07 December 2004

Are You Left Handed? Do You Know Someone Who Is?

Why Are Some People Left-Handed? What Does It Mean If They Are?

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.)

Hair Asymmetry

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 Posts

A 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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Anonymous said...

Yes, i am left handed and i am doing a science fair project on " what causes handedness" this site was very helpful. I encourage you to add on, this site has alot of information and is very informative.

Anonymous said...

My sister is left handed. both my parents are righties and so am I. I did a presentation in my science class for a bonus. The teacher loved the information and even the kids enjoyed it. Thanks!

Anonymous said...

im still confused. if handedness, is a typical mendelian trait then how is it possibly that both my parents are left handed including me but my brother is right handed. if r/r is recessive and it is a mendelian trait there is no possible way that he could be right-handed.?

Anonymous said...

@14 November, 2008 15:51

The blogpost also says a genetically left-handed person can be acquired right-handed due to social circumstances. Apparently the adaptivity of the brain also allows the opposite to occur. In other words, the "phenotype" is both determined by genes and by the environment, as with all genetic traits. More importantly, due to the inherent plasticity of neurons, phenotypes vary immensely opposed to other celltypes

Anonymous said...

did you know some people are both left and right handed?

Anonymous said...

I have to disagree with the whorl direction. I am left handed and my whorl direction goes clockwise and one of my friends is right handed and his whorl direction is ant-clockwise.

Anonymous said...

@ 14 November, 2008 15:51

The post say that r/r doesn't mean left handed. It means no hand preference. That means with r/r you could be left handed, right handed, or ambidextrous. If r/r meant just left handed, there would be no such thing as ambidextrous. Unless you taught yourself maybe but...

Anonymous said...

I don't get that I'm left handed and the rest of my family is right handed... What does that mean?

Anonymous said...

I am left handed and all my family is right handed. how weird is that?

Anonymous said...

Both my husband and I are right handed and both our children are left handed. How can this happen? We beleive that their great-grandmother was left handed but that is the only thing we can link it to.

Anonymous said...

What? I am confused. What do you mean? My friend is left-handed and said she is going to die early because her friend said that. Is that true?

Anonymous said...

If it is determined on genes then why am I left handed and my identical twin sister is right handed?

David said...

@ Anonymous 2 Aug 2012: Handedness isn't inherited in a simple Mendelian manner, or it would have been figured out long ago. It is probably like many other traits, like height and skin pigmentation: influenced by several genes and by developmental factors, resulting in a range of expression.

Maybe it's time for an updated post on left handedness.

Happy said...

Hello, I have been using my right hand for 24 years, I am a Muslim and our culture favors right handness, which it was encouraged by family, so I didn't really get the chance to use my hands in their natural order.
A recent experience made me notice that I was a naturally born Left-Handed.
I noticed that I was very strong with my left hand, I throw balls to long distance without effort with my left hand, but I lack accuracy, Also I write with my Right hand, which I was taught to do all these years, I found it hard to try to switch now to write with my left (lousy handwriting).
Would you consider me a Left-Handed or is just illusions?

Redfish1 said...

@ Happy

I had a somewhat similar experience as I was growing up: according to my mother, until I learned to write I was entirely ambidextrous, reaching for things with either hand, throwing, batting (baseball), eating. By the time I was being taught to write I was a contrary kid, and distinctly remember people trying to get me to use my right hand, so I used my left. Now I'm mostly left handed, but a chemical burn to my left hand forced me to write with my right hand, and I became proficient (but slow) and developed better penmanship with both hands. In addition, when I need to do something with great precision I use my right hand because I have better conscious control therewith.
Perhaps you, too, are ambidextrous!