ''defect'' is more beneficial than cooperating, all [[perfect rationality|rational]] players will play ''defect,'' all things being equal.
The classical prisoner's dilemma (PD) is as follows:
 
   
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In the '''iterated prisoner's dilemma''' the game is [[repeated game|played repeatedly]]. Thus each player has an opportunity to "punish" the other player for previous non-cooperative play. Cooperation may then arise as an equilibrium outcome. The incentive to defect is overcome by the threat of punishment, leading to the possibility of a cooperative outcome. So if the game is infinitely repeated, cooperation may be a [[subgame perfect Nash equilibrium]] although both players defecting always remains an equilibrium and there are many other equilibrium outcomes.
:Two suspects, A and B, are arrested by the police. The police have insufficient evidence for a conviction, and, having separated both prisoners, visit each of them to offer the same deal: if one testifies for the prosecution against the other and the other remains silent, the betrayer goes free and the silent accomplice receives the full 10-year sentence. If both stay silent, the police can sentence both prisoners to only six months in jail for a minor charge. If each betrays the other, each will receive a two-year sentence. Each prisoner must make the choice of whether to betray the other or to remain silent. However, neither prisoner knows for sure what choice the other prisoner will make. So the question this dilemma poses is: What will happen? How will the prisoners act?
 
   
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In casual usage, the label "prisoner's dilemma" may be applied to situations not strictly matching the formal criteria of the classic or iterative games; for instance, those in which two entities could gain important benefits from cooperating or suffer from the failure to do so, but find it merely difficult or expensive, not necessarily impossible, to coordinate their activities to achieve cooperation.
The dilemma can be summarised thus:
 
   
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==Strategy for the classical prisoner's dilemma==
<table border cellpadding="5" cellspacing="0" align="center">
 
<tr>
 
<td></td>
 
<th scope="col">Prisoner B Stays Silent</th>
 
<th scope="col">Prisoner B Betrays</th>
 
</tr>
 
<tr>
 
<th scope="row">Prisoner A Stays Silent</th>
 
<td>Both serve six months</td>
 
<td>Prisoner A serves ten years<br>Prisoner B goes free</td>
 
</tr>
 
<tr>
 
<th scope="row">Prisoner A Betrays</th>
 
<td>Prisoner A goes free<br>Prisoner B serves ten years</td>
 
<td>Both serve two years</td>
 
</table>
 
   
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The classical prisoner's dilemma can be summarized thus:
The dilemma arises when one assumes that both prisoners only care about minimising their own jail terms. Each prisoner has two options: to cooperate with his accomplice and stay quiet, or to betray his accomplice and give evidence. The outcome of each choice depends on the choice of the accomplice. However, neither prisoner knows the choice of his accomplice. Even if they were able to talk to each other, neither could be sure that he could trust the other.
 
   
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{| class="wikitable"
Let's assume the protagonist prisoner is working out his best move. If his partner stays quiet, his best move is to betray as he then walks free instead of receiving the minor sentence. If his partner betrays, his best move is still to betray, as by doing it he receives a relatively lesser sentence than staying silent. At the same time, the other prisoner's thinking would also have arrived at the same conclusion and would therefore also betray.
 
  +
! !! Prisoner B Stays Silent !! Prisoner B Betrays
  +
|-
  +
! Prisoner A Stays Silent
  +
| Each serves 6 months || Prisoner A: 10 years<br/>Prisoner B: goes free
  +
|-
  +
! Prisoner A Betrays
  +
| Prisoner A: goes free<br/>Prisoner B: 10 years || Each serves 5 years
  +
|}
   
If reasoned from the perspective of the optimal outcome for the group (of two prisoners), the correct choice would be for both prisoners to cooperate with each other, as this would reduce the total jail time served by the group to one year total. Any other decision would be worse for the two prisoners considered together. When the prisoners both betray each other, each prisoner achieves a worse outcome than if they had cooperated. This demonstrates very elegantly that in a [[non-zero sum game]] the [[Pareto optimum]] and the [[Nash Equilibrium]] can be opposite.
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In this game, regardless of what the opponent chooses, each player always receives a higher payoff (lesser sentence) by betraying; that is to say that betraying is the ''strictly [[dominant strategy]]''. For instance, Prisoner A can accurately say, "''No matter what Prisoner B does'', I personally am better off betraying than staying silent. Therefore, for my own sake, I should betray." However, if the other player acts similarly, then they both betray and both get a lower payoff than they would get by staying silent. Rational self-interested decisions result in each prisoner's being worse off than if each chose to lessen the sentence of the accomplice at the cost of staying a little longer in jail himself. Hence a seeming dilemma. In [[game theory]], this demonstrates very elegantly that in a [[non-zero sum game]] a [[Nash Equilibrium]] need not be a [[Pareto optimum]].
 
Alternately, the options are "confess" and "don't confess."
 
   
 
==Generalized form==
 
==Generalized form==
We can expose the skeleton of the game by stripping it of the Prisoners’ subtext. The generalized form of the game has been used frequently in [[experimental economics]]. The following rules give a typical realization of the game.
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We can expose the skeleton of the game by stripping it of the prisoner [[framing device]]. The generalized form of the game has been used frequently in [[experimental economics]]. The following rules give a typical realization of the game.
   
There are two players and a banker. Each player holds a set of two cards: one printed with the word "Cooperate", the other printed with "Defect" (the standard terminology for the game). Each player puts one card face-down in front of the banker. By laying them face down, the possibility of a player knowing the other player's selection in advance is eliminated (although revealing one's move does not affect the dominance analysis{{mn|Tell|1}}). At the end of the turn, the banker turns over both cards and gives out the payments accordingly.
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:There are two players and a banker. Each player holds a set of two cards: one printed with the word "Cooperate", the other printed with "Defect" (the standard terminology for the game). Each player puts one card face-down in front of the banker. By laying them face down, the possibility of a player knowing the other player's selection in advance is eliminated (although revealing one's move does not affect the dominance analysis<ref name="tell">A simple "[[Tell (poker)|tell]]" that partially or wholly reveals one player's choice&nbsp;— such as the Red player playing their Cooperate card face-up&nbsp;— does not change the fact that Defect is the dominant strategy. When one is considering the game itself, communication has no effect whatsoever. However, when the game is being played in real life considerations outside of the game itself may cause communication to matter. It is a point of utmost importance to the full implications of the dilemma that when we do not need to take into account external considerations, single-instance Prisoner's Dilemma is not affected in any way by communications. Even in single-instance Prisoner's Dilemma, meaningful prior communication about issues external to the game could alter the play environment, by raising the possibility of enforceable side contracts or credible threats. For example, if the Red player plays their Cooperate card face-up and simultaneously reveals a binding commitment to blow the jail up if and only if Blue Defects (with additional payoff <span style="color: #009">-11</span>,<span style="color: #900">-10</span>), then Blue's Cooperation becomes dominant. As a result, players are screened from each other and prevented from communicating outside of the game.</ref>). At the end of the turn, the banker turns over both cards and gives out the payments accordingly.
   
 
If player 1 (red) defects and player 2 (blue) cooperates, player 1 gets the Temptation to Defect payoff of 5 points while player 2 receives the Sucker's payoff of 0 points. If both cooperate they get the Reward for Mutual Cooperation payoff of 3 points each, while if they both defect they get the Punishment for Mutual Defection payoff of 1 point. The checker board [[payoff matrix]] showing the payoffs is given below.
 
If player 1 (red) defects and player 2 (blue) cooperates, player 1 gets the Temptation to Defect payoff of 5 points while player 2 receives the Sucker's payoff of 0 points. If both cooperate they get the Reward for Mutual Cooperation payoff of 3 points each, while if they both defect they get the Punishment for Mutual Defection payoff of 1 point. The checker board [[payoff matrix]] showing the payoffs is given below.
   
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{| class="wikitable"
{| border cellpadding="5" cellspacing="0" align="center">
 
|+ Canonical PD payoff matrix
+
|+ Example PD payoff matrix
 
|
 
|
 
!scope="col" style="color: #900"|Cooperate
 
!scope="col" style="color: #900"|Cooperate
Line 58: Line 48:
 
|-
 
|-
 
!scope="row" style="color: #009"|Cooperate
 
!scope="row" style="color: #009"|Cooperate
|<span style="color: #009">3</span>, <span style="color: #900">3
+
|<span style="color: #009">3</span>, <span style="color: #900">3</span>
|<span style="color: #009">0</span>, <span style="color: #900">5
+
|<span style="color: #009">0</span>, <span style="color: #900">5</span>
 
|-
 
|-
 
!scope="row" style="color: #009"|Defect
 
!scope="row" style="color: #009"|Defect
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In "win-lose" terminology the table looks like this:
 
In "win-lose" terminology the table looks like this:
   
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{| class="wikitable"
{| border cellpadding="5" cellspacing="0" align="center">
 
 
|
 
|
 
!scope="col" style="color: #900"|Cooperate
 
!scope="col" style="color: #900"|Cooperate
Line 74: Line 64:
 
|-
 
|-
 
!scope="row" style="color: #009"|Cooperate
 
!scope="row" style="color: #009"|Cooperate
|<span style="color: #009">win</span>-<span style="color: #900">win</span>
+
|<DIV ALIGN=CENTER><span style="color: #009">win</span>-<span style="color: #900">win</span>
|<span style="color: #009">lose much</span>-<span style="color: #900">win much</span>
+
|<span style="color: #009">lose much</span>-<span style="color: #900">win much</span></DIV>
 
|-
 
|-
 
!scope="row" style="color: #009"|Defect
 
!scope="row" style="color: #009"|Defect
|<span style="color: #009">win much</span>-<span style="color: #900">lose much</span>
+
|<DIV ALIGN=CENTER><span style="color: #009">win much</span>-<span style="color: #900">lose much</span></DIV>
|<span style="color: #009">lose</span>-<span style="color: #900">lose</span>
+
|<DIV ALIGN=CENTER><span style="color: #009">lose</span>-<span style="color: #900">lose</span></DIV>
 
|}
 
|}
   
These point assignments are given arbitrarily for illustration. It is possible to generalize them. Let T stand for Temptation to Defect, R for Reward for Mutual Cooperation, P for Punishment for Mutual Defection and S for Suckers Payoff. The following inequalities must hold:
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These point assignments are given arbitrarily for illustration. It is possible to generalize them, as follows:
   
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{| class="wikitable"
T>R>P>S
 
  +
|+ Canonical PD payoff matrix
  +
|
  +
!scope="col" style="color: #900"|Cooperate
  +
!scope="col" style="color: #900"|Defect
  +
|-
  +
!scope="row" style="color: #009"|Cooperate
  +
|<span style="color: #009">R</span>, <span style="color: #900">R</span>
  +
|<span style="color: #009">S</span>, <span style="color: #900">T</span>
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|-
  +
!scope="row" style="color: #009"|Defect
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|<span style="color: #009">T</span>, <span style="color: #900">S</span>
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|<span style="color: #009">P</span>, <span style="color: #900">P</span>
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|}
   
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Where ''T'' stands for ''Temptation to defect'', ''R'' for ''Reward for mutual cooperation'', ''P'' for ''Punishment for mutual defection'' and ''S'' for ''Sucker's payoff''. To be defined as Prisoner's dilemma, the following inequalities must hold:
If the game is iterated (played more than once in a row), the mutual cooperation total payment must exceed the temptation total payment, because otherwise the iterated game would not have a different Nash Equilibrium (see section on iterated version):
 
   
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''T'' > ''R'' > ''P'' > ''S''
2R > T+S
 
   
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This condition ensures that the equilibrium outcome is defection, but that cooperation Pareto dominates equilibrium play. In addition to the above condition, if the game is repeatedly played by two players, the following condition should be added.<ref>{{cite book
These rules were established by cognitive scientist [[Douglas Hofstadter]] and form the formal canonical description of a typical game of Prisoners Dilemma.
 
  +
| last = Dawkins
  +
| first = Richard
  +
| authorlink = Richard Dawkins
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| title = [[The Selfish Gene]]
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| publisher = [[Oxford University Press]]
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| date = 1989
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| id = ISBN 0-19-286092-5}} Page: 204 of Paperback edition</ref>
   
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2 ''R'' > ''T'' + ''S''
==A similar but different game==
 
[[Douglas Hofstadter|Hofstadter]]{{mn|DH|2}} once suggested that people often find problems such as the PD problem easier to understand when it is illustrated in the form of a simple game, or trade-off. One of several examples he used was "closed bag exchange":
 
: Two people meet and exchange closed bags, with the understanding that one of them contains money, and the other contains a purchase. Either player can choose to honour the deal by putting into his bag what he agreed, or he can defect by handing over an empty bag.
 
   
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If that condition does not hold, then full cooperation is not necessarily [[Pareto optimal]], as the players are collectively better off by having each player alternate between cooperate and defect.
In this game, defection is always the best course, implying that rational agents will never play, and that "closed bag exchange" will be a [[missing market]] due to [[adverse selection]].
 
   
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These rules were established by cognitive scientist [[Douglas Hofstadter]] and form the formal canonical description of a typical game of Prisoner's Dilemma.
In a variation, popular among [[hacker]]s and programmers, each bag-exchanging agent is given a memory (or access to a collective memory), and many exchanges are repeated over time.
 
   
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A simple special case occurs when the advantage of defection over cooperation is independent of what the co-player does and cost of the co-players defection is independent of one's own action, i.e. ''T''+''S'' = ''P''+''R''.
As noted, without this introduction of time and memory, there is not much meaning to this game. Not much is explained about the behaviour of actual systems and groups of people, except for describing interactions which don't happen. Yet more complexity is introduced here than might be expected. The programmer (especially the [[functional program]]mer) will pick up right away on the significance of introducing [[time]] and [[state]] (memory). But without any background on writing programs or modelling these kinds of systems, the various choices that one would have to make can be seen. How big is the memory of each actor? What is the strategy of each actor? How are actors with various strategies distributed and what determines who interacts with whom and in what order?
 
   
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==Human behavior in the Prisoner's Dilemma==
One may become frustrated by the complexity involved in creating any model which is meaningful at all, but some very interesting and worthy technical and philosophical issues are raised.
 
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One experiment based on the simple dilemma found that approximately 40% of participants played "cooperate" (i.e., stayed silent).<ref>{{cite book |last=Tversky |first=Amos |title=Preference, Belief, and Similarity: Selected Writings |publisher=MIT Press |year=2004 |isbn=026270093X}}</ref>
   
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==The iterated prisoner's dilemma==
The pregnancy of this problem is suggested by the fact that this discussion has not even mentioned the possibility of the formation (spontaneous or otherwise) of conglomerates of actors, negotiating their bag-exchanges collectively. And what about agents, who charge a fee for organising these bag exchanges? Or agents (journalists?) who collect and exchange information about the bag exchanges themselves?
 
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If two players play Prisoner's Dilemma more than once in succession, having memory of at least one previous game, it is called iterated Prisoner's Dilemma. Amongst results shown by Nobel Prize winner [[Robert Aumann]] in his 1959 paper, rational players repeatedly interacting for indefinitely long games can sustain the cooperative outcome. Popular interest in the iterated prisoners dilemma (IPD) was kindled by [[Robert Axelrod]] in his book ''[[The Evolution of Cooperation]]'' (1984). In this he reports on a tournament he organized in which participants have to choose their mutual strategy again and again, and have memory of their previous encounters. Axelrod invited academic colleagues all over the world to devise computer strategies to compete in an [[IPD tournament]]. The programs that were entered varied widely in algorithmic complexity, initial hostility, capacity for forgiveness, and so forth.
   
