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The mystery of left-handedness

We're the only species with such a dramatic preference for the right arm. Can genetics explain it?


Rik Smits
October 29, 2011 8:00PM (UTC)
This article is an edited excerpt from the new book "The Puzzle of Left-handedness" from University of Chicago Press.

'My husband's left-handed too.' The young woman sounded worried. 'Is he going to die young?' I couldn't help laughing; here we go again. The story that the average left-hander dies an untimely death has been around for decades. It's a dark fairy tale that's proven tenacious for two reasons. Every once in a while a scientist comes along who's happy  to breathe fresh life into it simply for the sake of causing a sensation, but more important, perhaps, is the eagerness with which the vast majority of right-handed people embrace the story every time it crops up. What could be nicer than a good safe shudder? Whether or not it's true isn't really the point. It's mainly a matter of guilt-free gloating.

The ease with which such stories circulate on the global rumour mill suggests that people don't really believe them. The version most commonly heard is that left-handers die an average of nine years sooner, and that by about the age of forty most have disappeared. Were that truly the case, then left-handedness would be an extremely grave affliction, a scourge claiming a huge number of young lives – the kind of illness people talk about in veiled terms, preferably in acronyms.

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'Have you heard? Caroline's got LH!'

'What? Her too?'

'Yes. You didn't hear this from me, but . . .'

'Gee ... That family sure has it hard.'

This is not the sort of thing people say.

It all started in 1992, when a Canadian professor of psychology by the name of Stanley Coren published a book in which, based on his own research, he drew attention to the horrifying fact that left-handed people have lifespans nine years shorter than average. He called it The Left-Hander Syndrome. There's no denying that Coren had a feeling for drama; he made rather a name for himself with his book and it earned him a tidy fortune. Then people simply got on with their lives.

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No politicians asked questions in the House; no specialists appeared in the media to inform the public. We didn't even see any worried left-handers forming action groups, or fearful parents-to-be demanding early pre­natal testing for this fatal disorder. Nobody pressed for further research. Nowhere in the world was there even a well-meaning govern­ment information campaign. Speculation about left-handed mortality was and remains public entertainment – which is not something real illnesses are.

Coren's book fitted a pattern that often emerges when left-handedness makes news: something grabs our attention, but it's never more than a flash in the pan and left-handers themselves usually decline to comment. That's another thing right-handed people always seem to overlook. In 1998 Australian biologist and journalist Geoff Burchfield asked the psychologist Michael Corballis in an interview: 'Why do you think the subject of handedness triggers such strong responses in people, especially lefties?' Corballis began in all seriousness to articulate a detailed answer, but he soon got bogged down in meaningless platitudes. Not that Corballis is stupid, far from it, but the question was as natural as it was misconceived. The unconventional preference exhibited by left-handed people, whom Burchfield rather dismissively refers to as 'lefties', is in reality something that astonishes and preoccupies right-handers. They regard the left-handers they come across from time to time as peculiar – and indeed intriguing and faintly creepy – whereas for left-handed people it's the most normal thing in the world. They don't see themselves and their like as in any way odd, and they are entirely used to living in a right-handed world. From an early age they have grown accustomed to the strange fascination their aberrant hand preference engenders in right-handed people.

This is a pity, really, because although right-handers get thoroughly heated up about the problems they perceive to exist while left-handers merely shrug, all sorts of real puzzles remain. Along with language and cooking, the fact that the vast majority of human beings have a distinct preference for the right hand is one of relatively few characteristics that are unique to our species. Preference for one hand or paw over the other is not unusual in itself – in fact it can be found in a wide range of animals, even in mice – but the uneven distribution of left- and right-handedness is a different matter. Only one in ten people regard them­selves as left-handed, whereas among animals it seems that a roughly equal number favor the left paw as favor the right.

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No one can yet say with any certainty just how it comes about that one person is right-handed and another left-handed. Equally puzzling are the reasons why left-handedness arose in the first place. It must have happened a very long time ago, because another puzzle is why down through the centuries, all over the world, there has always been a stable left-handed minority of around ten per cent. That's not all.

Hand preference reflects a deeper, all-pervasive quality of human beings, namely the fact that each cerebral hemisphere has its own distinct functions, yet in most left-handed people the layout of the brain seems to differ little if at all from that of right-handed people.

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As if all this wasn't confusing enough, the concepts 'left' and 'right' are problematic in themselves. We find it hard to learn which is which, and even in adulthood we can quite easily mistake one for the other. Driving instructors are all too well aware of the possibility that a nervous pupil will yank the steering wheel in the wrong direction. Never­theless, there are innumerable ways in which the distinction between left and right determines how we order, experience and comprehend the world. Photographs, drawings and paintings comply with implicit laws of orientation, as do films and comic strips.

