handedness
About 90% of us are better at doing things with the right than with the left hand. We write with the right hand, throw with it, and use it to hold implements such as scissors, a spoon, a knife, chopsticks, a sword, or a racquet. There are some activities that require both hands, but it is usually the right hand that plays the dominant role, as in threading a needle or unscrewing the lid of a jar. It is, of course, the greater skill of the right hand, in most people, that gives rise to the term dexterity.
This dominance of the right hand is a characteristic of the human species. Other animals may show a systematic preference for one hand or paw when reaching for things, or using implements, but over the population of the species as a whole there is typically no overall preference for one or other hand or paw. Even individual mice typically show a systematic preference for one paw when reaching for food in a glass tube, and the preferred paw usually has the stronger grip; but there are as many left-pawed as right-pawed mice. Chimpanzees in the wild have been observed to use sticks to extract termites from their nests, and individual animals vary with respect to which hand they prefer to use to hold the stick, but again there appears to be no overall preference. There is evidence for a slight majority of right-handers among captive chimpanzees for some activities, such as feeding, but the overall bias is only about 67%, significantly lower than the proportion of right-handed humans. The only species known to show a bias comparable to that in humans is the parrot, since roughly 90% of them prefer the left foot when picking up small objects or bits of food!
Right-handedness appears to be universal among humans. There is slight variation across cultures, but this is largely restricted to activities in which there are strong sanctions against using the left hand. For instance, in some traditional cultures it is forbidden to eat with the left hand, which in Hindu society, for instance, is regarded as ‘the hand of the privy’. Until quite recently, there were strong pressures to force left-handed children to write with the right hand, although it is now generally recognized that this is likely to do more harm than good. Forced right-handers may be more than usually prone to stuttering, and may fail to realize their full potential for manual and verbal skills.
It was once thought that the ancient Israelites must have been predominantly left-handed because Hebrew is written from right to left. This Eurocentric view ignored the many other right-to-left scripts: until about ad 1500 there were about as many right-to-left scripts as left-to-right ones. The present-day prevalence of left-to-right scripts almost certainly owes more to European expansionism than to handedness. Again, it has been argued that the ancient Egyptians were left-handed, because Egyptian art typically portrays humans in right profile, whereas the natural tendency of right handers is to draw faces and bodies in left profile. But the preference for right-sided profiles in ancient Egypt was probably due to a belief that the left side of the body is inferior and should be hidden. Indeed, the very universality of such beliefs is testimony to the universality of right-handedness itself. Evidence from works of art suggests that the proportion of right-handers has been roughly constant for at least 5000 years. Close examination of the design and wear of ancient stone tools suggests that right-handers may have been in the majority throughout the Stone Age, going back two million years or more.
Given the overwhelming majority of right-handers, it is all too easy to overlook the left-handed, or to infer that left-handedness is in some way pathological. Yet there is virtually no indication that left-handers are in any way inferior in intellect or skill, and indeed they often seem to excel. The Italian Renaissance artist and inventor Leonardo da Vinci, one of the greatest geniuses of all time, was left-handed, and confused the right-handed world by writing, in mirror-image fashion, from right to left. Several US presidents, including Harry Truman, Gerald Ford, Ronald Reagan, and Bill Clinton, join such historical figures as Alexander the Great, Julius Caesar, Cicero, Charlemagne, and King David in being left-handed. Left-handers in the entertainment world include Charlie Chaplin, Harpo Marx, Rock Hudson, Paul McCartney, and Ringo Starr. A disproportionate fraction of prominent sportspersons have been left-handed, especially in sports involving racquets and bats: Rod Laver, Jimmy Connors, John McEnroe, and Martina Navratilova in tennis; Babe Ruth in baseball; David Gower in cricket.
It is not always clear whether the prominence of left-handers in some walks of life has some biological cause, or whether being left-handed simply makes people want to prove themselves in a right-handed world. The advantage they often seem to show in competitive sport may also be due to the unexpectedness, to right-handers, of their movements and actions.