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Axelrod discovered that when these encounters were repeated over a long period of time with many players, each with different strategies, greedy strategies tended to do very poorly in the long run while more [[altruism|altruistic]] strategies did better, as judged purely by self-interest. He used this to show a possible mechanism for the evolution of altruistic behaviour from mechanisms that are initially purely selfish, by [[natural selection]].
==Real-life examples==
 
These particular examples, involving prisoners and bag switching and so forth, may seem contrived, but there are in fact many examples in human interaction as well as interactions in nature that have the same payoff matrix. The prisoner's dilemma is therefore of interest to the [[social science]]s such as [[economics]], [[politics]] and [[sociology]], as well as to the biological sciences such as [[ethology]] and [[evolutionary biology]]. Many natural processes have been abstracted into models in which living beings are engaged in endless games of Prisoner's Dilemma. This wide applicability of the PD gives the game its substantial importance.
 
   
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The best [[deterministic algorithm|deterministic]] strategy was found to be "[[Tit for Tat]]," which [[Anatol Rapoport]] developed and entered into the tournament. It was the simplest of any program entered, containing only four lines of BASIC, and won the contest. The strategy is simply to cooperate on the first iteration of the game; after that, the player does what his opponent did on the previous move. Depending on the situation, a slightly better strategy can be "Tit for Tat with forgiveness." When the opponent defects, on the next move, the player sometimes cooperates anyway, with a small probability (around 1%-5%). This allows for occasional recovery from getting trapped in a cycle of defections. The exact probability depends on the line-up of opponents.
In [[political science]], for instance, the PD scenario is often used to illustrate the problem of two states engaged in an [[arms race]]. Both will reason that they have two options, either to increase [[military expenditure]] or to make an agreement to reduce weapons. Neither state can be certain that the other one will keep to such an agreement; therefore, they both incline towards military expansion. The [[paradox]] is that both states are acting "[[rational]]ly", but producing an apparently "irrational" result. This could be considered a [[corollary]] to [[deterrence theory]].
 
   
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By analysing the top-scoring strategies, Axelrod stated several conditions necessary for a strategy to be successful.
Another interesting example concerns a well-known concept in [[cycling]] races, for instance in the [[Tour de France]]. Consider two cyclists halfway in a race, with the [[peloton]] (larger group) at great distance behind them. The two cyclists often work together (''mutual cooperation'') by sharing the tough load of the front position, where there is no shelter from the wind. If neither of the cyclists makes an effort to stay ahead, the peloton will soon catch up (''mutual defection''). An often-seen scenario is one cyclist doing the hard work alone (''cooperating''), keeping the two ahead of the peloton. In the end, this will likely lead to a victory for the second cyclist (''defecting'') who has an easy ride in the first cyclist's [[slipstream]].
 
   
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; Nice: The most important condition is that the strategy must be "nice", that is, it will not defect before its opponent does (this is sometimes referred to as an "optimistic" algorithm). Almost all of the top-scoring strategies were nice; therefore a purely selfish strategy will not "cheat" on its opponent, for purely utilitarian reasons first.
An occurrence of the prisoner’s dilemma in real life can be found in business. Two competing firms must decide how many resources to devote to advertisement. The effectiveness of Firm A’s advertising is partially determined by the advertising conducted by Firm B. Likewise, the profit derived from advertising for Firm B is affected by the advertising conducted by Firm A. If both Firm A and Firm B choose to advertise during a given period the advertising cancels out, receipts remain constant, and expenses increase due to the cost of advertising. Both firms would benefit from a reduction in advertising. However, should Firm B choose not to advertise, Firm A could benefit greatly by advertising. A prisoners' dilemma occurs when both Firm A and Firm B have dominant strategies, so the outcome is easy to predict. A dominant strategy is an action that will give the best result no matter what decision the rival firm makes. It is unlikely in a true prisoners' dilemma that Firm A and Firm B will cooperate because there is too much incentive for both sides to "cheat" in order to get their best outcome. It is also true that both sides will end up worse off than if they had cooperated. However, sometimes cooperative behaviours emerge in business situations which are surprisingly beneficial to the masses {{mn|Trust|6}}.
 
  +
; Retaliating: However, Axelrod contended, the successful strategy must not be a blind optimist. It must sometimes retaliate. An example of a non-retaliating strategy is Always Cooperate. This is a very bad choice, as "nasty" strategies will ruthlessly exploit such players.
  +
; Forgiving: Successful strategies must also be forgiving. Though players will retaliate, they will once again fall back to cooperating if the opponent does not continue to play defects. This stops long runs of revenge and counter-revenge, maximizing points.
  +
; Non-envious: The last quality is being non-envious, that is not striving to score more than the opponent (impossible for a ‘nice’ strategy, i.e., a 'nice' strategy can never score more than the opponent).
   
  +
Therefore, Axelrod reached the [[oxymoron]]-sounding conclusion that selfish individuals for their own selfish good will tend to be nice and forgiving and non-envious.
William Poundstone, in a book about the Prisoner's Dilemma (see References below), describes a situation in New Zealand where newspaper boxes are left unlocked. It is possible for someone to take a paper without paying (''defecting'') but very few do, recognising the resultant harm if everybody stole newspapers (''mutual defection''). Since the pure PD is simultaneous for all players (with no way for any player's action to have an effect on another's strategy) this widespread line of reasoning is called "[[magical thinking]]".{{mn|Magic|3}}
 
   
  +
The optimal (points-maximizing) strategy for the one-time PD game is simply defection; as explained above, this is true whatever the composition of opponents may be. However, in the iterated-PD game the optimal strategy depends upon the strategies of likely opponents, and how they will react to defections and cooperations. For example, consider a population where everyone defects every time, except for a single individual following the Tit-for-Tat strategy. That individual is at a slight disadvantage because of the loss on the first turn. In such a population, the optimal strategy for that individual is to defect every time. In a population with a certain percentage of always-defectors and the rest being Tit-for-Tat players, the optimal strategy for an individual depends on the percentage, and on the length of the game.
The theoretical conclusion of PD is one reason why, in many countries, [[plea bargain]]ing is forbidden. Often, precisely the PD scenario applies: it is in the interest of both suspects to confess and testify against the other prisoner/suspect, even if each is innocent of the alleged crime. Arguably, the worst case is when only one party is guilty &mdash; here, the innocent one is unlikely to confess, while the guilty one is likely to confess and testify against the innocent.
 
   
  +
A strategy called Pavlov (an example of [[Win-Stay, Lose-Switch]]) cooperates at the first iteration and whenever the player and co-player did the same thing at the previous iteration; Pavlov defects when the player and co-player did different things at the previous iteration. For a certain range of parameters, Pavlov beats all other strategies by giving preferential treatment to co-players which resemble Pavlov.
Many real-life dilemmas involve multiple players. Although metaphorical, [[Garrett Hardin|Hardin's]] [[tragedy of the commons]] may be viewed as an example of a multi-player generalisation of the PD: Each villager makes a choice for personal gain or restraint. The collective reward for unanimous (or even frequent) defection is very low payoffs (representing the destruction of the "commons"). However, such multi-player PDs are not formal as they can always be decomposed into a set of classical two-player games.
 
   
  +
Deriving the optimal strategy is generally done in two ways:
==The iterated prisoner's dilemma==
 
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# [[Bayesian game#Bayesian Nash equilibrium|Bayesian Nash Equilibrium]]: If the statistical distribution of opposing strategies can be determined (e.g. 50% tit-for-tat, 50% always cooperate) an optimal counter-strategy can be derived analytically.<ref name="bne">For example see the 2003 study [http://econ.hevra.haifa.ac.il/~mbengad/seminars/whole1.pdf “Bayesian Nash equilibrium; a statistical test of the hypothesis”] for discussion of the concept and whether it can apply in real [[economic]] or strategic situations (from [[Tel Aviv University]]).</ref>
In his book ''[[The Evolution of Cooperation]]'' (1984), [[Robert Axelrod]] explored an extension to the classical PD scenario, which he called the ''iterated prisoner's dilemma'' (IPD). In this, participants have to choose their mutual strategy again and again, and have memory of their previous encounters. Axelrod invited academic colleagues all over the world to devise computer strategies to compete in an [[IPD tournament]]. The programs that were entered varied widely in algorithmic complexity; initial hostility; capacity for forgiveness; and so forth.
 
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# [[Monte Carlo method|Monte Carlo]] simulations of populations have been made, where individuals with low scores die off, and those with high scores reproduce (a [[genetic algorithm]] for finding an optimal strategy). The mix of algorithms in the final population generally depends on the mix in the initial population. The introduction of mutation (random variation during reproduction) lessens the dependency on the initial population; empirical experiments with such systems tend to produce Tit-for-Tat players (see for instance Chess 1988), but there is no analytic proof that this will always occur.
   
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Although Tit-for-Tat is considered to be the most robust basic strategy, a team from [[Southampton University]] in England (led by Professor Nicholas Jennings [http://www.ecs.soton.ac.uk/~nrj] and consisting of Rajdeep Dash, Sarvapali Ramchurn, Alex Rogers, Perukrishnen Vytelingum) introduced a new strategy at the 20th-anniversary Iterated Prisoner's Dilemma competition, which proved to be more successful than Tit-for-Tat. This strategy relied on cooperation between programs to achieve the highest number of points for a single program. The University submitted 60 programs to the competition, which were designed to recognize each other through a series of five to ten moves at the start. Once this recognition was made, one program would always cooperate and the other would always defect, assuring the maximum number of points for the defector. If the program realized that it was playing a non-Southampton player, it would continuously defect in an attempt to minimize the score of the competing program. As a result,<ref name="southamptontrick">[http://www.prisoners-dilemma.com/results/cec04/ipd_cec04_full_run.html The 2004 Prisoner's Dilemma Tournament Results] show [[University of Southampton]]'s strategies in the first three places, despite having fewer wins and many more losses than the GRIM strategy. (Note that in a PD tournament, the aim of the game is not to “win” matches&nbsp;— that can easily be achieved by frequent defection). It should also be pointed out that even without implicit collusion between [[computer program|software strategies]] (exploited by the Southampton team) tit-for-tat is not always the absolute winner of any given tournament; it would be more precise to say that its long run results over a series of tournaments outperform its rivals. (In any one event a given strategy can be slightly better adjusted to the competition than tit-for-tat, but tit-for-tat is more robust). The same applies for the tit-for-tat-with-forgiveness variant, and other optimal strategies: on any given day they might not 'win' against a specific mix of counter-strategies.An alternative way of putting it is using the Darinian [[Evolutionarily stable strategy|ESS]] simulation. In such a simulation Tit-for-Tat will almost always come to dominate, though nasty strategies will drift in and out of the population because a Tit-for-Tat population is penetratable by non-retaliating nice strategies which in turn are easy prey for the nasty strategies. Richard Dawkins showed that here no static mix of strategies form a stable equilibrium and the system will always oscillate between bounds.</ref> this strategy ended up taking the top three positions in the competition, as well as a number of positions towards the bottom.
Axelrod discovered that when these encounters were repeated over a long period of time with many players, each with different strategies, "greedy" strategies tended to do very poorly in the long run while more "[[altruism|altruistic]]" strategies did better, as judged purely by self-interest. He used this to show a possible mechanism for the evolution of altruistic behaviour from mechanisms that are initially purely selfish, by [[natural selection]].
 
   
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This strategy takes advantage of the fact that multiple entries were allowed in this particular competition, and that the performance of a team was measured by that of the highest-scoring player (meaning that the use of self-sacrificing players was a form of [[minmaxing]]). In a competition where one has control of only a single player, Tit-for-Tat is certainly a better strategy. Because of this new rule, this competition also has little theoretical significance when analysing single agent strategies as compared to Axelrod's seminal tournament. However, it provided the framework for analysing how to achieve cooperative strategies in multi-agent frameworks, especially in the presence of noise. In fact, long before this new-rules tournament was played, Richard Dawkins in his book ''[[The Selfish Gene]]'' pointed out the possibility of such strategies winning if multiple entries were allowed, but remarked that most probably Axelrod would not have allowed them if they had been submitted. It also relies on circumventing rules about the prisoner's dilemma in that there is no communication allowed between the two players. When the Southampton programs engage in an opening "ten move dance" to recognize one another, this only reinforces just how valuable communication can be in shifting the balance of the game.
The best [[deterministic algorithm|deterministic]] strategy was found to be "[[Tit for Tat]]", which [[Anatol Rapoport]] developed and entered into the tournament. It was the simplest of any program entered, containing only four lines of BASIC, and won the contest. The strategy is simply to cooperate on the first iteration of the game; after that, the player does what his opponent did on the previous move. A slightly better strategy is "Tit for Tat with forgiveness". When the opponent defects, on the next move, the player sometimes cooperates anyway, with a small probability (around 1%-5%). This allows for occasional recovery from getting trapped in a cycle of defections. The exact probability depends on the line-up of opponents. "Tit for Tat with forgiveness" is best when miscommunication is introduced to the game &mdash; when one's move is incorrectly reported to the opponent.
 
   
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If an iterated PD is going to be iterated exactly N times, for some known constant N, then it is always game theoretically optimal to defect in all rounds. The only possible [[Nash equilibrium]] is to always defect. The proof goes like this: one might as well defect on the last turn, since the opponent will not have a chance to punish the player. Therefore, both will defect on the last turn. Thus, the player might as well defect on the second-to-last turn, since the opponent will defect on the last no matter what is done, and so on. For cooperation to emerge between game theoretic rational players, the total number of rounds must be random, or at least unknown to the players. However, even in this case always defect is no longer a strictly dominant strategy, only a Nash equilibrium. The [[superrational]] strategy in this case is to cooperate against a superrational opponent, and in the limit of large fixed N, experimental results on strategies agree with the superrational version, not the game-theoretic rational one.
By analysing the top-scoring strategies, Axelrod stated several conditions necessary for a strategy to be successful.
 
   
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Another odd case is "play forever" prisoner's dilemma. The game is repeated infinitely many times, and the player's score is the average (suitably computed).
; Nice: The most important condition is that the strategy must be "nice", that is, it will not defect before its opponent does. Almost all of the top-scoring strategies were nice. Therefore a purely selfish strategy for purely selfish reasons will never hit its opponent first.
 
; Retaliating: However, Axelrod contended, the successful strategy must not be a blind optimist. It must always retaliate. An example of a non-retaliating strategy is Always Cooperate. This is a very bad choice, as "nasty" strategies will ruthlessly exploit such softies.
 
; Forgiving: Another quality of successful strategies is that they must be forgiving. Though they will retaliate, they will once again fall back to cooperating if the opponent does not continue to play defects. This stops long runs of revenge and counter-revenge, maximising points.
 
; Non-envious: The last quality is being non-envious, that is not striving to score more than the opponent (impossible for a ‘nice’ strategy, i.e., a 'nice' strategy can never score more than the opponent).
 
   
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The prisoner's dilemma game is fundamental to certain theories of human cooperation and trust. On the assumption that the PD can model transactions between two people requiring trust, cooperative behaviour in populations may be modelled by a multi-player, iterated, version of the game. It has, consequently, fascinated many scholars over the years. In 1975, Grofman and Pool estimated the count of scholarly articles devoted to it at over 2,000. The iterated prisoner's dilemma has also been referred to as the "[[Peace war game|Peace-War game]]".<ref>Shy, O., 1996, ''[[industrial organization|Industrial Organization]]: Theory and Applications'', Cambridge, Mass.: The [[MIT]] Press.</ref>
Therefore, Axelrod reached the [[Utopia]]n-sounding conclusion that selfish individuals for their own selfish good will tend to be nice and forgiving and non-envious. One of the most important conclusions of Axelrod's study of IPDs is that Nice guys can finish first.
 
   
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===Continuous Iterated Prisoner's Dilemma===
Reconsider the arms-race model given in the [[prisoner's dilemma#The classical prisoner's dilemma|classical PD section]] above: It was concluded that the only rational strategy was to build up the military, even though both nations would rather spend their [[GDP]] on butter than guns. Interestingly, attempts to show that rival states actually compete in this way (by [[regression|regressing]] "high" and "low" military spending between periods under ''iterated PD assumptions'') often show that the posited arms race is not occurring as expected. (For example [[Greece|Greek]] and [[Turkey|Turkish]] military spending does not appear to follow a [[Tit for Tat|tit-for-tat]] iterated-PD arms-race, but is more likely driven by domestic politics.) This may be an example of rational behaviour differing between the one-off and iterated forms of the game.
 
