The Puzzle of Left-Handedness

by Rik Smits

  We can perhaps safely conclude that the left and right cerebral hemispheres both do their work in their own way, sometimes alone, sometimes in collaboration. The left brain is primarily skilled at aspects of tasks that have to do with counting, language production and comprehension, timing and consciousness of time – all tasks in which sequencing is important. The right brain seems to be more active in the recognition of situations and in associating them with each other. A competitor in a quiz who unfailingly recognizes any song from its first chord can probably attribute that talent primarily to the right side of the brain, which identifies the unique combination of timbre, pitch and so forth that occurs only in the opening bars of that particular number. If the competitor then taps out the rhythm, the left brain becomes fully involved, since he or she is engaging in an activity organized in time.

  These differences in the way the two halves of the brain work seem to be little more than tendencies. Strong tendencies, certainly, but they don’t amount to an absolute distinction. There’s room for considerable variation, for a greater or lesser contrast, for more or less obvious lateralization. It seems left-handedness can somehow be a consequence of this variation, but it needn’t be. We shouldn’t forget that as far as the separation of functions goes, the great majority of left-handed people show the same pattern as almost all right-handed people. Nor should we forget that despite the effectiveness of modern technology, we still have little firm knowledge about the brain.

  The two cerebral hemispheres differ not only in what they do and in how they function but anatomically, which is to say in their material form. A great many parts of the brain are larger on one side than the other in most people. Sometimes they differ in shape as well. In left-handers these differences seem on average slightly less pronounced, and in a few cases the proportions are actually the reverse of those found in most right-handers. When it comes to brain anatomy, left-handers also demonstrate rather more variation, but again this is no more than a tendency and the majority are no different from the average right-hander in the physical shape and composition of their brains. There may therefore be a weak connection between hand preference and brain shape, but there is not, to use phrenological terminology, a bump in the skull representing hand preference.

  There is another possible anatomical cause, which strictly speaking lies outside the brain. It has sometimes been suggested that hand preference arises from differences in the nerve bundles that connect the brain with the arms and legs. The left half of the brain directly controls not only the right arm and hand but the whole of the right side of the body, and vice versa. In practically all respects, that is. The connections between one half of the brain and the muscles or, for example, the sense of touch on the opposite half of the body take the form of a thick bundle of nerves that runs from the brain down the spinal column and from there to all the extremities on that side. But there are also connections between each cerebral hemisphere and the body on the same side. This bundle of nerves is nowhere near as thick, although it does mean that if one side of the brain is damaged, the side of the body it controls won’t necessarily be completely paralysed and totally numbed.

  Vesalius, Fabrica (1543). The main features of the nervous system are shown here, including the loose bundles of nerves that lead to the arms and hands and the long connections that lead via the spine to the torso and legs.

  From each half of the brain, then, two bundles of nerves lead to the spinal column, a thick bundle that connects with the opposite side of the body and a thin one straight down. The thick bundles cross each other just below the brain, reaching the side they control immediately above the spinal cord. These two thick bundles, despite the fact that each has roughly the same amount of body to control, can differ in size considerably. The same goes for the thinner bundles. In eight out of ten people both the thick and the thin bundles that run to the right side of the body are thicker than their counterparts that serve the left side. Eight out of ten; a very suggestive proportion. You might almost suspect that hand preference arises out of a better infrastructure that makes traffic between the brain and that half of the body smoother and more precise. Almost, because this turns out not to be the case. There is no connection at all between hand preference and the difference in the thickness of these nerve bundles.

  * In 1865 Dax’s son succeeded in having his father’s work published, but by then the credit had gone to Broca. Not entirely unfairly, since Broca had a far more precise idea than Dax as to where speech functions were located.

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  Animal Crackers

  When in the course of the nineteenth century it gradually became evident that in humans real differences exist between the two halves of the brain, a great many neurologists immediately hurled themselves upon the animals of the woods and fields in order to find out whether perhaps there was evidence of similar specializations. The crude neurological techniques of the time made it impossible to use healthy humans in research, so scientists were limited to examining people who happened to have suffered brain damage, or carefully tinkering with those who for one reason or another needed to undergo brain surgery. There were fewer difficulties with animals, which were enthusiastically ex -perimented and operated upon, sometimes with astonishing results. Contrary to expectations, animals often turned out to have a preference for one paw over the other and to stick to that preference with great determination.

  In one experiment rats were put into a cage with a tube attached, in which lay a tasty treat. The tube was mounted in an extreme corner of the transparent front of the cage, up against the side, so that the rat had to use the paw on that side to reach into it. Some rats stubbornly kept trying to use the wrong paw. They clearly had a marked preference, so much so that it didn’t even occur to them that they could easily get at the treat with the other paw. There were less dogmatic rats too, for whom it made no difference where the tube was placed. They had no hesitation in using whichever paw was convenient.

