M.K. Holder is an affiliated scientist in the Center for the Integrative Study of Animal Behavior at Indiana University. She replies:
In the 160 years in which "handedness" has been studied we have learned quite a lot, but we still cannot precisely describe what causes humans preferentially to use one hand over the other, or why human populations are biased toward right-hand use rather than left-hand use.
Scientists disagree over what percentage of human populations are "right-handed" or "left-handed" because there is no standard, empirical definition for measuring "handedness"; our criteria vary, and are based on various theoretical explanations because we are still trying to understand the mechanisms involved. But I can describe in general terms what we do know.
Most humans (say 70 percent to 95 percent) are right-handed, a minority (say 5 percent to 30 percent) are left-handed, and an indeterminate number of people are probably best described as ambidextrous. This appears to be universally true for all human populations anywhere in the world. There is evidence for genetic influence for handedness; however, it is non-Mendelian and geneticists cannot agree on the exact process. There is evidence that handedness can be influenced (and changed) by social and cultural mechanisms. For instance, teachers have been known to force children to switch from using their left hand to using their right hand for writing. Also, some more restrictive societies show less left-handedness in their populations than other more permissive societies.
Some researchers argue there is evidence for cases of "pathological" left-handedness related to brain trauma during birth. And many researchers trace the cause of handedness back to pre-natal, interuterine developmental processes, back to the time when the fetal brain is first developing distinct cerebral hemispheres. In the 1860s the French surgeon Paul Broca noted a relationship between right-handedness and left-hemispheric brain specialization for language abilities. But the hand-brain association is neither a simple, nor reliable, correlation. Studies conducted in the 1970s showed that most left-handers have the same left-hemispheric brain specialization for language typical of all humans--only a portion of left-handers have different patterns of language specialization.
So the bottom line is, we have a good general idea of the causes of right-handedness in human populations, but we have yet to work out the precise details, including why the direction is right instead of left.
Do other primates show a similar tendency to favor one hand over the other?
The second question (do non-human primates show handedness) is currently a controversial one. It is important to note the difference between an individual animal being left- or right-handed, and most of the animals in an entire population being either left- or right-handed. It is not unusual for individual animals to show a preferential use of one hand over the other, to develop an individual hand preference. But there is no consensus among researchers that any non-human species shows the same species-level handedness found in humans.
There are a few researchers who argue for this, but most of these work with animals in laboratory or captive settings, performing manual tasks that are very different from how animals use their hands in the wild.
In addition to studying handedness in humans, I have also studied hand usage in mountain gorillas (in Rwanda) as well as chimpanzees, red colobus monkeys, redtail monkeys and grey-cheeked mangabeys (in Uganda). My own research shows that individual monkeys and apes often develop individual preferences (both left and right) for manual tasks, but I have found no evidence for population-level handedness, as seen in humans.
Answer originally posted August 18, 1997.
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in which there are hypotheses on handedness
Hi! I am right handed as is everybody I am descended from as far as I know. My husband and his mother are both lefties (his mother used to get caned in school for using her lefthand, the poor thing). We have a 6 week old baby who I believe is already preferring his left hand. I was wondering if in some cases there is a simple genetic inheritance (eg, rL or ll) mechanism? (I never went past yr12 biology, forgive my ignorance!) Is lefthandedness maybe a dominant, but uncommon gene?
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Justine.
The issue of handedness has recently arisen amongst my social circle as a result of a small animation involving a rotating dance. Generally left-handers saw her rotating clockwise, right-handers in the other direction as this is to do with which side of your brain is dominant. However, the vast majority of right-handers on my course (architecture) could see her rotate both ways. this is because although being right-handers who use the left side of their brain more (logical side) they are studying a course that involves continually thinking spatially which is governed by the right side of the brain (used predominantly by left-handers). I feel that maybe as humans we generally evolve intellectually through logical and mathematics (left side of the brain) hence why there is a strong preference towards right-handedness. I have no scientific background to prove this this is just my personal observation.
Reply | Report Abuse | Link to thisI just wanted to share this with you. In our Muslim society it is highly recommended to use your right hand especially when you eat and off course when you shake hand. So even if you born left-handed you can use your right hand for eating and use your left hand to other activties. My brother is left handed he uses his hand to write and his left leg when he plays soccer however he naturally uses his right hand when eating.
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My mom's left-handed, but both my father and I are right-handed. My cousin has two right-handed parents and she is left-handed. It says in the article that handedness is "non-Mendelian," so you wouldn't be able to predict the chances of a child's handedness with something as simple as a Punnett square like you could eye color or hairline.
Reply | Report Abuse | Link to thisI'm sure glad I'm not part of your society. I would find it difficult as well as insulting to be forced to eat with my right hand because your religion finds it somehow "wrong" to eat left-handed.
Reply | Report Abuse | Link to thisMy Opinion : There are more Right Handed over Left Handed people in the world maybe because of Neurological
Reply | Report Abuse | Link to this(Brain Structure) reasons than Genetic (Hereditary) factors.
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Prashant S Akerkar