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Most work on the iterated prisoner's dilemma has focused on the discrete case, in which players either cooperate or defect, because this model is relatively simple to analyze. However, some researchers have looked at models of the continuous iterated prisoner's dilemma, in which players are able to make a variable contribution to the other player. Le and Boyd<ref>
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Le, S. and R. Boyd (2007) "Evolutionary Dynamics of the Continuous Iterated Prisoner's Dilemma" Journal of Theoretical Biology, Volume 245, 258–267.</ref> found that in such situations, cooperation is much harder to evolve than in the discrete iterated prisoner's dilemma. The basic intuition for this result is straigh&shy;tforward: in a continuous prisoner's dilemma, if a population starts off in a non-cooperative equilibrium, players who are only marginally more cooperative than non-cooperators get little benefit from assorting with one another. By contrast, in a discrete prisoner's dilemma, Tit-for-Tat cooperators get a big payoff boost from assorting with one another in a non-cooperative equilibrium, relative to non-cooperators. Since Nature arguably offers more opportunities for variable cooperation rather than a strict dichotomy of cooperation or defection, the continuous prisoner's dilemma may help explain why real-life examples of Tit-for-Tat-like cooperation are extremely rare in Nature (ex. Hammerstein<ref>Hammerstein, P. (2003). Why is reciprocity so rare in social animals? A protestant appeal. In: P. Hammerstein, Editor, Genetic and Cultural Evolution of Cooperation, MIT Press. pp. 83–94.
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</ref>) even though Tit-for-Tat seems robust in theoretical models.
   
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===Learning psychology and game theory===
The optimal (points-maximising) strategy for the one-time PD game is simply defection; as explained above, this is true whatever the composition of opponents may be. However, in the iterated-PD game the optimal strategy depends upon the strategies of likely opponents, and how they will react to defections and cooperations. For example, consider a population where everyone defects every time, except for a single individual following the Tit-for-Tat strategy. That individual is at a slight disadvantage because of the loss on the first turn. In such a population, the optimal strategy for that individual is to defect every time. In a population with a certain percentage of always-defectors and the rest being Tit-for-Tat players, the optimal strategy for an individual depends on the percentage, and on the length of the game.
 
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Where game players can learn to estimate the likelihood of other players defecting, their own behaviour is influenced by their experience of the others' behaviour. Simple statistics show that inexperienced players are more likely to have had, overall, atypically good or bad interactions with other players. If they act on the basis of these experiences (by defecting or cooperating more than they would otherwise) they are likely to suffer in future transactions. As more experience is accrued a truer impression of the likelihood of defection is gained and game playing becomes more successful. The early transactions experienced by immature players are likely to have a greater effect on their future playing than would such transactions affect mature players. This principle goes part way towards explaining why the formative experiences of young people are so influential and why, for example, those who are particularly vulnerable to bullying sometimes become bullies themselves.
   
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The likelihood of defection in a population may be reduced by the experience of cooperation in earlier games allowing [[Trust (sociology)|trust]] to build up.<ref name="trust" /> Hence self-sacrificing behaviour may, in some instances, strengthen the moral fibre of a group. If the group is small the positive behaviour is more likely to feed back in a mutually affirming way, encouraging individuals within that group to continue to cooperate. This is allied to the twin dilemma of encouraging those people whom one would aid to indulge in behaviour that might put them at risk. Such processes are major concerns within the study of [[reciprocal altruism]], [[group selection]], [[kin selection]] and [[Ethics (philosophy)|moral philosophy]].
Deriving the optimal strategy is generally done in two ways:
 
# Bayesian [[Nash Equilibrium]]: If the statistical distribution of opposing strategies can be determined (e.g. 50% tit-for-tat, 50% always cooperate) an optimal counter-strategy can be derived mathematically{{mn|BNE|4}}.
 
# [[Monte Carlo method|Monte Carlo]] simulations of populations have been made, where individuals with low scores die off, and those with high scores reproduce (a [[genetic algorithm]] for finding an optimal strategy). The mix of algorithms in the final population generally depends on the mix in the initial population. The introduction of mutation (random variation during reproduction) lessens the dependency on the initial population; empirical experiments with such systems tend to produce Tit-for-Tat players (see for instance Chess 1988), but there is no analytic proof that this will always occur.
 
   
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=== [[Douglas Hofstadter]]'s [[Superrationality]] ===
Although Tit-for-Tat was long considered to be the most solid basic strategy, a team from [[Southampton University]] in England (led by Professor Nicholas Jennings [http://www.ecs.soton.ac.uk/~nrj], and including Rajdeep Dash, Sarvapali Ramchurn, Alex Rogers and Perukrishnen Vytelingum) introduced a new strategy at the 20th-anniversary Iterated Prisoner's Dilemma competition, which proved to be more successful than Tit-for-Tat. This strategy relied on cooperation between programs to achieve the highest number of points for a single program. The University submitted 60 programs to the competition, which were designed to recognise each other through a series of five to ten moves at the start. Once this recognition was made, one program would always cooperate and the other would always defect, assuring the maximum number of points for the defector. If the program realised that it was playing a non-Southampton player, it would continuously defect in an attempt to minimise the score of the competing program. As a result{{mn|SouthamptonTrick|5}}, this strategy ended up taking the top three positions in the competition, as well as a number of positions towards the bottom.
 
   
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[[Douglas Hofstadter]] in his [[Metamagical Themas]] proposed that the definition of "rational" that led "rational" players to defect is faulty. He proposed that there is another type of rational behavior, which he called "[[superrationality|superrational]]", where players take into account that the other person is presumably superrational, like them. Superrational players behave identically, and know that they will behave identically. They take that into account before they maximize their payoffs, and they therefore cooperate.
Although this strategy is notable in that it proved more effective than Tit-for-Tat, it takes advantage of the fact that multiple entries were allowed in this particular competition. In a competition where one has control of only a single player, Tit-for-Tat is certainly a better strategy. It also relies on circumventing rules about the prisoner's dilemma in that there is no communication allowed between the two players. When the Southampton programs engage in an opening "ten move dance" to recognize one another, this only reinforces just how valuable communication can be in shifting the balance of the game.
 
   
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This view of the one-shot PD leads to cooperation as follows:
If an iterated PD is going to be iterated exactly N times, for some known constant N, then there is another interesting fact. The [[Nash equilibrium]] is to defect every time. That is easily proved by induction; one might as well defect on the last turn, since the opponent will not have a chance to punish the player. Therefore, both will defect on the last turn. Thus, the player might as well defect on the second-to-last turn, since the opponent will defect on the last no matter what is done, and so on. For cooperation to remain appealing, then, the future must be indeterminate for both players. One solution is to make the total number of turns N random. The shadow of the future must be indeterminably long.
 
   
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* Any superrational strategy will be the same for both superrational players, since both players will think of it.
Another odd case is "play forever" prisoner's dilemma. The game is repeated infinitely many times, and the player's score is the average (suitably computed).
 
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* therefore the superrational answer will lie on the diagonal of the payoff matrix
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* when you maximize return from solutions on the diagonal, you cooperate
   
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However, if a superrational player plays against a rational opponent, he will serve a 10-year sentence, and the rational player will go free.
The prisoner's dilemma game is fundamental to certain theories of human cooperation and trust. On the assumption that the PD can model transactions between two people requiring trust, cooperative behaviour in populations may be modelled by a multi-player, iterated, version of the game. It has, consequently, fascinated many, many scholars over the years. In 1975, Grofman and Pool estimated the count of scholarly articles devoted to it at over 2000.
 
   
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One-shot cooperation is observed in human culture, wherever religious and ethical codes exist.
===Learning psychology and game theory===
 
Where game players can learn to estimate the likelihood of other players defecting, their own behaviour is influenced by their experience of that of others. Simple statistics show that inexperienced players are more likely to have had, overall, atypically good or bad interactions with other players. If they act on the basis of these experiences (by defecting or cooperating more than they would otherwise) they are likely to suffer in future transactions. As more experience is accrued a truer impression of the likelihood of defection is gained and game playing becomes more successful. The early transactions experienced by immature players are likely to have a greater effect on their future playing than would such transactions affect mature players. This principle goes part way towards explaining why the formative experiences of young people are so influential and why they are particularly vulnerable to bullying, sometimes ending up as bullies themselves.
 
   
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Superrationality is not studied by academics, as rationality excludes any superrational behavior.
The likelihood of defection in a population may be reduced by the experience of cooperation in earlier games allowing [[Trust (sociology)|trust]] to build up{{mn|Trust|6}}. Hence self-sacrificing behaviour may, in some instances, strengthen the moral fibre of a group. If the group is small the positive behaviour is more likely to feedback in a mutually affirming way encouraging individuals within that group to continue to cooperate. This is allied to the twin dilemma of encouraging those people whom one would aid to indulge in behaviour that might put them at risk. Such processes are major concerns within the study of [[reciprocal altruism]], [[group selection]], [[kin selection]] and [[moral philosophy]].
 
   
==Friend or Foe?==
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===Morality===
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While it is sometimes thought that [[morality]] must involve the constraint of self-interest, [[David Gauthier]] famously argues that co-operating in the prisoners dilemma on moral principles is consistent with self-interest and the axioms of game theory.{{Fact|date=April 2008}} In his opinion, it is most prudent to give up straigh&shy;tforward maximizing and instead adopt a disposition of constrained maximization, according to which one resolves to cooperate in the belief that the opponent will respond with the same choice, while in the classical PD it is explicitly stipulated that the response of the opponent does not depend on the player's choice. This form of [[contractarianism]] claims that good moral thinking is just an elevated and subtly strategic version of basic means-end reasoning.
''[[Friend or Foe?]]'' is a game show that aired from 2002 to 2005 on the [[Game Show Network]] in the [[United States]]. It is an example of the prisoner's dilemma game tested by real people, but in an artificial setting. On the game show, three pairs of people compete. As each pair is eliminated, they play a game of Prisoner's Dilemma to determine how their winnings are split. If they both cooperate ("Friend"), they share the winnings 50-50. If one cooperates and the other defects ("Foe"), the defector gets all the winnings and the cooperator gets nothing. If both defect, both leave with nothing. Notice that the payoff matrix is slightly different from the standard one given above, as the payouts for the "both defect" and the "cooperate while the opponent defects" cases are identical. This makes the "both defect" case a weak equilibrium, compared with being a strict equilibrium in the standard prisoner's dilemma. If you know your opponent is going to vote "Foe", then your choice does not affect your winnings. In a certain sense, "Friend or Foe" has a payoff model between "Prisoner's Dilemma" and "[[Game of chicken|Chicken]]".
 
   
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[[Douglas Hofstadter]] expresses a strong personal belief that the mathematical symmetry is reinforced by a moral symmetry,<!-- are you sure that not the other way around? --> along the lines of the [[Kant]]ian [[categorical imperative]]: defecting in the hope that the other player cooperates is morally indefensible.{{Fact|date=April 2008}} If players treat each other as they would treat themselves, then they will cooperate.
The payoff matrix is
 
*''If both players cooperate, each gets +1.''
 
*''If both defect, each gets 0.''
 
*''If A cooperates and B defects, A gets +0 and B gets +2.''
 
   
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==Real-life examples==
Friend or Foe would be useful for someone who wanted to do a real-life analysis of prisoner's dilemma. Notice that participants only get to play once, so all the issues involving repeated playing are not present and a "tit for tat" strategy cannot develop.
 
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These particular examples, involving prisoners and bag switching and so forth, may seem contrived, but there are in fact many examples in human interaction as well as interactions in nature that have the same payoff matrix. The prisoner's dilemma is therefore of interest to the [[social science]]s such as [[economics]], [[politics]] and [[sociology]], as well as to the biological sciences such as [[ethology]] and [[evolutionary biology]]. Many natural processes have been abstracted into models in which living beings are engaged in endless games of Prisoner's Dilemma (PD). This wide applicability of the PD gives the game its substantial importance.
   
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===In politics===
In Friend or Foe, each player is allowed to make a statement to convince the other of his friendliness before both make the secret decision to cooperate or defect. One possible way to 'beat the system' would be for a player to tell his rival, "I am going to choose foe. If you trust me to split the winnings with you later, choose friend. Otherwise, if you choose foe, we both walk away with nothing." A greedier version of this would be "I am going to choose foe. I am going to give you X%, and I'll take (100-X)% of the total prize package. So, take it or leave it, we both get something or we both get nothing." (As in the [[Ultimatum game]].) Now, the trick is to minimise X such that the other contestant will still choose friend. Basically, the player has to know the threshold at which the utility his opponent gets from watching him receive nothing exceeds the utility he gets from the money he stands to win if he just went along.
 
   
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In [[political science]], for instance, the PD scenario is often used to illustrate the problem of two states engaged in an [[arms race]]. Both will reason that they have two options, either to increase [[military expenditure]] or to make an agreement to reduce weapons. Neither state can be certain that the other one will keep to such an agreement; therefore, they both incline towards military expansion. The [[paradox]] is that both states are acting [[Rationality|rational]]ly, but producing an apparently irrational result. This could be considered a [[corollary]] to [[deterrence theory]].
This approach was never tried in the game; it's possible that the judges might not allow it, and that even if they did, [[inequity aversion]] would produce a lower expected payoff from using the tactic. ([[Ultimatum game]]s in which this approach was attempted have led to rejections of high but unequal offers – in some cases up to two weeks wages have been turned down in preference to both players receiving nothing.)
 
   
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===In science===
(The published rules for the TV show disallowed splitting; the contestants had to sign a document saying that if they tried to split the winnings, they would forfeit the prize.)
 