  Even more remarkable results were achieved when rats with a strong paw preference had the part of the brain controlling that paw delib -erately damaged. After a few days they were able to move their prefer -red paw again, but it functioned less well than before. Nonetheless, the rats continued to favour it. This seemed very similar to the way people respond. Whatever may cause hand or paw preference, once it’s become established there’s an extreme reluctance to switch.

  Yet however surprising the results of experiments like these may be, they produce little of relevance to human hand preference. It turns out to be almost impossible, for example, to find a specialization in one half of the brain of an animal, whether ape or frog or anything in between, that resembles the kind of specialization we see in humans. In birds such as sparrows and canaries, the left brain does seem to be more involved in singing than the right, but bird brains differ so fundamentally from those of mammals – and therefore people – that this doesn’t tell us a great deal. Another problem is that hardly any criteria are imaginable that could be used to measure a bird’s preferences. In rats, reaching for treats serves as a relevant task, but how do you measure foot or wing preference in a sparrow or a seagull?

  To a greater degree even than experiments involving people, animal experiments are hard to interpret. Appearances can be deceptive, increasingly so the more an animal differs from us. The renowned scientific journal Nature reported in 1996 that ‘hand’ preference had been found in an amphibian for the first time: the European common toad. Researchers had stuck pieces of wet paper over the animals’ noses and mouths, or pulled small balloons over their heads, and watched to see which of their front feet (‘hands’) the animals used in their initial attempts to get rid of the annoying obstruction.

  The Italian-Australian research team reported sensational results from its toad-baiting. The creatures seemed more like people than any other species. No fewer than six out of ten were right-handed, while only one in six favoured the left. The remaining quarter had no clear preference. This
was extraordinary, since if chance determined hand preference then a quarter would be left-handed, a quarter right-handed and the rest indifferent. Toads were demonstrating a distinctly uneven distribution, just like humans, such that the left-handed, at around 15 per cent, were proportionately almost as rare as left-handed people. For a short time it seemed as if lateralization, the presumed source of our hand preference, could be traced back to the common ancestor of humans and toads.

  Unfortunately the scientists had spoken too soon. Tomio Naitoh of Shimani University in Japan and Richard Wassersug of Dalhousie University in Halifax, Canada, took up their pens to disabuse the readers of Nature. Toads, they wrote, have a habit of removing poisonous and indigestible food remains via their mouths. They do this by vomiting up their entire stomachs and wiping the stomach lining clean with a front hand. Since a toad’s stomach is asymmetrical, with a shorter membrane on one side, it always hangs out of the right side of its mouth, which makes it easier by far to reach with the right hand. It was very likely that the toads were reacting to foreign bodies on their faces as if they were cleaning their stomachs. You could still call this a hand preference, but its cause lies in the structure of a toad’s intestines, not in its brain. So much for the right-handed common ancestor of man and toad.

  A few years earlier, in 1990, other researchers had claimed that rhesus macaques were right-handed, based on the fact that the bones in their right arms were usually slightly longer and stronger than those in the left. But oddly, the researchers added, the difference in males became smaller with age and the difference in females larger. They cautiously concluded that something else might be at work here. Two years later New Zealander Rachel Baskerville identified what it was. Changes to the skeleton that coincide with age and sex often have a hormonal cause. In humans, for example, asymmetries were found long ago in the vicinity of the shoulder, a result of the fact that testosterone acts slightly more on the bone on one side than the other. Testosterone levels fall in men over the course of a lifetime, whereas in women they increase slightly, a pattern neatly reflected in monkey bones.

  The differences between limbs in rhesus macaques could also be a side effect of small irregularities in the symmetry of the torso as a whole. In people with back problems caused by malformations or by a tilted pelvis, one leg is often found to be slightly shorter than the other, by way of compensation. Whatever the precise reason for the difference found in the monkeys’ arms, it probably has nothing to do with paw preference.

  A multiplicity of animal experiments has nevertheless produced some results. First of all it’s clear that paw preference does exist in mammals, but in a way consistently different from the picture in humans. In animals with a paw preference, the right-pawed group is always roughly similar in size to the left-pawed group, while around half of individuals lack a clear preference. We never see anything like the uneven distribution so characteristic of man.

  An even more intriguing result is that breeding for paw preference, in contrast to breeding for colour and all kinds of other characteristics, has proven impossible. It was tried for many years with mice to no avail. In generation after generation the picture remained the same: there were always roughly as many left- as right-pawed individuals plus a large group that showed no preference. This proved, if nothing else, that paw preference is not passed down simply according to Mendel’s laws of inheritance. If we persist in thinking that paw preference in animals has something in common with hand preference in people, then the laboratory rat has only succeeded in making hand preference slightly more of a puzzle than it already was.

