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Handedness is an attribute of human beings defined by their unequal distribution of fine motor skill between the left and right hands. An individual who is more dexterous with the right hand is called right-handed, and one who is more skilled with the left is said to be left-handed. A minority of people are equally skilled with both hands, and are termed ambidextrous. People who demonstrate awkwardness with both hands are said to be ambilevous or ambisinister. Ambisinistrous motor skills or a low level of dexterity may be the result of a debilitating physical condition. There are four main types of handedness:
- Right-handedness is most common. Right-handed people are more dexterous with their right hands when performing a task.
- Left-handedness is less common than right-handedness. Left-handed people are more dexterous with their left hands when performing a task. About 8-15% of people are left-handed.
- Mixed-handedness, also known as cross-dominance, is being able to do different tasks better with different hands. For example, mixed-handed persons might write better with their right hand but throw a ball more efficiently with their left hand. This could also refer to pianists, because different sections of pieces are usually composed to fit the abilities of the different hands, considering that the right hand is usually for melodies and the left-hand, harmonies. However, many writers define handedness by the hand used for writing, so Mixed-handedness is often neglected.
- Ambidexterity is exceptionally rare, although it can be learned. A true ambidextrous person is able to do any task equally well with either hand. Those who learn it still tend to sway towards their originally dominant hand.
No one knows for certain why the human population is right-hand-dominant, but a number of theories have been proposed.
Theories of handedness
Newer theories of handedness look at handedness in different ways than previously. The newer view is that handedness is not a simple preference for one hand, because the two hands actually work together in more subtle ways. For example, when writing it is not a simple matter of one hand being dominant and writing on the paper. For a right-handed person, the left hand is involved in important ways: it orients and grips the paper and provides the context from which the right hand operates. Thus the right hand appears specialised for finer movements and the left for broader, contextual movements.
Evolution by natural selection is asserted to reinforce prevailing behaviors and deselect minority traits (unless the minority traits are linked in some way with desirable traits). However, all human populations continue to 'maintain' a minority of left-handers. The implications are that:
- any disadvantages associated with the minority trait (an increased likelihood of contracting certain diseases, for instance) are outweighed by a benefit to the left-handed individual.
- there is some sort of frequency dependent cost/benefit of being left- or right-handed according to the relative frequency of each type in the population.
- handedness is inextricably linked to some other cost/benefit expressed in inherited traits.
This theory is explored in a 2004 study by Faurie and Raymond. The researchers complement ethnographic data with a discussion of the success of left-handers in certain sports, to demonstrate that left-handed individuals have a competitive advantage in combat. The rate of left-handedness appears to correlate with the amount of violence in a given society (taking homicide rates as a measure). It is said to follow that the minority left-handed population has, historically, played a crucial role in the evolution of individual societies. The counter-conclusion—that increased violence in a society generates a larger left-handed population—is not, however, borne out by the researchers.
Brain hemisphere division of labor
Division of labor is the most commonly accepted theory of handedness. The premise of this theory is that since both speaking and handy-work require fine motor skills, having one hemisphere of the brain do both would be more efficient than having it divided up. Also, if all functions were carried out in both hemispheres, the size of the brain and its energy consumption would increase, which is not affordable. Since in most people, the left side of the brain controls speaking, right-handedness would prevail. It also predicts that left-handed people would have a reversed brain division of labor.
- It does not explain why the left hemisphere would always control language. Why not 50% of the population left and 50% right?
- While 95% of right-handers do indeed use the left side of the brain for speaking, it is more variable for left-handers. Some do use the right for linguistic skills, some use the left hemisphere, and others use both. On the balance, it appears that this theory could well explain some left-handedness, but it has too many gaps to explain all left-handedness.
- In primates and even sheep, brain lateralization has been found (e.g., right hemisphere dominance for face processing). [dubious — see talk page]
Advantage in sports
The advantage to players in one-on-one sports such as tennis, boxing, fencing or judo is that in a population containing perhaps 10% left-handers and 90% right-handers, the left-hander plays 90% of his or her games against right-handed opponents and is well practiced at dealing with this asymmetry. The right-hander plays 90% of their games against other right-handers — thus when confronted with a left-hander is less practiced. When a left-hander plays another left-hander, they are both likely to be at the same level of practice, as when right-handers play other right-handers. This explains why a disproportionately high number of left-handers are found in sports where direct one-on-one action predominates. In other sports such as golf, this advantage is not present since the one-on-one action is indirect; the handedness of one player has no effect on the other. The difficulty of obtaining left-handed golf clubs puts also the left-handers at an early disadvantage.