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In [[sociology]] or [[criminology]], the PD may be applied to an actual dilemma facing two inmates. The game theorist Marek Kaminski, a former political prisoner, analysed the factors contributing to payoffs in the game set up by a prosecutor for arrested defendants (cf. [[#References|References]]). He concluded that while the PD is the ideal game of a prosecutor, numerous factors may strongly affect the payoffs and potentially change the properties of the game.
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In [[environmental studies]], the PD is evident in crises such as global [[climate change]]. All countries will benefit from a stable climate, but any single country is often hesitant to curb [[Carbon dioxide|{{co2}}]] emissions. The benefit to an individual country to maintain current behavior is greater than the benefit to all countries if behavior was changed, therefore explaining the current impasse concerning climate change.<ref> The Economist (2007) [http://www.economist.com/finance/displaystory.cfm?story_id=9867020]. </ref>
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In program management and technology development, the PD applies to the relationship between the customer and the developer. Capt Dan Ward, an officer in the US Air Force, examined ''The Program Manager's Dilemma'' in an article published in Defense AT&L, a defense technology journal.<ref> Ward, D. (2004) [http://www.dau.mil/pubs/dam/05_06_2004/war-mj04.pdf The Program Manager's Dilemma The Program Manager's Dilemma] (Defense AT&L, Defense Acquisition University Press). </ref>
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===In sports===
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PD frequently occurs in [[cycling]] races, for instance in the [[Tour de France]]. Consider two cyclists halfway in a race, with the [[peloton]] (larger group) at great distance behind them. The two riders often work together (''mutual cooperation'') by sharing the tough load of the front position, where there is no shelter from the wind. If neither of the riders makes an effort to stay ahead, the peloton will soon catch up (''mutual defection''). An often-seen scenario is one rider doing the hard work alone (''cooperating''), keeping the two ahead of the peloton. Nearer to the finish (where the threat of the peloton has disappeared), the game becomes a simple [[zero-sum game]], with each rider trying to avoid at all costs giving a slipstream advantage to the other rider. If there was a (single) defecting rider in the preceding prisoners' dilemma, it is usually he who will win this zero-sum game, having saved energy in the cooperating rider's [[slipstream]]. The cooperating rider's attitude may seem extremely naive, but he often has no other choice when both riders have different physical profiles. The cooperating rider typically has an [[endurance]] profile, whereas the defecting rider will more likely be a [[Cycling sprinter|sprinter]]. When continuously taking the head position of the twosome, the 'cooperating' rider is merely trying to ride away from the defecting sprinter using his endurance advantage over long distance, thus avoiding a sprint duel at the finish, which he would be bound to lose, even if the sprinting rider had cooperated. Just after the escape from the peloton, the endurance-sprinter difference is less of importance, and it is therefore at this stage of the race that mutual cooperation PD can usually be observed. Arguably, it is this almost unavoidable presence of PD (and its transition in zero-sum games) that (unconsciously) makes cycling an exciting sport to watch.
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PD hardly applies to [[running]] sports, because of the negligible importance of air resistance (and shelter from it).
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In high school wrestling, sometimes participants intentionally lose unnaturally large amounts of weight so as to compete against lighter opponents. In doing so, the participants are clearly not at their top level of physical and athletic fitness and yet often end up competing against the same opponents anyway, who have also followed this practice (''mutual defection''). The result is a reduction in the level of competition. Yet if a participant maintains their natural weight (''cooperating''), they will most likely compete against a stronger opponent who has lost considerable weight.
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===In economics===
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Advertising is sometimes cited as a real life example of the prisoner’s dilemma. When [[cigarette]] advertising was legal in the United States, competing cigarette manufacturers had to decide how much money to spend on advertising. The effectiveness of Firm A’s advertising was partially determined by the advertising conducted by Firm B. Likewise, the profit derived from advertising for Firm B is affected by the advertising conducted by Firm A. If both Firm A and Firm B chose to advertise during a given period the advertising cancels out, receipts remain constant, and expenses increase due to the cost of advertising. Both firms would benefit from a reduction in advertising. However, should Firm B choose not to advertise, Firm A could benefit greatly by advertising. Nevertheless, the optimal amount of advertising by one firm depends on how much advertising the other undertakes. As the best strategy is dependent on what the other firm chooses there is no dominant strategy and this is not a prisoner's dilemma but rather is an example of a [[stag hunt]]. The outcome is similar, though, in that both firms would be better off were they to advertise less than in the equilibrium. Sometimes cooperative behaviors do emerge in business situations. For instance, cigarette manufacturers endorsed the creation of laws banning cigarette advertising, understanding that this would reduce costs and increase profits across the industry.<ref name="trust">This argument for the development of cooperation through trust is given in '' [[The Wisdom of Crowds]] '', where it is argued that long-distance [[capitalism]] was able to form around a nucleus of [[Religious Society of Friends|Quaker]]s, who always dealt honourably with their business partners. (Rather than defecting and reneging on promises&nbsp;— a phenomenon that had discouraged earlier long-term unenforceable overseas contracts). It is argued that dealings with reliable merchants allowed the [[meme]] for cooperation to spread to other traders, who spread it further until a high degree of cooperation became a profitable strategy in general [[commerce]] </ref> This analysis is likely to be pertinent in many other business situations involving advertising.
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Without enforceable agreements, members of a [[cartel]] are also involved in a (multi-player) prisoners' dilemma.<ref name=NicholsonIntermediateMicroEd8>{{citation|last1=Nicholson|first=Walter|authorlink=Walter Nicholson|year=2000|title=Intermediate Microeconomics|edition=8th|publisher=Harcourt}}</ref> 'Cooperating' typically means keeping prices at a pre-agreed minimum level. 'Defecting' means selling under this minimum level, instantly stealing business (and profits) from other cartel members. Ironically, [[anti-trust]] authorities want potential cartel members to mutually defect, ensuring the lowest possible prices for [[consumers]].
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===In law===
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The theoretical conclusion of PD is one reason why, in many countries, [[plea bargain]]ing is forbidden. Often, precisely the PD scenario applies: it is in the interest of both suspects to confess and testify against the other prisoner/suspect, even if each is innocent of the alleged crime. Arguably, the worst case is when only one party is guilty&nbsp;— here, the innocent one is unlikely to confess, while the guilty one is likely to confess and testify against the innocent.
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===In the media===
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In the 2008 edition of [[Big Brother (UK)]], the dilemma was applied to two of the housemates. A prize fund of £50,000 was available. If housemates chose to share the prize fund, each would receive £25,000. If one chose to share, and the other chose to take, the one who took it would receive the entire £50,000. If both chose to take, both housemates would receive nothing. The housemates had a minute to discuss their decision, and were given the possibility to lie. Both housemates declared they would share the prize fund, but either could have potentially been lying. When asked to give their final answers by big brother, both housemates did indeed choose to share, and so won £25,000 each.
  +
  +
===Multiplayer dilemmas===
  +
  +
Many real-life dilemmas involve multiple players. Although metaphorical, [[Garrett Hardin|Hardin's]] [[tragedy of the commons]] may be viewed as an example of a multi-player generalization of the PD: Each villager makes a choice for personal gain or restraint. The collective reward for unanimous (or even frequent) defection is very low payoffs (representing the destruction of the "commons"). Such multi-player PDs are not formal as they can always be decomposed into a set of classical two-player games. The commons are not always exploited: [[William Poundstone]], in a book about the Prisoner's Dilemma (see References below), describes a situation in New Zealand where newspaper boxes are left unlocked. It is possible for someone to [[Excludability|take a paper without paying]] (''defecting'') but very few do, feeling that if they do not pay then neither will others, destroying the system.
  +
  +
Because there is [[causality|no mechanism]] for personal choice to influence others' decisions, this type of thinking relies on correlations between behavior, not on causation. Because of this property, those who do not understand [[superrationality]] often mistake it for [[magical thinking]]. Without [[superrationality]], not only petty theft, but voluntary [[voting]] requires widespread [[magical thinking]], since a non-voter is a free rider on a democratic system.
  +
  +
  +
  +
==Related games==
  +
===Closed-bag exchange===
  +
[[Douglas Hofstadter|Hofstadter]]<ref name="dh">{{cite book | first=Douglas R. | last=Hofstadter| authorlink=Douglas Hofstadter | title= [[Metamagical Themas]]: questing for the essence of mind and pattern | publisher= Bantam Dell Pub Group| year=1985 | id=ISBN 0-465-04566-9}} - see Ch.29 ''The Prisoner's Dilemma Computer Tournaments and the Evolution of Cooperation''.</ref> once suggested that people often find problems such as the PD problem easier to understand when it is illustrated in the form of a simple game, or trade-off. One of several examples he used was "closed bag exchange":
  +
: Two people meet and exchange closed bags, with the understanding that one of them contains money, and the other contains a purchase. Either player can choose to honour the deal by putting into his bag what he agreed, or he can defect by handing over an empty bag.
  +
  +
In this game, defection is always the best course, implying that rational agents will never play. However, in this case both players cooperating and both players defecting actually give the same result, so chances of mutual cooperation, even in repeated games, are few.
  +
  +
===Friend or Foe?===
  +
''[[Friend or Foe?]]'' is a game show that aired from 2002 to 2005 on the [[Game Show Network]] in the [[United States]]. It is an example of the prisoner's dilemma game tested by real people, but in an artificial setting. On the game show, three pairs of people compete. As each pair is eliminated, they play a game of Prisoner's Dilemma to determine how their winnings are split. If they both cooperate (Friend), they share the winnings 50-50. If one cooperates and the other defects (Foe), the defector gets all the winnings and the cooperator gets nothing. If both defect, both leave with nothing. Notice that the payoff matrix is slightly different from the standard one given above, as the payouts for the "both defect" and the "cooperate while the opponent defects" cases are identical. This makes the "both defect" case a weak equilibrium, compared with being a strict equilibrium in the standard prisoner's dilemma. If you know your opponent is going to vote Foe, then your choice does not affect your winnings. In a certain sense, ''Friend or Foe'' has a payoff model between "Prisoner's Dilemma" and "[[Game of chicken|Chicken]]".
  +
  +
The payoff matrix is
  +
{| class="wikitable"
  +
|
  +
!scope="col" style="color: #900"|Cooperate
  +
!scope="col" style="color: #900"|Defect
  +
|-
  +
!scope="row" style="color: #009"|Cooperate
  +
|<span style="color: #009">1</span>, <span style="color: #900">1
  +
|<span style="color: #009">0</span>, <span style="color: #900">2
  +
|-
  +
!scope="row" style="color: #009"|Defect
  +
|<span style="color: #009">2</span>, <span style="color: #900">0</span>
  +
|<span style="color: #009">0</span>, <span style="color: #900">0</span>
  +
|}
  +
  +
This payoff matrix was later used on the [[United Kingdom|British]] [[television]] programmes ''[[Shafted]]'' and ''[[Golden Balls]]''.
   
==References==
 
* [[Robert Axelrod|Axelrod, R.]] (1981). The Evolution of Cooperation. ''Science'', 211(4489):1390-6
 
* [[Robert Axelrod|Axelrod, R.]] (1984). ''[[The Evolution of Cooperation]]''. ISBN 0465021212
 
* David M. Chess (1988). Simulating the evolution of behavior: the iterated prisoners' dilemma problem. Complex Systems, 2:663-670.
 
* Dresher, M. (1961). ''The Mathematics of Games of Strategy: Theory and Applications'' Prentice-Hall, Englewood Cliffs, NJ.
 
* Flood, M.M. (1952). Some experimental games. Research memorandum RM-789. [[RAND]] Corporation, Santa Monica, CA. <!--(Research Memoranda do not appear for sale at the RAND [http://www.rand.org/pubs/authors/f/flood_merrill_m.html store])-->
 
* Poundstone, W. (1992) ''Prisoner's Dilemma'' Doubleday, NY NY.
 
* Greif, A. (2006). ''Institutions and the Path to the Modern Economy: Lessons from Medieval Trade.'' [[Cambridge University Press]], [[Cambridge]], UK.
 
   
 
==See also==
 
==See also==
  +
* [[Cellular automata]]
 
 
* [[Centipede game]]
 
* [[Centipede game]]
  +
* [[Conflict resolution research]]
 
* [[Diner's dilemma]]
 
* [[Diner's dilemma]]
  +
* [[Entrapment games]]
 
* [[Evolutionarily stable strategy]]
 
* [[Evolutionarily stable strategy]]
  +
* [[Folk theorem (game theory)]]
 
* [[Nash equilibrium]]
 
* [[Nash equilibrium]]
  +
* [[Non zero sum game]]
 
* [[Neuroeconomics]]
 
* [[Neuroeconomics]]
  +
* [[Price equation]]
 
* [[Reciprocal altruism]]
 
* [[Reciprocal altruism]]
 
* [[Rendezvous problem]]
 
* [[Rendezvous problem]]
Line 198: Line 269:
 
* [[Tragedy of the commons]]
 
* [[Tragedy of the commons]]
 
* [[Tragedy of the anticommons]]
 
* [[Tragedy of the anticommons]]
  +
* [[Traveler's dilemma]]
 
* [[Trust (sociology)]]
 
* [[Trust (sociology)]]
  +
* [[Social trap]]
  +
* [[War of attrition (game)]]
  +
* [[Zero-sum]]
   
 
==Notes==
 
==Notes==
  +
<!--This article uses the Cite.php citation mechanism. If you would like more information on how to add references to this article, please see http://meta.wikimedia.org/wiki/Cite/Cite.php -->
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  +
{{Reflist|2}}
<div class="references-small">
 
   
  +
==References==
{{mnb|Tell|1}} A simple "[[Tell (poker)|tell]]" that partially or wholly reveals one player's choice – such as the Red player playing their Cooperate card face-up – does not change the fact that Defect is the dominant strategy. When one is considering the game itself, communication has no effect whatsoever. However, when the game is being played in real life considerations outside of the game itself may cause communication to matter. It is a point of utmost importance to the full implications of the dilemma that when we do not need to take into account external considerations, single-instance Prisoner's Dilemma is not affected in any way by communications.
 
  +
{{Sourcesstart}}
  +
* [[Robert Aumann]], “Acceptable points in general cooperative n-person games”, in R. D. Luce and A. W. Tucker (eds.), Contributions to the Theory 23 of Games IV, Annals of Mathematics Study 40, 287–324, Princeton University Press, Princeton NJ.
  +
* [[Robert Axelrod|Axelrod, R.]] (1984). ''[[The Evolution of Cooperation]]''. ISBN 0-465-02121-2
  +
* [[Cristina Bicchieri|Bicchieri, Cristina]] (1993). Rationality and Coordination. Cambridge University Press
  +
* [[Kenneth Binmore]], Fun and Games.
  +
* David M. Chess (1988). Simulating the evolution of behavior: the iterated prisoners' dilemma problem. Complex Systems, 2:663–670.
  +
* [[Melvin Dresher|Dresher, M.]] (1961). ''The Mathematics of Games of Strategy: Theory and Applications'' Prentice-Hall, Englewood Cliffs, NJ.
  +
* [[Merrill M. Flood|Flood, M.M.]] (1952). Some experimental games. Research memorandum RM-789. [[RAND]] Corporation, Santa Monica, CA. <!--(Research Memoranda do not appear for sale at the RAND [http://www.rand.org/pubs/authors/f/flood_merrill_m.html store])-->
  +
* Kaminski, Marek M. (2004) ''Games Prisoners Play'' Princeton University Press. ISBN 0-691-11721-7 http://webfiles.uci.edu/mkaminsk/www/book.html
  +
* Poundstone, W. (1992) ''Prisoner's Dilemma'' Doubleday, NY NY.
  +
* Greif, A. (2006). ''Institutions and the Path to the Modern Economy: Lessons from Medieval Trade.'' [[Cambridge University Press]], [[Cambridge]], UK.
  +
* [[Anatol Rapoport|Rapoport, Anatol]] and Albert M. Chammah (1965). ''Prisoner's Dilemma''. [[University of Michigan Press]].
  +
* S. Le and R. Boyd (2007) "Evolutionary Dynamics of the Continuous Iterated Prisoner's Dilemma" Journal of Theoretical Biology, Volume 245, 258–267. [http://letuhuy.bol.ucla.edu/academic/cont_ipd_Le_Boyd_JTB.pdf Full text]
  +
* A. Rogers, R. K. Dash, S. D. Ramchurn, P. Vytelingum and N. R. Jennings (2007) “Coordinating team players within a noisy iterated Prisoner’s Dilemma tournament” Theoretical Computer Science 377 (1-3) 243-259. [http://users.ecs.soton.ac.uk/nrj/download-files/tcs07.pdf]
  +
{{Sourcesend}}
   
  +
== Further reading ==
Even in single-instance Prisoner's Dilemma, meaningful prior communication could alter the play environment, by raising the possibility of enforceable side contracts or credible threats. For example, if the Red player plays their Cooperate card face-up and simultaneously reveals a binding commitment to blow the jail up if and only if Blue Defects (with additional payoff <span style="color: #009">-11</span>,<span style="color: #900">-10</span>), then Blue's Cooperation becomes dominant. As a result, players are screened from each other and prevented from communicating outside of the game.
 