Despite how successful some left-handers are, and how inconspicuous most remain, throughout history left-handedness has been associated with clumsiness and with unpleasant traits such as untrustworthiness and insincerity. The Latin word for left, sinister, has all kinds of dark, dismal and ominous connotations that have come into every one of the many languages related to Latin. Science has thrown its weight into the ring at regular intervals. At the end of the nineteenth century infamous skull-measurer Cesare Lombroso had no hesitation in saying that left-handedness was a sign of a criminal personality, and in the mid-twentieth century prominent American psychoanalyst Abram Blau announced that left-handedness was tantamount to 'infantile negativism', the equivalent of a refusal to eat everything on your plate. Neither of these eminent gentlemen had the slightest evidence for their harsh judgements, but they weren't going to let that spoil their fun.

The actions and attitudes of Coren and his ilk demonstrate that as far as rushing to conclusions goes, little has changed. Left-handedness was and is associated with maladies of all kinds, including mental retard­ation, alcoholism, asthma, hay fever, homosexuality, cancer, diabetes, insomnia, suicidal urges and criminality. In most cases there's a complete absence of solid evidence for any such association, although it is true that groups of people with minor ailments, disabilities or stains on their character generally include more left-handers than average, whether they be hay-fever sufferers, breast cancer patients or prison inmates. But higher than average percentages of left-handers can also be found in art colleges and architecture schools, and among the highly gifted.

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Then again, it turns out that within randomly composed groups of people, no differentiation based on character traits or failings has ever been found to coincide with a division into left-handed and right-handed.

The puzzles and paradoxes continue to stack up. There are plenty of indications that left-handed people are entirely normal, except in their hand preference, yet the fuss that's made even today about writing with the left hand implies otherwise. It's the subject of endless philosophizing and theorizing and often described as contrary to nature. Much is made of the 'serious problems' faced by left-handed six-year-olds who are called upon to 'imitate right-handed writing using their left hands,' as Dutch handwriting guru A. van Engen puts it. Oddly, despite all this concern, teacher training colleges fail to pay serious attention to left-handedness even today. Left-handed six-year-olds generally have to find out for themselves how to master handwriting – the most difficult skill taught in primary school – based on a model that shows them the reverse of what they are trying to do. Most manage it so successfully, despite all the problems they're presumed to encounter, that in no time at all they can write as well and as quickly as their right-handed classmates.

After a century of research, we're left pretty much empty-handed, since none of the three best explan­ations for the existence of left- and right-handedness is satisfactory.

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General trauma theories have a long list of shortcomings, of which the most important is that they require us to assume that in newborns the world over, the incidence of brain damage is horrendously high, although we barely notice it in the course of people's lives. The 2009 discovery that differences between left- and right-handers in the arrangement of features in the two sides of their brains can be far greater and more general than was once thought makes it harder than ever to assume that some kind of trauma lies behind all left-handedness.

[Famed neurologist Dr. Norman] Geschwind gets us a long way with his testosterone theory, but it doesn't solve the twins problem. He's therefore unable to explain the existence of left-handedness as such, although the suggestion that testos­ter­one is a driving force behind pathological switches of hand preference seems perfectly sensible.

So we're left with genetic explanations. However, standard explanations in terms of mutations and alleles [forms of genes] won't work either. Mutation is the motor of evolution: without mutation there is no new variation and without variation there is no selection. But that means that the proportions of carriers of different alleles are inherently unstable, whereas the proportion of left-handers to right-handers has remained remarkably constant throughout the world over a very long time.

For all the mystery, there are one or two things that we do know for certain. We can be sure, for example, that there is an inherited aspect to hand preference and that the basic probability of its manifestation is an unchanging 10 percent or thereabouts. We also know that left-handedness in a parent doubles the likelihood that a child will be left-handed. Oddly, every characteristic that conforms to these stipulations turns out to be self-stabilizing, as long as the product of the basic probability of its occurring multiplied by the influence of both parents remains below 100 per cent. Beyond that, the characteristic will spread unstoppably across all individuals.

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Take a ship full of colonists on a remote island. When they start to reproduce they form pairs that will sometimes consist of two right-handers, sometimes of a right-hander and a left-hander and sometimes of two left-handers. How many pairs of each composition there will be depends on the size of the group and the percentage of left-handers on board. But if every child that results has a 10 per cent chance of being left-handed and if that probability doubles with each left-handed parent, then after a while the colony will consistently produce left-handedness at a rate of about one in ten, no matter how many left-handers arrived with the ship and irrespective of whether the group grows, shrinks or maintains its original numbers. It doesn't even make any difference if there was only one left-handed person on the original ship, or only one right-hander, or 50 per cent of each. In all cases, after a handful of generations, 12.7 per cent of the population will be left-handed, a percentage that from then on will not change at all.

