Left-handers have always been a mystery to science: why are people left- or right-handed at all? Scientists now realise that handedness is not only intriguing – it might also provide insights into a range of perplexing diseases.
By Wilson da Silva
LEFT-HANDERS are different. Scientists have long known this. But not why.
It may sound like a silly area of research, but understanding handedness could potentially help crack enduring problems in biomedicine. Take immune disorders: they are more prevalent in lefties. Studies suggest that being left-handed more than doubles one’s chances of picking up a range of diseases, especially thyroid and bowel disorders.
Lefties are nine times more likely to suffer learning disorders, and strong left-handers are 11 times more likely to have dyslexia than strong right-handers.
How could these be related? About 90 per cent of people are right-handed, a figure that is stable across all regional and racial groups. And handedness usually exhibits itself in other parts of the body too: left-eyedness, left-footedness and left-earedness. But the how and why of handedness is a mystery, and the reason it is largely absent in animals (except chimpanzees, where it is 50/50) may well be a peculiarity of civilisation and our propensity for tools, painting and writing.
Originally, handedness was thought to be a learned trait. Since most people are right-handed, they induce – through copying or coercion – right-handedness. But a wealth of recent work has swung the argument to handedness being ‘hard-wired’ in the brain. The battleground is now over what causes this wiring to set.
One emerging but still contested hypothesis is the so-called Geschwind-Behan-Galaburda theory. First proposed in 1987, it posits that right-handedness is ‘normal’. Left-handedness arises when a change in the chemical composition of the womb, perhaps as a result of complication, leads to a rewiring of the brain during development that triggers left-handedness.
Its leading proponent is Stanley Coren of the University of British Columbia, who argues the cause is probably elevated levels of testosterone. Testosterone is known to retard the development of the left hemisphere (controller of the right hand) in early brain development: an excess might mean the right hemisphere becomes dominant and left-handedness arises.
Testosterone might also explain why left-handers have a high risk of developing certain illnesses such as allergies, since the hormone slows the growth of the thymus that produces white blood cells. It might explain why left-handedness is more common in men, why more than half of premature babies are left-handed and, since language is centred on the left hemisphere, and why language disorders are more common in boys. Left-handedness might therefore be the common thread in problems long thought to be unrelated.
Not everyone is sold. Some argue that genetics plays a big part. If both parents are right-handed, the chance of offspring being a leftie is 8 per cent. This rises to 17 per cent if one of the parents is left-handed, and shoots up to 50 per cent if both parents are left-handed. Nevertheless, genes may not completely account for handedness. If they did, why do 18 per cent of identical twins – who share exactly the same genes – have different handedness?
Amar Klar, a geneticist at the National Cancer Institute in the United States, thinks genetics can explain both. He believes that a single gene determines handedness; those with the gene are right-handed while those without have a 50/50 chance of being left-handed. He argues a non-functional allele causes this 50/50 split, and this random element explains the statistics.
In April this year, scientists at the University of California at Los Angeles buttressed his argument. Daniel Geschwind and colleagues used MRI scans to compare the brain size and structure of 72 sets of identical twins to 67 pairs of fraternal twins. Identical right-handed twins displayed very similar brain structure sizes, while identical twins with at least one left-hander showed less similar brains. Fraternal twins – who share only 50 percent of their genes – did not demonstrate this difference, suggesting that handedness is highly influenced by genetics.
Whatever the cause, it’s clear that handedness is not straight-forward. But studying it is worth the effort, since its association with a range of seemingly unrelated disorders may well provide a window of understanding for future research.
That doesn’t mean that left-handers should be seen as oddities: any group that includes Leonardo da Vinci, Isaac Newton, Pablo Picasso, Mark Twain, Albert Einstein, Ludwig von Beethoven and H.G. Wells can’t be all bad.
Wilson da Silva is a science journalist in Sydney and a former editor of the magazines Newton, Science Spectra and 21C.