  +
===Books===
  +
*Banfield, G., & Christodoulou, C. (2005). Can Self-Control Be Explained Through Games? River Edge, NJ: World Scientific Publishing Co.
  +
*Bendor, J., Kramer, R. M., & Stout, S. (2005). When in Doubt... Cooperation in a Noisy Prisoner's Dilemma. Northampton, MA: Edward Elgar Publishing.
  +
* [[Cristina Bicchieri|Bicchieri, Cristina]] and Mitchell Green (1997) "Symmetry Arguments for Cooperation in the Prisoner's Dilemma", in G. Holmstrom-Hintikka and R. Tuomela (eds.), Contemporary Action Theory: The Philosophy and Logic of Social Action, Kluwer.
  +
*Goren, H., & Bornstein, G. (1999). Reciprocation and learning in the Intergroup Prisoner's Dilemma Game. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  +
*Insko, C. A., & Schopler, J. (1998). Differential distrust of groups and individuals. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  +
*Komorita, S. S., & Parks, C. D. (1999). Reciprocity and cooperation in social dilemmas: Review and future directions. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  +
*Larsen, J. D. (2002). Prisoner's dilemma as a model for understanding decisions. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  +
*Liebrand, W. B. G., & Messick, D. M. (1999). Dynamic and static theories of costs and benefits of cooperation. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  +
*McCabe, K. A., & Smith, V. L. (2001). Goodwill Accounting and the process of exchange. Cambridge, MA: The MIT Press.
  +
*Schulz, U. (1994). Mathematical models of gaming behavior. Lisse, Netherlands: Swets & Zeitlinger Publishers.
   
{{mnb|DH|2}} {{cite book | first=Douglas R. | last=Hofstadter| authorlink=Douglas Hofstadter | title= [[Metamagical Themas]]: questing for the essence of mind and pattern | publisher= Bantam Dell Pub Group| year=1985 | id=ISBN 0-46-504566-9}} - see Ch.29 ''The Prisoner's Dilemma Computer Tournaments and the Evolution of Cooperation''.
 
   
  +
===Papers===
{{mnb|Magic|3}} As well as being an explanation for the lack of petty-theft, [[magical thinking]] has been used to explain such things as voluntary [[voting]] (where a non-voter is considered a [[free rider]]). Potentially, it might be used to explain [[Wikipedia]] contributions: Text may be added under the assumption that if contributions are not made, then similar people will also fail to contribute (i.e. arguing from effect to cause). Alternatively, the explanation could depend on expected future actions (and not require a magical connection). Modelling future interactions requires the addition of the temporal dimension, as given in the [[prisoner's dilemma#The iterated prisoner's dilemma|Iterated prisoner’s dilemma section]].
 
  +
Aaftink, J. (1989). Far-sighted equilibria in 2x2, non-cooperative, repeated games: Theory and Decision Vol 27(3) Nov 1989, 175-192.
   
{{mnb|BNE|4}} For example see the 2003 study [http://econ.hevra.haifa.ac.il/~mbengad/seminars/whole1.pdf “Bayesian Nash equilibrium; a statistical test of the hypothesis”] for discussion of the concept and whether it can apply in real [[economic]] or strategic situations (from [[Tel Aviv University]]).
 
   
  +
*Acevedo, M., & Krueger, J. I. (2005). Evidential reasoning in the prisoner's dilemma: American Journal of Psychology Vol 118(3) Fal 2005, 431-457.
{{mnb|SouthamptonTrick|5}}[http://www.prisoners-dilemma.com/results/cec04/ipd_cec04_full_run.h
 
  +
*Ajzen, I. (1971). Attitudinal vs. normative messages: An investigation of the differential effects of persuasive communications on behavior: Sociometry Vol 34(2) Jun 1971, 263-280.
</div>
 
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  +
*Axelrod, R. (1980). More effective choice in the Prisoner's Dilemma: Journal of Conflict Resolution Vol 24(3) Sep 1980, 379-403.
  +
*Axelrod, R. (1990). The emergence of cooperation among egoists. New York, NY: Cambridge University Press.
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*Axelrod, R., & Dion, D. (1988). The further evolution of cooperation: Science Vol 242(4884) Dec 1988, 1385-1390.
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*Axelrod, R., & Hamilton, W. D. (1981). The evolution of cooperation: Science Vol 211(4489) Mar 1981, 1390-1396.
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*Axelrod, R., Riolo, R. L., & Cohen, M. D. (2002). Beyond geography: Cooperation with persistent links in the absence of clustered neighborhoods: Personality and Social Psychology Review Vol 6(4) 2002, 341-346.
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*Baker, F., & Rachlin, H. (2002). Self-control by pigeons in the prisoner's dilemma: Psychonomic Bulletin & Review Vol 9(3) Sep 2002, 482-488.
   
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*Bartholdi, J. J., Butler, C. A., & Trick, M. A. (1986). More on the evolution of cooperation: Journal of Conflict Resolution Vol 30(1) Mar 1986, 129-140.
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*[http://www.nature.com/npp/journal/v31/n5/full/1300932a.html Effects of Tryptophan Depletion on the Performance of an Iterated Prisoner's Dilemma Game in Healthy Adults] - Nature Neuropsychopharmacology
 
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*Clements, K. C., & Stephens, D. W. (1995). Testing models of non-kin cooperation: Mutualism and the Prisoner's Dilemma: Animal Behaviour Vol 50(2) Aug 1995, 527-535.
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*Van Lange, P. A., Liebrand, W. B., & Kuhlman, D. M. (1990). Causal attribution of choice behavior in three N-Person Prisoner's Dilemmas: Journal of Experimental Social Psychology Vol 26(1) Jan 1990, 34-48.
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*Vega-Redondo, F. (1994). Bayesian boundedly rational agents play the Finitely Repeated Prisoner's Dilemma: Theory and Decision Vol 36(2) Mar 1994, 187-206.
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*Vlaev, I., & Chater, N. (2007). Context effects in games: Local versus global sequential effects on choice in the prisoner's dilemma game: Judgment and Decision Making Vol 2(6) Dec 2007, 380-389.
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*Vlaev, I., & Chater, N. (2008). Debiasing context effects in strategic decisions: Playing against a consistent opponent can correct perceptual but not reinforcement: Judgment and Decision Making Vol 3(6) Aug 2008, 463-474.
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*Volij, O. (2002). In Defense of DEFECT: Games and Economic Behavior Vol 39(2) May 2002, 309-321.
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*Watabe, M., Terai, S., Hayashi, N., & Yamagishi, T. (1996). Cooperation in the one-shot Prisoner's Dilemma based on expectations of reciprocity: Japanese Journal of Experimental Social Psychology Vol 36(2) Dec 1996, 183-196.
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*Watanabe, Y., & Yamagishi, T. (1997). When "false consensus" stops being "false": An experimental study of a 1-shot prisoner's dilemma situation: Japanese Journal of Psychology Vol 67(6) Feb 1997, 421-428.
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*Wiseman, T., & Yilankaya, O. (2001). Cooperation, Secret Handshakes, and Imitation in the Prisoners' Dilemma: Games and Economic Behavior Vol 37(1) Oct 2001, 216-242.
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*Wong, R. Y.-m., & Hong, Y.-y. (2005). Dynamic Influences of Culture on Cooperation in the Prisoner's Dilemma: Psychological Science Vol 16(6) Jun 2005, 429-434.
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*Wu, J., & Axelrod, R. (1995). How to cope with noise in the iterated prisoner's dilemma: Journal of Conflict Resolution Vol 39(1) Mar 1995, 183-189.
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*Yabuuchi, M. (1986). Metagame theory and extension of conditional strategy in Prisoner's Dilemma game: Japanese Journal of Experimental Social Psychology Vol 26(1) Aug 1986, 57-65.
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*Yamagishi, T., Kanazawa, S., Mashima, R., & Terai, S. (2005). Separating trust from cooperation in a dynamic relationship: Prisoner's Dilemma with variable dependence: Rationality and Society Vol 17(3) Aug 2005, 275-308.
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*Yamagishi, T., & Kiyonari, T. (2000). The group as the container of generalized reciprocity: Social Psychology Quarterly Vol 63(2) Jun 2000, 116-132.
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*Yamagishi, T., Makimura, Y., Foddy, M., Matsuda, M., Kiyonari, T., & Platow, M. J. (2005). Comparisons of Australians and Japanese on group-based cooperation: Asian Journal of Social Psychology Vol 8(2) Aug 2005, 173-190.
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*Yamagishi, T., Terai, S., Kiyonari, T., Mifune, N., & Kanazawa, S. (2007). The social exchange heuristic: Managing errors in social exchange: Rationality and Society Vol 19(3) Aug 2007, 259-291.
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*Yamauchi, T. (1982). The effect of face-to-face playing on choice behavior in Prisoner's Dilemma game: Japanese Journal of Psychology Vol 52(6) Feb 1982, 337-343.
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*Yi, R., Buchhalter, A. R., Gatchalian, K. M., & Bickel, W. K. (2007). The relationship between temporal discounting and the prisoner's dilemma game in intranasal abusers of prescription opioids: Drug and Alcohol Dependence Vol 87(1) Feb 2007, 94-97.
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*Yi, R., Johnson, M. W., & Bickel, W. K. (2005). Relationship between cooperation in an iterated prisoner's dilemma game and the discounting of hypothetical outcomes: Learning & Behavior Vol 33(3) Aug 2005, 324-336.
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*Yi, R., & Rachlin, H. (2004). Contingencies of reinforcement in a five-person prisoner's dilemma: Journal of the Experimental Analysis of Behavior Vol 82(2) Sep 2004, 161-176.
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*Yoshida, F. (1978). An experimental study of interpersonal perceptions in social interaction: I. The cooperator and the noncooperator: Divergent vs convergent perceptions: Japanese Journal of Experimental Social Psychology Vol 18(1) Aug 1978, 11-20.
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*Yoshida, F. (1984). Intergroup orientations and intergroup perceptions in a Prisoner's Dilemma Game: Japanese Journal of Psychology Vol 55(5) Dec 1984, 282-288.
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*Yoshida, F., & Ohmoto, S. (1985). Comparison between interpersonal and intergroup behaviors and perceptions in a Prisoner's Dilemma Game: Japanese Journal of Psychology Vol 56(2) Jun 1985, 86-92.
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*Youngs, G. A. (1986). Patterns of threat and punishment reciprocity in a conflict setting: Journal of Personality and Social Psychology Vol 51(3) Sep 1986, 541-546.
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*Zhong, C.-B., Loewenstein, J., & Murnighan, J. K. (2007). Speaking the same language: The cooperative effects of labeling in the prisoner's dilemma: Journal of Conflict Resolution Vol 51(3) Jun 2007, 431-456.
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*Zoltan, J. (1992). The effect of recessivity and segregation in the frame of the prisoner's dilemma: Pszichologia: Az MTA Pszichologiai Intezetenek folyoirata Vol 12(4) 1992, 539-564.
  +
  +
===Dissertations===
  +
*Acevedo, M. (2002). The effects of social projection and payoff on cooperative behavior in the prisoner's dilemma. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
*Armstrong, S. H. (1975). A three-dimensional theory of group process in adolescent dyads: Dissertation Abstracts International.
  +
*Beale, D. K. (1974). Conflicting motives in the Prisoner's Dilemma game: Dissertation Abstracts International.
  +
*Beyer, R. A. (1981). An assessment of the effects of moral level and age on contract maintenance in prisoner's dilemma: Dissertation Abstracts International.
  +
*Bryant, W. P. (1978). Machiavellianism, perspective taking, and partner's response as predictors of interpersonal behavior measured by a modified Prisoner's Dilemma game: Dissertation Abstracts International.
  +
*Charlton, S. R. (2007). The relationship between behavioral measures of self-control: Temporal discounting and the single-player iterated prisoner's dilemma. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
*Christensen, R. L. (1976). An investigation of interpersonal trust and the effect of dogmatism and characteristics of the other person on subjective trust in the Prisoner's Dilemma game: Dissertation Abstracts International.
  +
*Conklin, M. M. (1983). An investigation of delinquent and nondelinquent adolescent performance on The Prisoner's Dilemma Game when paired with an adult confederate under three different strategy conditions: Dissertation Abstracts International.
  +
*Elliott, S. W. (1992). Steps towards a psychological calculus for game theory: Application of a model of categorization to the repeated prisoner's dilemma: Dissertation Abstracts International.
  +
*Finney, P. D. (1977). Responsibility attributions and the self system: Dissertation Abstracts International.
  +
*Foster, M. S. (1976). Sex differences in behavior as a function of experience and expectation: Dissertation Abstracts International.
  +
*Hamburger, F. M. (1976). On behavioral effects of normative and attitudinal communications: Dissertation Abstracts International.
  +
*Harkey, S. T. (2001). Choice propensity in single exposure/no-feedback prisoner dilemmas as a function of social orientation, payoffs, and expectations: Evaluation of a formal model. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
*Hartman, E. A. (1973). The motivational basis of sex differences in the Prisoner's Dilemma game: Dissertation Abstracts International Vol.
  +
*Helm, B. L. (1973). Locus of control and the exercise of coercive power: Dissertation Abstracts International.
  +
*Herrman, K. D. (2006). Inducing interpersonal trust and team learning using a prisoner's dilemma game. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
*Hoerl, R. T. (1972). Cooperation and competition as a function of communication and trust: Dissertation Abstracts International Vol.
  +
*Jacobs, M. K. (1976). Women's moral reasoning and behavior in Prisoner's Dilemma: Dissertation Abstracts International.
  +
*Kimmel, M. J. (1975). On distinguishing interpersonal trust from cooperative responding in the Prisoner's Dilemma game: Dissertation Abstracts International.
  +
*Koenig, K. K. (1980). Cooperation as a function of locus of control in a Prisoner's Dilemma game: Dissertation Abstracts International.
  +
*Krause, R. M. (1975). A general conflict simulation model for ecological generality using the Prisoner's Dilemma: Dissertation Abstracts International.
  +
*Lichtig, L. K. (1977). The development of interpersonal relationships: An experimental approach: Dissertation Abstracts International.
  +
*Lopez, L. C. (1977). The relationship between selected cognitive styles and cooperation in a Prisoner's Dilemma game situation: Dissertation Abstracts International.
  +
*Mayer, F. J. (1993). Nonlinear dynamics of strategic interactions: Dissertation Abstracts International.
  +
*McCarter, R. H. (1988). Self-integration and self-defeating behavior in the Prisoner's Dilemma Game: Dissertation Abstracts International.
  +
*Morikawa, T. (1994). A cognitive theory of how cooperation evolves: A simulation. Dissertation Abstracts International Section A: Humanities and Social Sciences.
  +
*Oman, R. N. (1978). The effect of unpredictable and malevolent strategies on the game playing of childhood schizophrenics: Dissertation Abstracts International.
  +
*O'Riordan, N. F. (1987). Behavior of low and high trust juvenile delinquents when playing a modified Prisoner's Dilemma Game: Dissertation Abstracts International.
  +
*Pincus, J. (1977). Cognitive factors and cooperation in the Prisoner's Dilemma: Dissertation Abstracts International.
  +
*Pivnick, W. P. (1973). Parental occupation and situational meaning as determinants of behavior in the Prisoner's Dilemma Game: Dissertation Abstracts International Vol.
  +
*Plous, S. L. (1986). Perceptual illusions and military realities: A social-psychological analysis of the nuclear arms race: Dissertation Abstracts International.
  +
*Pollard, S. W. (1975). Effects of power on cooperation: Dissertation Abstracts International.
  +
*Romero, C. (1978). Cooperative behavior of prison inmates towards peers and institutional staff utilizing the prisoner dilemma game: Dissertation Abstracts International.
  +
*Sachteleben, T. L. (2006). The effect of opportunity cost, level of cooperation, and social value orientation on preferences for structural solutions to the N-person Prisoner's Dilemma. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
*Sanabria, F. (2004). Pigeons in an operant prisoner's dilemma: New experimental paradigms. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
*Sell, R. L. (1977). Cooperation and competition as a function of residential environment, consequences of game strategy choices and perceived control: Dissertation Abstracts International.
  +
*Setzman, E. J. (1974). Cooperation and competition between men and women in a dyadic game-playing situation: Dissertation Abstracts International.
  +
*Shebalin, P. V. (1997). Collective learning and cooperation between intelligent software agents: A study of artificial personality and behavior in autonomous agents playing the infinitely repeated prisoner's dilemma game. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
*Speyer, E. C. (1985). Trust level within a dyad as influenced by observed trust level of a more powerful person without and within the dyad during the Prisoner's Dilemma Game: Dissertation Abstracts International.
  +
*Stahelski, A. J. (1981). The interaction of cooperators and competitors in a conversational simulation of the Prisoners Dilemma game: Dissertation Abstracts International.
  +
*Stuart, H. W. (1993). Non-equilibrium and non-procedural approaches to game theory: Dissertation Abstracts International.
  +
*Swan, G. A. (1974). Machiavellianism, impulsivity, field dependence-independence, and performance on the Prisoners' Dilemma game: Dissertation Abstracts International.
  +
*Talley, M. B. (1975). Effects of asymmetry of payoff and asymmetry of information in a Prisoner's Dilemma game: Dissertation Abstracts International.
  +
*Tempey, D. F. (1975). Trust, choice behavior, and activity description: Dissertation Abstracts International.
  +
*Ugis-Upton, T. A. (1981). An investigation of the effectiveness of the communication of confession of guilt and restitution on the reduction of distrust: Dissertation Abstracts International.
  +
*Whitehill, M. B. (1987). Psychopathy and interpersonal relationships: Dissertation Abstracts International.
  +
*Widom, C. S. (1974). Interpersonal conflict and cooperation in psychopaths: Dissertation Abstracts International.
  +
*Winkel, M. H. (1983). Effects of situational, attitudinal, and mediational factors on cooperative behavior: Dissertation Abstracts International.
  +
*Winquist, J. R. (2001). Sources of the discontinuity effect: Being in a group, playing against a group, and between-sides communication. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
*Wright, T. L. (1972). Situational and personality parameters of interpersonal trust in a modified Prisoner's Dilemma game: Dissertation Abstracts International Vol.
  +
*Yi, R. (2003). A modified tit-for-tat strategy in a 5-person prisoner's dilemma. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  +
  +
  +
== External links ==
  +
{{Spoken Wikipedia|Prisoners_Dilemma.ogg|2007-06-25}}
  +
* [http://plato.stanford.edu/entries/prisoner-dilemma/ Prisoner's Dilemma (Stanford Encyclopedia of Philosophy)]
  +
* [http://www.nature.com/npp/journal/v31/n5/full/1300932a.html Effects of Tryptophan Depletion on the Performance of an Iterated Prisoner's Dilemma Game in Healthy Adults] - Nature Neuropsychopharmacology
  +
*[http://www.egwald.ca/operationsresearch/prisonersdilemma.php Is there a "dilemma" in Prisoner's Dilemma] by Elmer G. Wiens
  +
* [http://webfiles.uci.edu/mkaminsk/www/book.html "Games Prisoners Play"] - game-theoretic analysis of interactions among actual prisoners, including PD.
  +
*Play an [http://kane.me.uk/ipd/ iterated prisoner's dilemma game].
  +
*Another version of the [http://www.gametheory.net/Web/PDilemma/ iterated prisoner's dilemma game]
  +
*[http://www.paulspages.co.uk/hmd/ Iterated prisoner's dilemma game] applied to Big Brother TV show situation.
  +
*[http://www.msri.org/ext/larryg/pages/15.htm The Bowerbird's Dilemma] The Prisoner's Dilemma in ornithology&nbsp;— mathematical cartoon by Larry Gonnick.
  +
*[http://www.economics.li/downloads/egefdile.pdf Examples of Prisoners' dilemma]
  +
*[http://www.gohfgl.com/ Multiplayer game based on prisoner dilemma] Play Prisoner's Dilemma over IRC or internet&nbsp;— by Axiologic Research.
  +
*[http://www.rte.ie/tv/theview/archive/20080331.html The Edge cites Robert Axelrod's book and discusses the success of U2 following the principles of IPD.]
   