It barely matters at all whether a trait of this type is advantageous or disadvantageous in evolutionary terms. Which is nice, because one of the puzzling aspects of the existence of left-handedness alongside right-handedness has always been that no one could think what positive effect on reproductive success might ensure that left-handedness continued to exist. If hand preference were a matter of the emergence of gene variation under selective pressure, then there would have to be a positive effect of some kind. It might perhaps be something like the advantage of having sickle cell anaemia in regions where malaria is prevalent, but no one has ever been able to think of anything concrete. Now that we know that left- and right-handedness cannot be dependent on a duo of alleles, we are liberated from the requirement to come up with this elusive advantage. Evolutionary quality is of hardly any significance in the case of self-stabilizing traits.

To understand this we need to look at another human characteristic, one that has nothing to do with hand preference but does exhibit the same kind of stable distribution. That characteristic is sexual orientation.

Like hand preference, sexual preference can be divided, roughly speak­ing, into two categories, in the same ratio of one to nine. The majority of us are attracted to the opposite sex, while 10 per cent of us prefer people of the same sex. As with hand preference, the division is not razor-sharp and absolute, but most people are preponderantly one way or the other. It seems as if in sexual orientation there are more cases of explicitly mixed preference, but appearances may be deceptive. There are for example people who label themselves bisexual for ideological reasons. As with the number of left-handers, the percentage of homosexuals, male and female, seems to have been more or less constant for many centuries, all over the world. There is one clear difference, however. While it remains an open question whether hand preference is linked to any evolutionary advantage or disadvantage, there can be no doubt that homosexuality is a textbook case of a trait that makes people less likely to reproduce.

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A confirmed homosexual has no interest in intercourse with the opposite sex and therefore in Darwinian terms he or she fails to compete. In fact from the point of view of both natural selection and sexual selection, a homosexual lifestyle is so disastrous that it's mystifying how this trait ever became established and why the numbers have remained constant even in the face of outright persecution.

We need to note at this point that the repression of homosexuality may actually have contributed to its survival. Throughout history homo­sexuality has been viewed with almost universal contempt, or indeed simply denied and oppressed. It was at best something engaged in by men at the margins of society. We need only look at the way people in strictly sexually segregated societies today turn a blind eye to homosexual contact as a safety valve for frustrations among young men. All men, homosexual or not, were expected to start a family and care for their children. Women were given no choice; they were simply presented with some equivalent of the Victorian adage 'close your eyes and think of England'. In many societies, sadly, they still are.

So for millennia most homosexual men and women had little chance of finding fulfilment, if we assume this would have involved sticking to relationships that suited their sexual orientation. They were forced to engage in family formation and reproduction to a more or less normal degree. The situation is probably little different today, not only in countries where homosexuality officially does not exist but even in Western Europe and America. So the negative effect on reproductive success has probably been considerably reduced by social pressures. It's far from negligible all the same. Which of us doesn't know at least one confirmed bachelor, not to mention a middle-aged man who has never flown the parental coop, house-sharing sisters, elderly spinsters or even a nun or a hermit? These are all lifestyles that have traditionally provided unsuspected, if perhaps unfulfilled homosexuals with a way to live.

As a self-stabilizing trait like left-handedness, homosexuality could easily acquire a place for itself within the human population. In fact as soon as it existed in a few individuals, for whatever reason, it would inevitably acquire such a place. The negative effect on reproductive success does not alter this, it only means that a slightly higher average number of children need to be born, across the board, to maintain the population level.

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Take a concrete example. We begin with the same values as those for left-handedness, in other words a basic probability of homosexuality in any given child of roughly one in ten, double that if one of the parents is homosexual. We then assume that all pairs made up of two homo­sexuals fail to reproduce at all and that half of all couples of which one partner is homosexual are also childless. These are strict conditions, and they reduce reproductive chances considerably, but even then the average number of children per couple needed to maintain the popu­lation at its existing level only rises from two to 2.25. The number of homosexuals in the population will remain absolutely steady at a little over 11 per cent.

It's more than possible that there are other traits of this kind, occurring at some constant frequency through all times and all circumstances, immune to selective pressures. We don't know of any, but that could be because they are uninteresting or inconspicuous, or occur invisibly, deep inside our bodies.

This article is excerpted with permission from "The Puzzle of Left-handedness" from University of Chicago Press.

Rik Smits is a linguist, a science journalist and the author of books on a variety of subjects.


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