Advantage in hand-to-hand combat
A variant of the above argument says that left-handed people have an advantage in fighting without weapons, because of the "surprise" factor. (This fact is well-known to boxers and was employed to world-record effect on November 4, 1947, when Mike Collins, a natural left-hander, emerged from his corner in a right-handed stance before suddenly shifting left and delivering the fight's first and last punch, knocking out opponent Pat Brownson in 4 seconds.)
A 2004 study by Charlotte Faurie and Michael Raymond of the University of Montpellier II in France, published in the Proceedings of the Royal Society, argues that there is such a link. To prove their theory, Faurie and Raymond surveyed nine primitive societies in five separate continents. Through a mix of direct observation and existing data, they estimated the number of left-handed people within each population. They also looked at murder rates, thinking that those communities with higher murder rates might favor populations with more left-handed people, if left-handedness is a trait associated with greater fitness with regard to combat.
Among these samples, they found strong support for the idea that, at least in primitive societies with higher levels of violence, left-handed people are more numerous.
In neither of the previous two theories is the origin of handedness explained, nor is the prevalence of right-handedness.
There is strong evidence that prenatal testosterone contributes to brain organization. One theory is that high levels of prenatal testosterone results in a higher incidence of left-handedness. This could be why there are more left-handed males than females and also the increased incidence of left-handedness in male twins. See Geschwind-Galaburda Hypothesis.
Asymmetry of internal organs
While the external organs are highly symmetric, the internal organs such as heart and stomach are highly asymmetric. Perhaps the asymmetric brain piggybacks onto this.
Some ambidextrous individuals note that they prefer sleeping on their right side, putting less overall weight on the heart-side of the body. Perhaps this unconscious preference to elevate the left side of the chest cavity to reduce the amount of work that the heart must exert during sleep favors the right side by supplying extra blood (due to gravity), widening the diameter of vessels within the right side of the body to compensate for the increase in pressure, thus, over time increasing that side's muscle efficiency.[How to reference and link to summary or text]
Its basic premise is that left-handedness is due to brain damage during the birth process. Some statistics support this theory. Difficult or stressful births happen far more commonly among babies who grow up to be left-handed or ambidextrous. Birth stress is also associated with a number of birth defects and complications, including cerebral palsy and autism. A problem with this theory is that the dramatic reduction of traumatic births due to superior medical technology and obstetrics has failed to lower the proportion of left handed people. It would be a dramatic coincidence if improved medical technology allowed more children to survive stressful births in the exact proportion.[How to reference and link to summary or text]
A popular theory is that ultrasound scans may affect the brain of unborn children, causing higher rates of left-handedness in mothers who have ultrasound scans compared to those who do not. This is probably based on a few studies where this relation is studied. In one of these the authors claim:
"...we found a possible association between routine ultrasonography in utero and subsequent non-right handedness among children in primary school."However later in the same article the authors state that "Thus the association ... may be due to chance," and:
"the result was not significant, suggesting that the study had insufficient statistical power to resolve the relationship between ultrasonography and subsequent left handedness in the child."
Is left-handedness genetic?
In 2007, researchers discovered LRRTM1, the first gene linked to increased odds of being left-handed. The researchers also found evidence that possessing one particular form of this gene slightly raises the risk of psychotic mental illnesses such as schizophrenia. However handedness is not inherited from parents in a simple way. Even when both parents are left-handed, there is only a 26% chance of their child being left-handed.[How to reference and link to summary or text]
This rate of incidence is high enough that when members of the same family exhibit left-handedness by chance, it can look as though the trait is inherited. For instance, many members of the British royal family are left-handed, and their fame has led to observance of possibly inherited left-handedness. When a powerful family exhibits left-handedness they do not feel the same pressure to comply to the norm, and may instead glorify their difference, leading to a reverse discrimination. One of the many myths of left-handedness involves the genetics of the Clan Kerr. The predominantly left-handed Kerr noblemen of the Scottish Borders built fortified homes with counterclockwise spiral staircases, so that left-handed swordsmen would be better able to defend them (but perhaps at the same time making it easier for right-handed swordsmen to attack them). However, a 1993 study found no statistically significant increase in left-handedness among people with the family name Kerr or Carr. 