 
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Latest revision as of 12:03, 4 January 2009

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Prison

Will the two prisoners cooperate, or will both of them betray to lessen their own terms, ending up with longer ones?

The Prisoner's Dilemma constitutes a problem in game theory. It was originally framed by Merrill Flood and Melvin Dresher working at RAND in 1950. Albert W. Tucker formalized the game with prison sentence payoffs and gave it the "Prisoner's Dilemma" name (Poundstone, 1992).

In its "classical" form, the prisoner's dilemma (PD) is presented as follows:

Two suspects are arrested by the police. The police have insufficient evidence for a conviction, and, having separated both prisoners, visit each of them to offer the same deal. If one testifies ("defects") for the prosecution against the other and the other remains silent, the betrayer goes free and the silent accomplice receives the full 10-year sentence. If both remain silent, both prisoners are sentenced to only six months in jail for a minor charge. If each betrays the other, each receives a five-year sentence. Each prisoner must choose to betray the other or to remain silent. Each one is assured that the other would not know about the betrayal before the end of the investigation. How should the prisoners act?

If we assume that each player prefers shorter sentences to longer ones, and that each gets no utility out of lowering the other player's sentence, and that there are no reputation effects from a player's decision, then the prisoner's dilemma forms a non-zero-sum game in which two players may each "cooperate" with or "defect" from (i.e., betray) the other player. In this game, as in all game theory, the only concern of each individual player ("prisoner") is maximizing his/her own payoff, without any concern for the other player's payoff. The unique equilibrium for this game is a Pareto-suboptimal solution—that is, rational choice leads the two players to both play defect even though each player's individual reward would be greater if they both played cooperatively.

In the classic form of this game, cooperating is strictly dominated by defecting, so that the only possible equilibrium for the game is for all players to defect. In simpler terms, no matter what the other player does, one player will always gain a greater payoff by playing defect. Since in any situation playing defect is more beneficial than cooperating, all rational players will play defect, all things being equal.

In the iterated prisoner's dilemma the game is played repeatedly. Thus each player has an opportunity to "punish" the other player for previous non-cooperative play. Cooperation may then arise as an equilibrium outcome. The incentive to defect is overcome by the threat of punishment, leading to the possibility of a cooperative outcome. So if the game is infinitely repeated, cooperation may be a subgame perfect Nash equilibrium although both players defecting always remains an equilibrium and there are many other equilibrium outcomes.

In casual usage, the label "prisoner's dilemma" may be applied to situations not strictly matching the formal criteria of the classic or iterative games; for instance, those in which two entities could gain important benefits from cooperating or suffer from the failure to do so, but find it merely difficult or expensive, not necessarily impossible, to coordinate their activities to achieve cooperation.

Strategy for the classical prisoner's dilemma

The classical prisoner's dilemma can be summarized thus:

Prisoner B Stays Silent Prisoner B Betrays
Prisoner A Stays Silent Each serves 6 months Prisoner A: 10 years
Prisoner B: goes free
Prisoner A Betrays Prisoner A: goes free
Prisoner B: 10 years
Each serves 5 years

In this game, regardless of what the opponent chooses, each player always receives a higher payoff (lesser sentence) by betraying; that is to say that betraying is the strictly dominant strategy. For instance, Prisoner A can accurately say, "No matter what Prisoner B does, I personally am better off betraying than staying silent. Therefore, for my own sake, I should betray." However, if the other player acts similarly, then they both betray and both get a lower payoff than they would get by staying silent. Rational self-interested decisions result in each prisoner's being worse off than if each chose to lessen the sentence of the accomplice at the cost of staying a little longer in jail himself. Hence a seeming dilemma. In game theory, this demonstrates very elegantly that in a non-zero sum game a Nash Equilibrium need not be a Pareto optimum.

Generalized form

We can expose the skeleton of the game by stripping it of the prisoner framing device. The generalized form of the game has been used frequently in experimental economics. The following rules give a typical realization of the game.

There are two players and a banker. Each player holds a set of two cards: one printed with the word "Cooperate", the other printed with "Defect" (the standard terminology for the game). Each player puts one card face-down in front of the banker. By laying them face down, the possibility of a player knowing the other player's selection in advance is eliminated (although revealing one's move does not affect the dominance analysis[1]). At the end of the turn, the banker turns over both cards and gives out the payments accordingly.

If player 1 (red) defects and player 2 (blue) cooperates, player 1 gets the Temptation to Defect payoff of 5 points while player 2 receives the Sucker's payoff of 0 points. If both cooperate they get the Reward for Mutual Cooperation payoff of 3 points each, while if they both defect they get the Punishment for Mutual Defection payoff of 1 point. The checker board payoff matrix showing the payoffs is given below.

Example PD payoff matrix
Cooperate Defect
Cooperate 3, 3 0, 5
Defect 5, 0 1, 1

In "win-lose" terminology the table looks like this:

Cooperate Defect
Cooperate
win-win
lose much-win much
Defect
win much-lose much
lose-lose

These point assignments are given arbitrarily for illustration. It is possible to generalize them, as follows:

Canonical PD payoff matrix
Cooperate Defect
Cooperate R, R S, T
Defect T, S P, P

Where T stands for Temptation to defect, R for Reward for mutual cooperation, P for Punishment for mutual defection and S for Sucker's payoff. To be defined as Prisoner's dilemma, the following inequalities must hold:

T > R > P > S

This condition ensures that the equilibrium outcome is defection, but that cooperation Pareto dominates equilibrium play. In addition to the above condition, if the game is repeatedly played by two players, the following condition should be added.[2]

2 R > T + S

If that condition does not hold, then full cooperation is not necessarily Pareto optimal, as the players are collectively better off by having each player alternate between cooperate and defect.

These rules were established by cognitive scientist Douglas Hofstadter and form the formal canonical description of a typical game of Prisoner's Dilemma.

A simple special case occurs when the advantage of defection over cooperation is independent of what the co-player does and cost of the co-players defection is independent of one's own action, i.e. T+S = P+R.

Human behavior in the Prisoner's Dilemma

One experiment based on the simple dilemma found that approximately 40% of participants played "cooperate" (i.e., stayed silent).[3]

The iterated prisoner's dilemma

If two players play Prisoner's Dilemma more than once in succession, having memory of at least one previous game, it is called iterated Prisoner's Dilemma. Amongst results shown by Nobel Prize winner Robert Aumann in his 1959 paper, rational players repeatedly interacting for indefinitely long games can sustain the cooperative outcome. Popular interest in the iterated prisoners dilemma (IPD) was kindled by Robert Axelrod in his book The Evolution of Cooperation (1984). In this he reports on a tournament he organized in which participants have to choose their mutual strategy again and again, and have memory of their previous encounters. Axelrod invited academic colleagues all over the world to devise computer strategies to compete in an IPD tournament. The programs that were entered varied widely in algorithmic complexity, initial hostility, capacity for forgiveness, and so forth.

Axelrod discovered that when these encounters were repeated over a long period of time with many players, each with different strategies, greedy strategies tended to do very poorly in the long run while more altruistic strategies did better, as judged purely by self-interest. He used this to show a possible mechanism for the evolution of altruistic behaviour from mechanisms that are initially purely selfish, by natural selection.

The best deterministic strategy was found to be "Tit for Tat," which Anatol Rapoport developed and entered into the tournament. It was the simplest of any program entered, containing only four lines of BASIC, and won the contest. The strategy is simply to cooperate on the first iteration of the game; after that, the player does what his opponent did on the previous move. Depending on the situation, a slightly better strategy can be "Tit for Tat with forgiveness." When the opponent defects, on the next move, the player sometimes cooperates anyway, with a small probability (around 1%-5%). This allows for occasional recovery from getting trapped in a cycle of defections. The exact probability depends on the line-up of opponents.

By analysing the top-scoring strategies, Axelrod stated several conditions necessary for a strategy to be successful.

Nice
The most important condition is that the strategy must be "nice", that is, it will not defect before its opponent does (this is sometimes referred to as an "optimistic" algorithm). Almost all of the top-scoring strategies were nice; therefore a purely selfish strategy will not "cheat" on its opponent, for purely utilitarian reasons first.
Retaliating
However, Axelrod contended, the successful strategy must not be a blind optimist. It must sometimes retaliate. An example of a non-retaliating strategy is Always Cooperate. This is a very bad choice, as "nasty" strategies will ruthlessly exploit such players.
Forgiving
Successful strategies must also be forgiving. Though players will retaliate, they will once again fall back to cooperating if the opponent does not continue to play defects. This stops long runs of revenge and counter-revenge, maximizing points.
Non-envious
The last quality is being non-envious, that is not striving to score more than the opponent (impossible for a ‘nice’ strategy, i.e., a 'nice' strategy can never score more than the opponent).

Therefore, Axelrod reached the oxymoron-sounding conclusion that selfish individuals for their own selfish good will tend to be nice and forgiving and non-envious.

The optimal (points-maximizing) strategy for the one-time PD game is simply defection; as explained above, this is true whatever the composition of opponents may be. However, in the iterated-PD game the optimal strategy depends upon the strategies of likely opponents, and how they will react to defections and cooperations. For example, consider a population where everyone defects every time, except for a single individual following the Tit-for-Tat strategy. That individual is at a slight disadvantage because of the loss on the first turn. In such a population, the optimal strategy for that individual is to defect every time. In a population with a certain percentage of always-defectors and the rest being Tit-for-Tat players, the optimal strategy for an individual depends on the percentage, and on the length of the game.

A strategy called Pavlov (an example of Win-Stay, Lose-Switch) cooperates at the first iteration and whenever the player and co-player did the same thing at the previous iteration; Pavlov defects when the player and co-player did different things at the previous iteration. For a certain range of parameters, Pavlov beats all other strategies by giving preferential treatment to co-players which resemble Pavlov.

Deriving the optimal strategy is generally done in two ways:

  1. Bayesian Nash Equilibrium: If the statistical distribution of opposing strategies can be determined (e.g. 50% tit-for-tat, 50% always cooperate) an optimal counter-strategy can be derived analytically.[4]
  2. Monte Carlo simulations of populations have been made, where individuals with low scores die off, and those with high scores reproduce (a genetic algorithm for finding an optimal strategy). The mix of algorithms in the final population generally depends on the mix in the initial population. The introduction of mutation (random variation during reproduction) lessens the dependency on the initial population; empirical experiments with such systems tend to produce Tit-for-Tat players (see for instance Chess 1988), but there is no analytic proof that this will always occur.

Although Tit-for-Tat is considered to be the most robust basic strategy, a team from Southampton University in England (led by Professor Nicholas Jennings [2] and consisting of Rajdeep Dash, Sarvapali Ramchurn, Alex Rogers, Perukrishnen Vytelingum) introduced a new strategy at the 20th-anniversary Iterated Prisoner's Dilemma competition, which proved to be more successful than Tit-for-Tat. This strategy relied on cooperation between programs to achieve the highest number of points for a single program. The University submitted 60 programs to the competition, which were designed to recognize each other through a series of five to ten moves at the start. Once this recognition was made, one program would always cooperate and the other would always defect, assuring the maximum number of points for the defector. If the program realized that it was playing a non-Southampton player, it would continuously defect in an attempt to minimize the score of the competing program. As a result,[5] this strategy ended up taking the top three positions in the competition, as well as a number of positions towards the bottom.

This strategy takes advantage of the fact that multiple entries were allowed in this particular competition, and that the performance of a team was measured by that of the highest-scoring player (meaning that the use of self-sacrificing players was a form of minmaxing). In a competition where one has control of only a single player, Tit-for-Tat is certainly a better strategy. Because of this new rule, this competition also has little theoretical significance when analysing single agent strategies as compared to Axelrod's seminal tournament. However, it provided the framework for analysing how to achieve cooperative strategies in multi-agent frameworks, especially in the presence of noise. In fact, long before this new-rules tournament was played, Richard Dawkins in his book The Selfish Gene pointed out the possibility of such strategies winning if multiple entries were allowed, but remarked that most probably Axelrod would not have allowed them if they had been submitted. It also relies on circumventing rules about the prisoner's dilemma in that there is no communication allowed between the two players. When the Southampton programs engage in an opening "ten move dance" to recognize one another, this only reinforces just how valuable communication can be in shifting the balance of the game.