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  Other Asymmetries and Preferences

  Although in most cases we couldn’t say straight off whether the people around us, even those we know well, are left- or right-handed, there is one category of individuals concerning whom careful records are kept: sportspeople. Left-handedness is a far from trivial advantage to those engaged in sports in which contestants enter into single combat, such as tennis, fencing and boxing, and indeed baseball, which in essence comes down to a duel between pitcher and batter. The reason is not hard to discover; it lies in the simple fact that left-handers constitute a small minority.

  Take training. Most of the time spent by anyone seriously involved in a sport is devoted to rigorous practice. Since most people are right-handed, each player has a right-handed opponent to train with around nine times out of ten. For left-handers this is somewhat awkward initially, since they’re faced with asymmetry, but after a while they become entirely used to playing against a right-hander. If they find themselves facing a fellow left-hander in a match, they have little difficulty coping; the situation may be slightly unfamiliar, but they’re suddenly in their element. At last the constellation is symmetrical. For well-trained left-handers it therefore matters little which hand an opponent prefers.

  Not so right-handers. Like left-handers they train with right-handed people nine times out of ten and usually this suits them fine, but when they occasionally have to take on a left-hander they’re doubly disadvantaged. They’re forced to engage in an asymmetrical battle for which they’re poorly prepared, against an opponent who’s a dab hand at dealing with this type of asymmetry. No wonder that in the official rankings of boxing, tennis and fencing champions, left-handers are significantly over-represented, as they are on the lists of top scorers in baseball.

  The sports world tells us something else too: hand preference is not the only kind of asymmetry people encounter. A minority of soccer players are left-footed. They prefer to use the right foot to stand on while kicking the ball with the left. Such players are in demand because they can shoot from angles that the majority of footballers find hard to deal with, and because they’re in a minority they’re just that bit more unpredictable and difficult for right-footed players to mark.

  As well as right- and left-footed soccer players, there are some who can hold their ground equally well with either foot. They are fairly few in number but more common than two-handers. If people exist who can truly do everything just as easily with either hand – while still being reasonably dextrous – then they’re as scarce as hens’ teeth. ‘Two-footedness’ is encountered relatively frequently, possibly because the tasks of a preferred foot are a good bit simpler than the performances we expect from a hand. In the end it’s really only a matter of ball control, the equivalent of accurate throwing. Feet don’t have any other skills, if only because all our toes do is help us walk, or keep our balance when standing still, tasks that involve both feet equally. A single specific skill, however complex, can be mastered by the non-preferred side of the body as long as we train long and hard. Left-handers who have been forced to learn to write with their right hands are living proof of this, as are left-handed musicians who play right-handedly.

  Yet most soccer players stick anxiously to their preferred foot, and many never manage to train the other to anything like the same standard, so a kick with the wrong boot often fails completely. This was demonstrated on one occasion by the legendary, pig-headed and exclusively left-footed Dutch footballer Willem van Hanegem, renowned for his curved balls, as they’re known. Once, in his prime, when he was selected to take a penalty that could have made him the country’s top scorer, he took it with his right foot out of sheer perversity – and missed by a mile. Vintage Van Hanegem.

  If we have both a hand preference and a foot preference, might there not be other, similar kinds of asymmetry elsewhere in the human body? Aside from our arms and legs there is just one other body part that serves mainly to carry out active tasks so delicate or complex that it allows us to speak of a preference asymmetry: the tongue. True, we have only one, but it is symmetrical, and each half is controlled by the opposite half of the brain. It’s called upon to perform some of the most difficult jobs we face.

  First of all the tongue is responsible for an efficient preliminary treatment of the food we put into our mouths. It has to ensure that nothing escapes the grinding force of the teeth while i
tself remaining out of their reach. When this occasionally goes wrong we’re instantly reminded why it’s important. Its second task is to form speech sounds, which rely upon the subtle curves and differences in shape assumed by the tongue one after the other at impressive speed, in close coordination with movements of the lips, lower jaw and vocal cords. Considering that most of us can speak at a rate of 180 words a minute without too much difficulty, and that on average a word consists of four or five different sounds, it’s clear that some fairly impressive acrobatic feats go on between our teeth.

  Some people do indeed seem to have a preferred side. In normal circumstances we’re not aware of having a tongue preference, but it’s easy to identify. The trick is to place one side of the tongue, then the other, gently between the molars and hold it there while singing your national anthem. The tongue side that is free when the words of the song take the least effort to sing and the result sounds best is the preferred side. Sadly no information is available as to how many people have a clear preference, what proportion of people are right- or left-tongued, or whether there is any connection between that figure and the proportion of left- and right-handers.