The least controversial etiology of left-handedness is that of the pathological left-hander. Left-handers comprise almost 20% of the mentally retarded population and 28% of the severely and profoundly mentally retarded population.[How to reference and link to summary or text] It is believed that in these individuals, both their left-handedness and their retardation are caused by brain damage to their left hemisphere as a result of a prenatal or postnatal event. It is also possible that a nutritional insult results in left hemisphere aberration. If the verbal processing area in the left hemisphere is damaged early in life, even partially, the right hemisphere would assume verbal processing functions, along with other hemispheric functions. This would account for the left-handers who process verbal material in their right hemisphere and, depending upon the severity of the brain damage, would also account for the higher proportion of left-handers found in the retarded population. There is no genetic component to this type of left-handedness.
The second type of left-hander is the natural or genetic left-hander. Such persons function normally but are more likely to process language (at least in part) in the right hemisphere.
The third type of left-hander is the learned left-hander. This left-hander writes with the left hand but has relatively poor handwriting, and shows dual hemispheric activation during verbal processing. Because preverbal children are not lateralized for hand use, these left-handers may have initially chanced to successfully manipulate some toy with their left hand and continued to use their left hand for toy manipulation. When eventually given a pencil or crayon, because of past reinforcement, they employ their left hand, and continue to use their left hand when they write even when they may be naturally right-handed. This, of course, is quite inefficient neurologically, as described above, and because of the additional processing time required, may be the reason quite a few left-handers stutter when they are young and have notoriously poor handwriting.[How to reference and link to summary or text] It is believed that eventually these left-handers develop verbal processing function in their right hemisphere too, and that these individuals become the left-handers who naturally show dual hemispheric activation during verbal processing. So far there is no clear explaination why humans are left-brained for verbal processing by default.
Parental and societal pressure
This theory explains right-handed dominance by claiming that since people are mostly right-handed, parental pressure essentially teaches this behavior as normal. In this way, the right-handed dominance continues. This idea assumes that environmental pressures can dominate over a genetic tendency, indicated by the handedness of children being more closely related to their biological parents than to adoptive parents, and the fact that the percentage of left-handed people has remained virtually unchanged [How to reference and link to summary or text].
Social stigma and repression of left-handedness
Left handed people live in a world dominated by right-handed people, and many tools and procedures are designed to facilitate use by right-handed people, often without even realising difficulties placed on the left-handed. However, as well as inconvenience, left-handed people have been considered unlucky or even malicious for their difference by the right-handed majority. In many European languages, including English, the word for the direction "right" also means "correct" or "proper". Throughout history, being left-handed was considered as negative. The Latin word sinistra meant "left" as well as "unlucky" and this double meaning survives in Euopean derivatives of Latin, including the English word "sinister.' There are many negative connotations associated with the phrase "left-handed": clumsy, awkward, unlucky, insincere, sinister, malicious, and so on. In French, gauche means both "left" and "awkward" or "clumsy", while droit(e) (cognate to English direct) means both "right" and "straight", as well as "law" and the legal sense of "right". The name "Dexter" derives from the Latin for "right", as does the word "dexterity" meaning manual skill. As these are all very old words, they would tend to support theories indicating that the predominance of right-handedness is an extremely old phenomenon.
Until very recently in Taiwan, left-handed people were strongly encouraged to switch to being right-handed, or at least switch to writing with the right hand. It is considered more difficult to write legible Chinese characters with the left hand than it is to write Latin letters, though difficulty is subjective and depends on the person in question. Because writing when moving one's hand away from its side of the body can cause smudging if the outward side of the hand is allowed to drag across the writing, it is considered easier to write the Latin alphabet with the right hand than with the left. Conversely, right-to-left alphabets such as the Arabic and Hebrew are considered easier to write with the left hand in general.
In Christianity, right and left are taken up as agents of symbolism. Christian Scripture depicts Jesus sitting at the "right hand" of God; sin is depicted as deviation to the "left hand side" of Christ.
In Islam it is encouraged to use the right hand for acts considered good, like eating and drinking. The use of the left hand is encouraged when the act is considered base, such as cleaning oneself after urination or defecation. This is attested in various Hadiths by the Prophet Muhammad.
- Edinburgh Handedness Inventory
- Handedness and sexual orientation
- Handedness and mathematical ability
- Lateral dominance
- Sinistral and dextral
- Nurse Minerva answers questions from readers about handedness
- David Wolman's lively book exploring the causes and consequences of handedness
- Handedness and Earnings / Higher paychecks: a left-handed compliment?
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