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:
No one knows for certain why the human population is right-hand-dominant, but a number of theories have been proposed.
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:
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.
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.
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.
"...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."
In 2007, researchers discovered LRRTM1, the first gene linked to increased odds of being left-handed. The researchers also claim that possessing this gene slightly raises the risk of psychotic mental illnesses such as schizophrenia. However handedness is not inherited from parents. Even when both parents are left-handed, there is only a 26% chance of their child being left-handed.
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. It is believed these individuals are both retarded and left-handed for the same reason: 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. 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.
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, frequent references reinforce the positive aspects of "right" and "right hand" side. The Bible indicates that Jesus sits at the right hand of God, or indeed is the right hand of God. Thus, faith may guide followers toward right-side preference. Christianity has had far-reaching effects on many societies throughout history, potentially strengthening the spread of right-side preference.
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.