If an iterated PD is going to be iterated exactly N times, for some known constant N, then it is always game theoretically optimal to defect in all rounds. The only possible Nash equilibrium is to always defect. The proof goes like this: one might as well defect on the last turn, since the opponent will not have a chance to punish the player. Therefore, both will defect on the last turn. Thus, the player might as well defect on the second-to-last turn, since the opponent will defect on the last no matter what is done, and so on. For cooperation to emerge between game theoretic rational players, the total number of rounds must be random, or at least unknown to the players. However, even in this case always defect is no longer a strictly dominant strategy, only a Nash equilibrium. The superrational strategy in this case is to cooperate against a superrational opponent, and in the limit of large fixed N, experimental results on strategies agree with the superrational version, not the game-theoretic rational one.

Another odd case is "play forever" prisoner's dilemma. The game is repeated infinitely many times, and the player's score is the average (suitably computed).

The prisoner's dilemma game is fundamental to certain theories of human cooperation and trust. On the assumption that the PD can model transactions between two people requiring trust, cooperative behaviour in populations may be modelled by a multi-player, iterated, version of the game. It has, consequently, fascinated many scholars over the years. In 1975, Grofman and Pool estimated the count of scholarly articles devoted to it at over 2,000. The iterated prisoner's dilemma has also been referred to as the "Peace-War game".[6]

Continuous Iterated Prisoner's Dilemma

Most work on the iterated prisoner's dilemma has focused on the discrete case, in which players either cooperate or defect, because this model is relatively simple to analyze. However, some researchers have looked at models of the continuous iterated prisoner's dilemma, in which players are able to make a variable contribution to the other player. Le and Boyd[7] found that in such situations, cooperation is much harder to evolve than in the discrete iterated prisoner's dilemma. The basic intuition for this result is straigh­tforward: in a continuous prisoner's dilemma, if a population starts off in a non-cooperative equilibrium, players who are only marginally more cooperative than non-cooperators get little benefit from assorting with one another. By contrast, in a discrete prisoner's dilemma, Tit-for-Tat cooperators get a big payoff boost from assorting with one another in a non-cooperative equilibrium, relative to non-cooperators. Since Nature arguably offers more opportunities for variable cooperation rather than a strict dichotomy of cooperation or defection, the continuous prisoner's dilemma may help explain why real-life examples of Tit-for-Tat-like cooperation are extremely rare in Nature (ex. Hammerstein[8]) even though Tit-for-Tat seems robust in theoretical models.

Learning psychology and game theory

Where game players can learn to estimate the likelihood of other players defecting, their own behaviour is influenced by their experience of the others' behaviour. Simple statistics show that inexperienced players are more likely to have had, overall, atypically good or bad interactions with other players. If they act on the basis of these experiences (by defecting or cooperating more than they would otherwise) they are likely to suffer in future transactions. As more experience is accrued a truer impression of the likelihood of defection is gained and game playing becomes more successful. The early transactions experienced by immature players are likely to have a greater effect on their future playing than would such transactions affect mature players. This principle goes part way towards explaining why the formative experiences of young people are so influential and why, for example, those who are particularly vulnerable to bullying sometimes become bullies themselves.

The likelihood of defection in a population may be reduced by the experience of cooperation in earlier games allowing trust to build up.[9] Hence self-sacrificing behaviour may, in some instances, strengthen the moral fibre of a group. If the group is small the positive behaviour is more likely to feed back in a mutually affirming way, encouraging individuals within that group to continue to cooperate. This is allied to the twin dilemma of encouraging those people whom one would aid to indulge in behaviour that might put them at risk. Such processes are major concerns within the study of reciprocal altruism, group selection, kin selection and moral philosophy.

Douglas Hofstadter's Superrationality

Douglas Hofstadter in his Metamagical Themas proposed that the definition of "rational" that led "rational" players to defect is faulty. He proposed that there is another type of rational behavior, which he called "superrational", where players take into account that the other person is presumably superrational, like them. Superrational players behave identically, and know that they will behave identically. They take that into account before they maximize their payoffs, and they therefore cooperate.

This view of the one-shot PD leads to cooperation as follows:

  • Any superrational strategy will be the same for both superrational players, since both players will think of it.
  • therefore the superrational answer will lie on the diagonal of the payoff matrix
  • when you maximize return from solutions on the diagonal, you cooperate

However, if a superrational player plays against a rational opponent, he will serve a 10-year sentence, and the rational player will go free.

One-shot cooperation is observed in human culture, wherever religious and ethical codes exist.

Superrationality is not studied by academics, as rationality excludes any superrational behavior.

Morality

While it is sometimes thought that morality must involve the constraint of self-interest, David Gauthier famously argues that co-operating in the prisoners dilemma on moral principles is consistent with self-interest and the axioms of game theory.[How to reference and link to summary or text] In his opinion, it is most prudent to give up straigh­tforward maximizing and instead adopt a disposition of constrained maximization, according to which one resolves to cooperate in the belief that the opponent will respond with the same choice, while in the classical PD it is explicitly stipulated that the response of the opponent does not depend on the player's choice. This form of contractarianism claims that good moral thinking is just an elevated and subtly strategic version of basic means-end reasoning.

Douglas Hofstadter expresses a strong personal belief that the mathematical symmetry is reinforced by a moral symmetry, along the lines of the Kantian categorical imperative: defecting in the hope that the other player cooperates is morally indefensible.[How to reference and link to summary or text] If players treat each other as they would treat themselves, then they will cooperate.

Real-life examples

These particular examples, involving prisoners and bag switching and so forth, may seem contrived, but there are in fact many examples in human interaction as well as interactions in nature that have the same payoff matrix. The prisoner's dilemma is therefore of interest to the social sciences such as economics, politics and sociology, as well as to the biological sciences such as ethology and evolutionary biology. Many natural processes have been abstracted into models in which living beings are engaged in endless games of Prisoner's Dilemma (PD). This wide applicability of the PD gives the game its substantial importance.

In politics

In political science, for instance, the PD scenario is often used to illustrate the problem of two states engaged in an arms race. Both will reason that they have two options, either to increase military expenditure or to make an agreement to reduce weapons. Neither state can be certain that the other one will keep to such an agreement; therefore, they both incline towards military expansion. The paradox is that both states are acting rationally, but producing an apparently irrational result. This could be considered a corollary to deterrence theory.

In science

In sociology or criminology, the PD may be applied to an actual dilemma facing two inmates. The game theorist Marek Kaminski, a former political prisoner, analysed the factors contributing to payoffs in the game set up by a prosecutor for arrested defendants (cf. References). He concluded that while the PD is the ideal game of a prosecutor, numerous factors may strongly affect the payoffs and potentially change the properties of the game.

In environmental studies, the PD is evident in crises such as global climate change. All countries will benefit from a stable climate, but any single country is often hesitant to curb [[Carbon dioxide|Template:Co2]] emissions. The benefit to an individual country to maintain current behavior is greater than the benefit to all countries if behavior was changed, therefore explaining the current impasse concerning climate change.[10]

In program management and technology development, the PD applies to the relationship between the customer and the developer. Capt Dan Ward, an officer in the US Air Force, examined The Program Manager's Dilemma in an article published in Defense AT&L, a defense technology journal.[11]

In sports

PD frequently occurs in cycling races, for instance in the Tour de France. Consider two cyclists halfway in a race, with the peloton (larger group) at great distance behind them. The two riders often work together (mutual cooperation) by sharing the tough load of the front position, where there is no shelter from the wind. If neither of the riders makes an effort to stay ahead, the peloton will soon catch up (mutual defection). An often-seen scenario is one rider doing the hard work alone (cooperating), keeping the two ahead of the peloton. Nearer to the finish (where the threat of the peloton has disappeared), the game becomes a simple zero-sum game, with each rider trying to avoid at all costs giving a slipstream advantage to the other rider. If there was a (single) defecting rider in the preceding prisoners' dilemma, it is usually he who will win this zero-sum game, having saved energy in the cooperating rider's slipstream. The cooperating rider's attitude may seem extremely naive, but he often has no other choice when both riders have different physical profiles. The cooperating rider typically has an endurance profile, whereas the defecting rider will more likely be a sprinter. When continuously taking the head position of the twosome, the 'cooperating' rider is merely trying to ride away from the defecting sprinter using his endurance advantage over long distance, thus avoiding a sprint duel at the finish, which he would be bound to lose, even if the sprinting rider had cooperated. Just after the escape from the peloton, the endurance-sprinter difference is less of importance, and it is therefore at this stage of the race that mutual cooperation PD can usually be observed. Arguably, it is this almost unavoidable presence of PD (and its transition in zero-sum games) that (unconsciously) makes cycling an exciting sport to watch.

PD hardly applies to running sports, because of the negligible importance of air resistance (and shelter from it).

In high school wrestling, sometimes participants intentionally lose unnaturally large amounts of weight so as to compete against lighter opponents. In doing so, the participants are clearly not at their top level of physical and athletic fitness and yet often end up competing against the same opponents anyway, who have also followed this practice (mutual defection). The result is a reduction in the level of competition. Yet if a participant maintains their natural weight (cooperating), they will most likely compete against a stronger opponent who has lost considerable weight.

In economics

Advertising is sometimes cited as a real life example of the prisoner’s dilemma. When cigarette advertising was legal in the United States, competing cigarette manufacturers had to decide how much money to spend on advertising. The effectiveness of Firm A’s advertising was partially determined by the advertising conducted by Firm B. Likewise, the profit derived from advertising for Firm B is affected by the advertising conducted by Firm A. If both Firm A and Firm B chose to advertise during a given period the advertising cancels out, receipts remain constant, and expenses increase due to the cost of advertising. Both firms would benefit from a reduction in advertising. However, should Firm B choose not to advertise, Firm A could benefit greatly by advertising. Nevertheless, the optimal amount of advertising by one firm depends on how much advertising the other undertakes. As the best strategy is dependent on what the other firm chooses there is no dominant strategy and this is not a prisoner's dilemma but rather is an example of a stag hunt. The outcome is similar, though, in that both firms would be better off were they to advertise less than in the equilibrium. Sometimes cooperative behaviors do emerge in business situations. For instance, cigarette manufacturers endorsed the creation of laws banning cigarette advertising, understanding that this would reduce costs and increase profits across the industry.[9] This analysis is likely to be pertinent in many other business situations involving advertising.

Without enforceable agreements, members of a cartel are also involved in a (multi-player) prisoners' dilemma.[12] 'Cooperating' typically means keeping prices at a pre-agreed minimum level. 'Defecting' means selling under this minimum level, instantly stealing business (and profits) from other cartel members. Ironically, anti-trust authorities want potential cartel members to mutually defect, ensuring the lowest possible prices for consumers.

In law

The theoretical conclusion of PD is one reason why, in many countries, plea bargaining is forbidden. Often, precisely the PD scenario applies: it is in the interest of both suspects to confess and testify against the other prisoner/suspect, even if each is innocent of the alleged crime. Arguably, the worst case is when only one party is guilty — here, the innocent one is unlikely to confess, while the guilty one is likely to confess and testify against the innocent.

In the media

In the 2008 edition of Big Brother (UK), the dilemma was applied to two of the housemates. A prize fund of £50,000 was available. If housemates chose to share the prize fund, each would receive £25,000. If one chose to share, and the other chose to take, the one who took it would receive the entire £50,000. If both chose to take, both housemates would receive nothing. The housemates had a minute to discuss their decision, and were given the possibility to lie. Both housemates declared they would share the prize fund, but either could have potentially been lying. When asked to give their final answers by big brother, both housemates did indeed choose to share, and so won £25,000 each.

Multiplayer dilemmas

Many real-life dilemmas involve multiple players. Although metaphorical, Hardin's tragedy of the commons may be viewed as an example of a multi-player generalization of the PD: Each villager makes a choice for personal gain or restraint. The collective reward for unanimous (or even frequent) defection is very low payoffs (representing the destruction of the "commons"). Such multi-player PDs are not formal as they can always be decomposed into a set of classical two-player games. The commons are not always exploited: William Poundstone, in a book about the Prisoner's Dilemma (see References below), describes a situation in New Zealand where newspaper boxes are left unlocked. It is possible for someone to take a paper without paying (defecting) but very few do, feeling that if they do not pay then neither will others, destroying the system.

Because there is no mechanism for personal choice to influence others' decisions, this type of thinking relies on correlations between behavior, not on causation. Because of this property, those who do not understand superrationality often mistake it for magical thinking. Without superrationality, not only petty theft, but voluntary voting requires widespread magical thinking, since a non-voter is a free rider on a democratic system.


Related games

Closed-bag exchange

Hofstadter[13] once suggested that people often find problems such as the PD problem easier to understand when it is illustrated in the form of a simple game, or trade-off. One of several examples he used was "closed bag exchange":

Two people meet and exchange closed bags, with the understanding that one of them contains money, and the other contains a purchase. Either player can choose to honour the deal by putting into his bag what he agreed, or he can defect by handing over an empty bag.

In this game, defection is always the best course, implying that rational agents will never play. However, in this case both players cooperating and both players defecting actually give the same result, so chances of mutual cooperation, even in repeated games, are few.

Friend or Foe?

Friend or Foe? is a game show that aired from 2002 to 2005 on the Game Show Network in the United States. It is an example of the prisoner's dilemma game tested by real people, but in an artificial setting. On the game show, three pairs of people compete. As each pair is eliminated, they play a game of Prisoner's Dilemma to determine how their winnings are split. If they both cooperate (Friend), they share the winnings 50-50. If one cooperates and the other defects (Foe), the defector gets all the winnings and the cooperator gets nothing. If both defect, both leave with nothing. Notice that the payoff matrix is slightly different from the standard one given above, as the payouts for the "both defect" and the "cooperate while the opponent defects" cases are identical. This makes the "both defect" case a weak equilibrium, compared with being a strict equilibrium in the standard prisoner's dilemma. If you know your opponent is going to vote Foe, then your choice does not affect your winnings. In a certain sense, Friend or Foe has a payoff model between "Prisoner's Dilemma" and "Chicken".

The payoff matrix is

Cooperate Defect
Cooperate 1, 1 0, 2
Defect 2, 0 0, 0

This payoff matrix was later used on the British television programmes Shafted and Golden Balls.


