Major depression is the leading cause of disability worldwide, and in 2000 was the fourth leading contributor to the global burden of disease (DALYs); it is also an important risk factor for suicide. It is understandable, then, that clinical depression is thought to be a pathology — a major dysfunction of the brain. Yet the epidemiology of clinical depression is something of an outlier when compared to the epidemiology of major dysfunctions of other organs like the heart, liver, lungs and kidneys[attribution needed]. In most cases, rates of organ dysfunction increase with age, with low rates in adolescents and young adults, and the highest rates in the elderly. These patterns are consistent with evolutionary theories of aging which posit that selection against dysfunctional traits decreases with age (because there is a decreasing probability of surviving to later ages).
In contrast to these patterns, prevalence of clinical depression is high in all age categories, including otherwise healthy adolescents and young adults. In one study of the US population, for example, the 12 month prevalence for a major depression episode was highest in the youngest age category (15-24 year olds). The high prevalence of depression is also an outlier when compared to the prevalence of major mental retardation, autism, and schizophrenia, all with prevalence rates about one tenth that of depression, or less.
The common occurrence and persistence of a trait like clinical depression with such negative effects early in life is difficult to explain. (Rates of infectious disease are high in young people, of course, but clinical depression is not thought to be caused by an infection.) Evolutionary psychology suggests ways in which behaviour and mental states, including seemingly harmful states such as depression, may be adaptations to recurring reproductive problems faced by our ancestors, actually having improved the fitness of either the individual or their relatives. It has been argued, for example, that Abraham Lincoln's life-long depression was a source of insight and strength. Some even suggest that "we aren’t designed to have happiness as our natural default" and so a state of depression is the evolutionary norm.
The following hypotheses attempt to identify a benefit of depression that outweighs its obvious costs. All take as their starting point the fact that one of the most potent, well-established causes of major depression is a severe negative life event.
Psychic pain hypothesis
One reason depression is thought to be a pathology is that it causes so much psychic pain and distress. However, physical pain is also very distressful, yet it has an evolved function: to inform the organism that it is suffering damage, to motivate it to withdraw from the source of damage, and to learn to avoid such damage-causing circumstances in the future.
According to the psychic pain hypothesis, depression is analogous to physical pain in that it informs the sufferer that current circumstances, such as the loss of a friend, are imposing a threat to biological fitness, it motivates the sufferer to cease activities that led to the costly situation, if possible, and it causes him or her to learn to avoid similar circumstances in the future. Proponents of this view tend to focus on low mood, and regard clinical depression as a dysfunctional extreme of low mood.
Rank theory: If an individual is involved in a lengthy fight for dominance in a social group and is clearly losing, depression causes the individual to back down and accept the submissive role. In doing so, the individual is protected from unnecessary harm. In this way, depression helps maintain a social hierarchy. This theory is a special case of a more general theory derived from the psychic pain hypothesis: that the cognitive response that produces modern-day depression evolved as a mechanism that allows people to assess whether they are in pursuit of an unreachable goal, and if they are, to motivate them to desist.
Honest signaling theory
Another reason depression is thought to be a pathology is that key symptoms, such as loss of interest in virtually all activities, are extremely costly to the sufferer. Biologists and economists have proposed, however, that signals with inherent costs can credibly signal information when there are conflicts of interest. In the wake of a serious negative life event, such as those that have been implicated in depression (e.g., death, divorce), 'cheap' signals of need, such as crying, might not be believed when social partners have conflicts of interest. The symptoms of major depression, such as loss of interest in virtually all activities and suicidality, are inherently costly, but, as costly signaling theory requires, the costs differ for individuals in different states. For individuals who are not genuinely in need, the fitness cost of major depression is very high because it threatens the flow of fitness benefits. For individuals who are in genuine need, however, the fitness cost of major depression is low because the individual is not generating many fitness benefits. Thus, only an individual in genuine need can afford to suffer major depression. Major depression therefore serves as an honest, or credible, signal of need.
To provide a specific example: Individuals suffering a severe loss such as the death of a spouse are often in need of help and assistance from others. Such individuals who have few conflicts with their social partners are predicted to experience grief -- a means, in part, to signal need to others. Such individuals who have many conflicts with their social partners, in contrast, are predicted to experience depression -- a means, in part, to credibly signal need to others who might be skeptical that the need is genuine. Put in simple language depression may function to enforce sincerity. 
The social navigation, bargaining, or niche change hypothesis  suggests that depression, operationally defined as a combination of prolonged anhedonia and psychomotor retardation or agitation, provides a focused sober perspective on socially imposed constraints hindering a person’s pursuit of major fitness enhancing projects. Simultaneously, publicly displayed symptoms, which reduce the depressive's ability to conduct basic life activities, serve as a social signal of need; the signal's costliness for the depressive certifies its honesty. Finally, for social partners who find it uneconomical to respond helpfully to an honest signal of need, the same depressive symptoms also have the potential to extort relevant concessions and compromises. Depression’s extortionary power comes from the fact that it retards the flow of just those goods and services such partners have come to expect from the depressive under status quo socioeconomic arrangements.
Thus depression may be a social adaptation especially useful in motivating a variety of social partners, all at once, to help the depressive initiate major fitness-enhancing changes in their socioeconomic life. There are extraordinarily diverse circumstances under which this may become necessary in human social life, ranging from loss of rank or a key social ally which makes the current social niche uneconomic to having a set of creative new ideas about how to make a livelihood which begs for a new niche. The social navigation hypothesis emphasizes that an individual can become tightly ensnared in an overly restrictive matrix of social exchange contracts, and that this situation sometimes necessitates a radical contractual upheaval that is beyond conventional methods of negotiation. Regarding the treatment of depression, this hypothesis calls into question any assumptions by the clinician that the typical cause of depression is related to maladaptive perverted thinking processes or other purely endogenous sources. The social navigation hypothesis calls instead for a penetrating analysis of the depressive’s talents and dreams, identification of relevant social constraints (especially those with a relatively diffuse non-point source within the social network of the depressive), and practical social problem-solving therapy designed to relax those constraints enough to allow the depressive to move forward with their life under an improved set of social contracts. This theory has been criticized by .