Evidence of handedness can be seen in stone age tools. The scraping implement held in a left hand was formed by careful chipping by a hammer stone held in a more dextrous right hand. The stone axe shows a twist suited to right-handed use. Less frequently left-handed tools are found
It is sometimes suggested that the prevalence of right-handedness is simply due to environmental pressure. We live in a right-handed world. The location of door handles, the way pages are arranged in books, the construction of objects such as scissors, corkscrews, nuts and bolts, golf clubs, and even access to the zipper on men's trousers (to the wearer of the trousers, that is), are all designed for the convenience of right-handers and are a source of frustration to left-handers. But the very universality of right-handedness, extending to cultures that have been isolated for tens of thousands of years, suggests that causation is the other way round: it is the biological disposition toward right-handedness that is responsible for the cultural pressure. Moreover, the stubborn persistence of left-handers as a roughly constant proportion of the population in all human societies suggests that handedness may be under genetic control.
No one has yet located a gene for handedness, if indeed there is one (or perhaps there are several) ; but there have been reasonably plausible attempts to capture some of the facts about human handedness in terms of a hypothetical gene. Children are more likely to be left-handed if one or both of their parents are left-handed, but handedness does not ‘breed true’, since the proportion of left-handers born to two left-handed parents is somewhat less than half. It has therefore been suggested that the handedness gene does not simply determine whether one will be right-handed or left-handed. Rather, one form (or allele) of the gene, which we may call D for dextral, may code for right-handedness, while the other allele, which we may call C for chance, leaves the direction of handedness to chance. Different versions of this idea have been proposed by Marian Annett of the University of Leicester, and by Christopher McManus of the University of London. The chance element may also explain why some people are ambidextrous rather than clearly right- or left-handed. The children of left-handed parents would then have a chance of receiving two C alleles, one from each parent, which perhaps explains why only about 50% (or less) are left-handed. The C allele also helps explain why identical twins, like Tweedledum and Tweedledee in Lewis Carroll's Through the Looking Glass, are often of opposite handedness, although other factors are likely to contribute as well.
Besides being right-handed, most of us are right-footed when kicking, right-eyed when aiming, and right-eared when listening on the telephone, and for most people it is the left side of the brain that controls speech and language. Since it is also the left brain that controls the right hand, this has led to the idea that the left side of the brain exerts a general dominance, especially over skilled actions like tool use or speech. This view has been tempered, however, by discoveries that the right brain is the more involved in other functions, such as spatial attention and orientation, and in non-verbal activities, such as art and music.
Nearly all right-handers are left-brained for language, suggesting that the D allele controls both handedness and brain dominance for language. Left-handers show a much more mixed pattern. Some are left-brained, some right-brained, and some appear to have speech represented on both sides. This is further evidence that the C allele, which is assumed to be present in the great majority of left handers, does not have a directional influence.
Throughout recorded history, and in virtually all human cultures, handedness has invited myth, superstition, and of course prejudice. One reason for this may be that handedness is not evident in the hands themselves. It is difficult to tell whether a person is left- or right-handed by physically inspecting the hands — although there may be some give-away signs like rings, or a watch, or differential signs of wear. Handedness is really only evident from the way people do things — a reflection of brain activity rather than of how the hands themselves are formed. The hidden sources of handedness may explain why it is often linked to supernatural or extrabodily sources. The right hand is often the hand of God or virtue, while the left hand is the hand of the Devil or wickedness. In the Bible there are over 100 favourable references to the right hand, and about 25 unfavourable references to the left.
The true nature of handedness is still not fully understood, and there is no reason to suppose that our modern theories are entirely free of age-old superstitions.
— Michael C. Corballis
Bibliography
- Corballis, M. C. (1991). The lopsided ape. Oxford University Press, New York
See also language and the brain; symmetry and asymmetry.