See also

Notes

  1. A simple "tell" that partially or wholly reveals one player's choice — such as the Red player playing their Cooperate card face-up — does not change the fact that Defect is the dominant strategy. When one is considering the game itself, communication has no effect whatsoever. However, when the game is being played in real life considerations outside of the game itself may cause communication to matter. It is a point of utmost importance to the full implications of the dilemma that when we do not need to take into account external considerations, single-instance Prisoner's Dilemma is not affected in any way by communications. Even in single-instance Prisoner's Dilemma, meaningful prior communication about issues external to the game could alter the play environment, by raising the possibility of enforceable side contracts or credible threats. For example, if the Red player plays their Cooperate card face-up and simultaneously reveals a binding commitment to blow the jail up if and only if Blue Defects (with additional payoff -11,-10), then Blue's Cooperation becomes dominant. As a result, players are screened from each other and prevented from communicating outside of the game.
  2. Dawkins, Richard (1989). The Selfish Gene, Oxford University Press. ISBN 0-19-286092-5. Page: 204 of Paperback edition
  3. Tversky, Amos (2004). Preference, Belief, and Similarity: Selected Writings, MIT Press.
  4. For example see the 2003 study “Bayesian Nash equilibrium; a statistical test of the hypothesis” for discussion of the concept and whether it can apply in real economic or strategic situations (from Tel Aviv University).
  5. The 2004 Prisoner's Dilemma Tournament Results show University of Southampton's strategies in the first three places, despite having fewer wins and many more losses than the GRIM strategy. (Note that in a PD tournament, the aim of the game is not to “win” matches — that can easily be achieved by frequent defection). It should also be pointed out that even without implicit collusion between software strategies (exploited by the Southampton team) tit-for-tat is not always the absolute winner of any given tournament; it would be more precise to say that its long run results over a series of tournaments outperform its rivals. (In any one event a given strategy can be slightly better adjusted to the competition than tit-for-tat, but tit-for-tat is more robust). The same applies for the tit-for-tat-with-forgiveness variant, and other optimal strategies: on any given day they might not 'win' against a specific mix of counter-strategies.An alternative way of putting it is using the Darinian ESS simulation. In such a simulation Tit-for-Tat will almost always come to dominate, though nasty strategies will drift in and out of the population because a Tit-for-Tat population is penetratable by non-retaliating nice strategies which in turn are easy prey for the nasty strategies. Richard Dawkins showed that here no static mix of strategies form a stable equilibrium and the system will always oscillate between bounds.
  6. Shy, O., 1996, Industrial Organization: Theory and Applications, Cambridge, Mass.: The MIT Press.
  7. Le, S. and R. Boyd (2007) "Evolutionary Dynamics of the Continuous Iterated Prisoner's Dilemma" Journal of Theoretical Biology, Volume 245, 258–267.
  8. Hammerstein, P. (2003). Why is reciprocity so rare in social animals? A protestant appeal. In: P. Hammerstein, Editor, Genetic and Cultural Evolution of Cooperation, MIT Press. pp. 83–94.
  9. 9.0 9.1 This argument for the development of cooperation through trust is given in The Wisdom of Crowds , where it is argued that long-distance capitalism was able to form around a nucleus of Quakers, who always dealt honourably with their business partners. (Rather than defecting and reneging on promises — a phenomenon that had discouraged earlier long-term unenforceable overseas contracts). It is argued that dealings with reliable merchants allowed the meme for cooperation to spread to other traders, who spread it further until a high degree of cooperation became a profitable strategy in general commerce
  10. The Economist (2007) [1].
  11. Ward, D. (2004) The Program Manager's Dilemma The Program Manager's Dilemma (Defense AT&L, Defense Acquisition University Press).
  12. Nicholson, Walter (2000), Intermediate Microeconomics (8th ed.), Harcourt 
  13. Hofstadter, Douglas R. (1985). Metamagical Themas: questing for the essence of mind and pattern, Bantam Dell Pub Group. ISBN 0-465-04566-9. - see Ch.29 The Prisoner's Dilemma Computer Tournaments and the Evolution of Cooperation.

References

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  • Robert Aumann, “Acceptable points in general cooperative n-person games”, in R. D. Luce and A. W. Tucker (eds.), Contributions to the Theory 23 of Games IV, Annals of Mathematics Study 40, 287–324, Princeton University Press, Princeton NJ.
  • Axelrod, R. (1984). The Evolution of Cooperation. ISBN 0-465-02121-2
  • Bicchieri, Cristina (1993). Rationality and Coordination. Cambridge University Press
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  1. redirect Template:Refend


Further reading

Books

  • Banfield, G., & Christodoulou, C. (2005). Can Self-Control Be Explained Through Games? River Edge, NJ: World Scientific Publishing Co.
  • Bendor, J., Kramer, R. M., & Stout, S. (2005). When in Doubt... Cooperation in a Noisy Prisoner's Dilemma. Northampton, MA: Edward Elgar Publishing.
  • Bicchieri, Cristina and Mitchell Green (1997) "Symmetry Arguments for Cooperation in the Prisoner's Dilemma", in G. Holmstrom-Hintikka and R. Tuomela (eds.), Contemporary Action Theory: The Philosophy and Logic of Social Action, Kluwer.
  • Goren, H., & Bornstein, G. (1999). Reciprocation and learning in the Intergroup Prisoner's Dilemma Game. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  • Insko, C. A., & Schopler, J. (1998). Differential distrust of groups and individuals. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  • Komorita, S. S., & Parks, C. D. (1999). Reciprocity and cooperation in social dilemmas: Review and future directions. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  • Larsen, J. D. (2002). Prisoner's dilemma as a model for understanding decisions. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  • Liebrand, W. B. G., & Messick, D. M. (1999). Dynamic and static theories of costs and benefits of cooperation. Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  • McCabe, K. A., & Smith, V. L. (2001). Goodwill Accounting and the process of exchange. Cambridge, MA: The MIT Press.
  • Schulz, U. (1994). Mathematical models of gaming behavior. Lisse, Netherlands: Swets & Zeitlinger Publishers.


Papers

Aaftink, J. (1989). Far-sighted equilibria in 2x2, non-cooperative, repeated games: Theory and Decision Vol 27(3) Nov 1989, 175-192.


  • Acevedo, M., & Krueger, J. I. (2005). Evidential reasoning in the prisoner's dilemma: American Journal of Psychology Vol 118(3) Fal 2005, 431-457.
  • Ajzen, I. (1971). Attitudinal vs. normative messages: An investigation of the differential effects of persuasive communications on behavior: Sociometry Vol 34(2) Jun 1971, 263-280.
  • Alvard, M. S., & Nolin, D. A. (2002). Rousseau's whale hunt? Coordination among big-game hunters: Current Anthropology Vol 43(4) Aug-Oct 2002, 533-559.
  • Antonides, G. (1994). Mental accounting in a sequential Prisoner's Dilemma game: Journal of Economic Psychology Vol 15(2) Jun 1994, 351-374.
  • Arce M, D. G. (1994). Stability criteria for social norms with applications to the prisoner's dilemma: Journal of Conflict Resolution Vol 38(4) Dec 1994, 749-765.
  • Aretz, H.-J. (2005). The Relevance of Value Commitments in the Supply of Public Goods: Zeitschrift fur Soziologie Vol 34(5) Oct 2005, 326-343.
  • Arfi, B. (2006). Linguistic Fuzzy-Logic Social Game of Cooperation: Rationality and Society Vol 18(4) Nov 2006, 471-537.
  • Armstrong, S., & Roback, H. (1977). An empirical test of Schutz' three-dimensional theory of group process in adolescent dyads: Small Group Behavior Vol 8(4) Nov 1977, 443-456.
  • Arribas, I., & Urbano, A. (2005). Repeated games with probabilistic horizon: Mathematical Social Sciences Vol 50(1) Jul 2005, 39-60.
  • Ashmore, T. M. (1987). The Prisoner's Dilemma: A computer adaptation: Western Journal of Speech Communication Vol 51(1) Win 1987, 117-126.
  • Au, W. T., & Komorita, S. S. (2002). Effects of initial choices in the prisoner's dilemma: Journal of Behavioral Decision Making Vol 15(4) Oct 2002, 343-359.
  • Axelrod, R. (1980). More effective choice in the Prisoner's Dilemma: Journal of Conflict Resolution Vol 24(3) Sep 1980, 379-403.
  • Axelrod, R. (1990). The emergence of cooperation among egoists. New York, NY: Cambridge University Press.
  • Axelrod, R., & Dion, D. (1988). The further evolution of cooperation: Science Vol 242(4884) Dec 1988, 1385-1390.
  • Axelrod, R., & Hamilton, W. D. (1981). The evolution of cooperation: Science Vol 211(4489) Mar 1981, 1390-1396.
  • Axelrod, R., Riolo, R. L., & Cohen, M. D. (2002). Beyond geography: Cooperation with persistent links in the absence of clustered neighborhoods: Personality and Social Psychology Review Vol 6(4) 2002, 341-346.
  • Baker, F., & Rachlin, H. (2001). Probability of reciprocation in repeated prisoner's dilemma games: Journal of Behavioral Decision Making Vol 14(1) Jan 2001, 51-67.
  • Baker, F., & Rachlin, H. (2002). Self-control by pigeons in the prisoner's dilemma: Psychonomic Bulletin & Review Vol 9(3) Sep 2002, 482-488.
  • Bartholdi, J. J., Butler, C. A., & Trick, M. A. (1986). More on the evolution of cooperation: Journal of Conflict Resolution Vol 30(1) Mar 1986, 129-140.
  • Basu, K. (1977). Information and strategy in iterated Prisoner's Dilemma: Theory and Decision Vol 8(3) Jul 1977, 293-298.
  • Batson, C. D., & Ahmad, N. (2001). Empathy-induced altruism in a prisoner's dilemma II: What if the target of empathy has defected? : European Journal of Social Psychology Vol 31(1) Jan-Feb 2001, 25-36.
  • Batson, C. D., & Moran, T. (1999). Empathy-induced altruism in a prisoner's dilemma: European Journal of Social Psychology Vol 29(7) Nov 1999, 909-924.
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Dissertations

  • Acevedo, M. (2002). The effects of social projection and payoff on cooperative behavior in the prisoner's dilemma. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  • Armstrong, S. H. (1975). A three-dimensional theory of group process in adolescent dyads: Dissertation Abstracts International.
  • Beale, D. K. (1974). Conflicting motives in the Prisoner's Dilemma game: Dissertation Abstracts International.
  • Beyer, R. A. (1981). An assessment of the effects of moral level and age on contract maintenance in prisoner's dilemma: Dissertation Abstracts International.
  • Bryant, W. P. (1978). Machiavellianism, perspective taking, and partner's response as predictors of interpersonal behavior measured by a modified Prisoner's Dilemma game: Dissertation Abstracts International.
  • Charlton, S. R. (2007). The relationship between behavioral measures of self-control: Temporal discounting and the single-player iterated prisoner's dilemma. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  • Christensen, R. L. (1976). An investigation of interpersonal trust and the effect of dogmatism and characteristics of the other person on subjective trust in the Prisoner's Dilemma game: Dissertation Abstracts International.
  • Conklin, M. M. (1983). An investigation of delinquent and nondelinquent adolescent performance on The Prisoner's Dilemma Game when paired with an adult confederate under three different strategy conditions: Dissertation Abstracts International.
  • Elliott, S. W. (1992). Steps towards a psychological calculus for game theory: Application of a model of categorization to the repeated prisoner's dilemma: Dissertation Abstracts International.
  • Finney, P. D. (1977). Responsibility attributions and the self system: Dissertation Abstracts International.
  • Foster, M. S. (1976). Sex differences in behavior as a function of experience and expectation: Dissertation Abstracts International.
  • Hamburger, F. M. (1976). On behavioral effects of normative and attitudinal communications: Dissertation Abstracts International.
  • Harkey, S. T. (2001). Choice propensity in single exposure/no-feedback prisoner dilemmas as a function of social orientation, payoffs, and expectations: Evaluation of a formal model. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  • Hartman, E. A. (1973). The motivational basis of sex differences in the Prisoner's Dilemma game: Dissertation Abstracts International Vol.
  • Helm, B. L. (1973). Locus of control and the exercise of coercive power: Dissertation Abstracts International.
  • Herrman, K. D. (2006). Inducing interpersonal trust and team learning using a prisoner's dilemma game. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  • Hoerl, R. T. (1972). Cooperation and competition as a function of communication and trust: Dissertation Abstracts International Vol.
  • Jacobs, M. K. (1976). Women's moral reasoning and behavior in Prisoner's Dilemma: Dissertation Abstracts International.
  • Kimmel, M. J. (1975). On distinguishing interpersonal trust from cooperative responding in the Prisoner's Dilemma game: Dissertation Abstracts International.
  • Koenig, K. K. (1980). Cooperation as a function of locus of control in a Prisoner's Dilemma game: Dissertation Abstracts International.
  • Krause, R. M. (1975). A general conflict simulation model for ecological generality using the Prisoner's Dilemma: Dissertation Abstracts International.
  • Lichtig, L. K. (1977). The development of interpersonal relationships: An experimental approach: Dissertation Abstracts International.
  • Lopez, L. C. (1977). The relationship between selected cognitive styles and cooperation in a Prisoner's Dilemma game situation: Dissertation Abstracts International.
  • Mayer, F. J. (1993). Nonlinear dynamics of strategic interactions: Dissertation Abstracts International.
  • McCarter, R. H. (1988). Self-integration and self-defeating behavior in the Prisoner's Dilemma Game: Dissertation Abstracts International.
  • Morikawa, T. (1994). A cognitive theory of how cooperation evolves: A simulation. Dissertation Abstracts International Section A: Humanities and Social Sciences.
  • Oman, R. N. (1978). The effect of unpredictable and malevolent strategies on the game playing of childhood schizophrenics: Dissertation Abstracts International.
  • O'Riordan, N. F. (1987). Behavior of low and high trust juvenile delinquents when playing a modified Prisoner's Dilemma Game: Dissertation Abstracts International.
  • Pincus, J. (1977). Cognitive factors and cooperation in the Prisoner's Dilemma: Dissertation Abstracts International.
  • Pivnick, W. P. (1973). Parental occupation and situational meaning as determinants of behavior in the Prisoner's Dilemma Game: Dissertation Abstracts International Vol.
  • Plous, S. L. (1986). Perceptual illusions and military realities: A social-psychological analysis of the nuclear arms race: Dissertation Abstracts International.
  • Pollard, S. W. (1975). Effects of power on cooperation: Dissertation Abstracts International.
  • Romero, C. (1978). Cooperative behavior of prison inmates towards peers and institutional staff utilizing the prisoner dilemma game: Dissertation Abstracts International.
  • Sachteleben, T. L. (2006). The effect of opportunity cost, level of cooperation, and social value orientation on preferences for structural solutions to the N-person Prisoner's Dilemma. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  • Sanabria, F. (2004). Pigeons in an operant prisoner's dilemma: New experimental paradigms. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  • Sell, R. L. (1977). Cooperation and competition as a function of residential environment, consequences of game strategy choices and perceived control: Dissertation Abstracts International.
  • Setzman, E. J. (1974). Cooperation and competition between men and women in a dyadic game-playing situation: Dissertation Abstracts International.
  • Shebalin, P. V. (1997). Collective learning and cooperation between intelligent software agents: A study of artificial personality and behavior in autonomous agents playing the infinitely repeated prisoner's dilemma game. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  • Speyer, E. C. (1985). Trust level within a dyad as influenced by observed trust level of a more powerful person without and within the dyad during the Prisoner's Dilemma Game: Dissertation Abstracts International.
  • Stahelski, A. J. (1981). The interaction of cooperators and competitors in a conversational simulation of the Prisoners Dilemma game: Dissertation Abstracts International.
  • Stuart, H. W. (1993). Non-equilibrium and non-procedural approaches to game theory: Dissertation Abstracts International.
  • Swan, G. A. (1974). Machiavellianism, impulsivity, field dependence-independence, and performance on the Prisoners' Dilemma game: Dissertation Abstracts International.
  • Talley, M. B. (1975). Effects of asymmetry of payoff and asymmetry of information in a Prisoner's Dilemma game: Dissertation Abstracts International.
  • Tempey, D. F. (1975). Trust, choice behavior, and activity description: Dissertation Abstracts International.
  • Ugis-Upton, T. A. (1981). An investigation of the effectiveness of the communication of confession of guilt and restitution on the reduction of distrust: Dissertation Abstracts International.
  • Whitehill, M. B. (1987). Psychopathy and interpersonal relationships: Dissertation Abstracts International.
  • Widom, C. S. (1974). Interpersonal conflict and cooperation in psychopaths: Dissertation Abstracts International.
  • Winkel, M. H. (1983). Effects of situational, attitudinal, and mediational factors on cooperative behavior: Dissertation Abstracts International.
  • Winquist, J. R. (2001). Sources of the discontinuity effect: Being in a group, playing against a group, and between-sides communication. Dissertation Abstracts International: Section B: The Sciences and Engineering.
  • Wright, T. L. (1972). Situational and personality parameters of interpersonal trust in a modified Prisoner's Dilemma game: Dissertation Abstracts International Vol.
  • Yi, R. (2003). A modified tit-for-tat strategy in a 5-person prisoner's dilemma. Dissertation Abstracts International: Section B: The Sciences and Engineering.


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