Depression is not only costly to the sufferer, it also imposes a significant burden on family, friends, and society at large -- yet another reason it is thought to be pathological. Yet if sufferers of depression have real but unmet needs, they might have to provide an incentive to others to address those needs.
The bargaining theory of depression is similar to the honest signaling, niche change, and social navigation theories of depression described above. It draws on theories of labor strikes developed by economists to basically add one additional element to honest signaling theory: The fitness of social partners is generally correlated. When a wife suffers depression and reduces her investment in offspring, for example, the husband's fitness is also put at risk. Thus, not only do the symptoms of major depression serve as costly and therefore honest signals of need, they also compel reluctant social partners to respond to that need in order to prevent their own fitness from being reduced.
- ↑ "The World Health Report 2001", Health Systems: Improving Performance, Geneva: World Health Organization, 2001
- ↑ Prevalence of Heart Disease --- United States, 2005. URL accessed on 2008-03-22. Prevalence of Chronic Kidney Disease and Associated Risk Factors --- United States, 1999--2004. URL accessed on 2008-03-22.
- ↑ Kessler RC, McGonagle KA, Swartz M, Blazer DG, Nelson CB (1993). Sex and depression in the National Comorbidity Survey. I: Lifetime prevalence, chronicity and recurrence. J Affect Disord 29 (2-3): 85–96.
- ↑ Epidemiologic Catchment Area, National Comorbidity Study
- ↑ Neese, Randolph M. (2005-10). The American Psychiatric Publishing Textbook of Mood Disorders - Chapter 10: Evolutionary Explanations for Mood and Mood Disorders, 159-175, Washington, DC: American Psychiatric Publishing. URL accessed 2007-10-23.
- ↑ Erica Goode (February 1, 2000), Viewing Depression As Tool for Survival, New York Times, http://query.nytimes.com/gst/fullpage.html?res=9800E0D8163FF932A35751C0A9669C8B63
- ↑ Natasha Mitchell (3 April 2004), The Evolution of Depression – Does it Have a Role?, Australian Broadcasting Corporation, http://www.abc.net.au/rn/science/mind/stories/s1077027.htm
- ↑ Julia M. Klein (12 February 2007), Depression as a survival tool? Some new treatments assume so, LA Times, http://www.biopsychiatry.com/depression/adaptive.html
- ↑ Joshua Wolf Shenk (2005), Lincoln's Melancholy: How Depression Challenged a President and Fueled His Greatness, Houghton Mifflin
- ↑ John Naish (January 19, 2008), Why the happiness industry can only lead to misery, The Times, http://women.timesonline.co.uk/tol/life_and_style/women/body_and_soul/article3209845.ece
- ↑ Thornhill NW, Thornhill R (1991). An evolutionary analysis of psychological pain following human (Homo sapiens) rape: IV. The effect of the nature of the sexual assault. J Comp Psychol 105 (3): 243–52.
- ↑ Thornhill, R., Thornhill, N.W. (1989). The evolution of psychological pain. Sociobiology and the Social Sciences: 73–103.
- ↑ 13.0 13.1 Neese, Randolph M. (2005-01). Is Depression an Adaptation?. Archives of General Psychology 57 (1): 14–20.
- ↑ Suarez, S.D., Gallup Jr, G.G. (1991). Depression as a response to reproductive failure. Journal of Social and Biological Structures 8: 279–287.
- ↑ Hagen, Edward H., Barrett, Clark (2007). Perinatal Sadness among Shuar Women: Support for an Evolutionary Theory of Psychic Pain. Medical Anthropology Quarterly: 22–40.
- ↑ Keller, Matthew C., Neese, Randolph M. (2005-05). Is low mood an adaptation? Evidence for subtypes with symptoms that match precipitants. Journal of Affective Disorders 86 (1): 27–35.
- ↑ Tooby, J., Cosmides, L. (1990). The past explains the present: Emotional adaptations and the structure of ancestral environments. Ethology and Sociobiology 11 (4-5): 375–424.
- ↑ 18.0 18.1 Hagen, E.H. (1999). The Functions of Postpartum Depression. Evolution and Human Behavior 20 (5): 325–359.
- ↑ 19.0 19.1 Nettle, D. (2004). Evolutionary origins of depression: a review and reformulation. Journal of Affective Disorders 81 (2): 91–102.
- ↑ Paul Gilbert (1992). Depression: The Evolution of Powerlessness, Psychology Press.
- ↑ 21.0 21.1 Hagen, E.H. (2003). The Bargaining Model of Depression. Genetic and Cultural Evolution of Cooperation.
- ↑ 22.0 22.1 22.2 Watson, P.J., Andrews, P.W. (2002). Toward a revised evolutionary adaptationist analysis of depression: the social navigation hypothesis. Journal of Affective Disorders 72 (1): 1–14.
- ↑ 23.0 23.1 Watson, Paul J. "An Evolutionary Adaptationist Theory of Unipolar Depression: Depression as an adaptation for social navigation, especially for overcoming costly contractual constraints of the individual's social niche"
- ↑ Hagen, E.H. (2002). Depression as bargaining The case postpartum. Evolution and Human Behavior 23 (5): 323–336.