Most people prefer one hand or the other for certain tasks, such as writing, throwing, handling eating utensils, and the like. About 90% of us are right handed. (Analysis of historical art shows this has been the percentage for thousands of years.)
Handedness, of course, is not in the hand, but in the brain. The motor centers of the brain control the hands, and if you are right handed the motor centers on the left side of your brain are more acute and dominant. (The left side of the brain controls the right side of the body, and vice versa.)
Is handedness is an inherited characteristic, determined by your genes, or a social characteristic determined by your social environment (learned), or some combination of both? Many theories have been argued over the years.
This question is part of a larger, and more controversial, one: How much of our behavior is determined by our genes?
Handedness Is Genetic
Recent results suggest a genetic explanation fits the data best. Handedness appears to be controlled by a single gene, called RHS ("relative hand skill"), located at chromosome 2p12-q11. This gene appears to work in typical Mendelian fashion: If you are R/R or R/r (R is the dominant), you are probably right handed. If you are r/r you have no innate hand preference, and you are just as likely to be right handed as left handed. (The location of the gene is shown here.)
One of the factors that helped show that handedness is genetic is that there is another trait controlled by the same gene: hair whorl direction.
Clockwise whorl on the right, anticlockwise on the left
[larger image here]
If you look at the direction of the whorl of hair on the back of someone's head, about 90% of the time it appears to radiate clockwise. This correlates strongly with being right handed. The hair whorl images above are from Klar, 2003 , the one on the right being Dr. Klar himself.
Hair whorl direction is not a learned or socially determined trait. If the inheritance of hair whorl direction is determined by the same gene as handedness (and if their occurrence is highly correlated) then handedness must be a genetically determined trait too. This is what recent research shows. [More recent research has cast doubt on this correlation between hair whorl direction and handedness.]
So there is at least one clear example of a behavioral trait under mendelian genetic control.
Left Brain? Right Brain?
There are some other traits that show lateral asymmetry. Structural asymmetries in the human brain have been known for many years. [review article, pdf format] The right frontal lobe is usually bigger than the left, while the left occipital lobe is usually bigger than the right. These asymmetries are said to be more pronounced in right handers.
Some of these physical asymmetries are also seen in the brains of fossil hominids and other primates, although they seem to be more developed in humans.
Many people are familiar with the idea that the two brain hemispheres are functionally specialized, with the language centers tending to be more developed in the left hemisphere, while emotional functions seem to be more "right-brain" oriented. (I am not convinced that a lot of this hemisphere-dominance/personality stuff is not largely new-age piffle. Here are some sites on the subject. [brainconnection] [ABC])
About 97% of right handers have their speech and language centers localized in their left hemispheres. About 70% of left handers do. Another way of saying this is that the left hemisphere is dominant for language in most right-handed people, but the right hemisphere can be the seat of language specialization more often in left-handers. (I personally would be skeptical of the precision of the left-hander numbers. Categorization of people as left-handed varies according to criteria used; some genetic left-handers have been converted socially to right-handers; and of course the left-handed sample is much smaller.)
There does not appear to be any advantage to the language centers being in the left or right hemisphere — performance is equal in people with each organization. Hemispheric functional specialization is also seen in non-human primates and many other animals (such as birds).
The connection between left-hemisphere language specialization and right handedness may be related to the motor functions of the language centers. They are also important in controlling gestures (usually made with the right hand, as in sign languages and ape gesturing) and facial expressions. Thus the centers that are largely devoted to language in the human brain correspond to centers controlling important motor functions in other animals. Of course spoken language involves delicate motor control of the lungs, throat, tongue, and face.
In any event the connection between handedness and hemispheric specialization is clearly complex, if there is any direct connection at all.
The Brains of Chimps
There is some evidence that other great apes, like humans, are mostly right handed. One would expect that the same genes are doing the same things in humans and other primates.
Recent studies of lateral brain specialization in chimpanzees indicate that handedness in chimps is associated with specific structures in the motor regions of the brain, rather than with areas homologous with the language regions. The same may well be found in humans. This suggests that handedness is not closely related to the development of language, since chimps don’t have the same language faculty as humans.
What about speculation that handedness is related to sex, sexual orientation, pathology or other attributes? Discussion will have to wait for another post.
Later PostsA more recent Science In Action post has more about left handedness.
And check out Great Left Handed People in this sister blog.
Further Reading. Here are some sources that might be interesting.
The Inheritance of Left-Handedness
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