On the Other Hand

On the other hand

Perri Klass

















Famous leftie ... Angelina Jolie.
Left-handedness still a mystery to scientists, writes Perri Klass.

Humans are asymmetric animals. Early in our embryonic development, the heart turns to the left. The liver develops on the right. The left and right lungs have distinct structures. There are certain rare syndromes in which the usual asymmetry of organs is reversed. I remember how disconcerting it was the first time I examined a child with dextrocardia, a heart pointed to the right side, and heard the heart sounds in unexpected places. But when it comes to handedness, another basic human asymmetry - which reflects the structure and function of the brain - the reversed pattern is relatively common but not easily understood.

Over the centuries, left-handers have been accused of criminality and dealings with the devil and children have been subjected to "re-education". In recent years, the stigma has largely vanished; among other things, four of the past seven US presidents - Ford, the elder Bush, Clinton and Obama - have been left-handed. (Reagan is sometimes cited as being ambidextrous.)

But the riddle of what underlies handedness remains. Its proportions - roughly 90 per cent of people are right-handed and 10 per cent left-handed - stay consistent over time. "This is really still mysterious," says Clyde Francks, the lead author of a 2007 study in which Oxford University researchers identified a genetic variant linked to left-handedness.
Hand dominance (whether left or right) is related to brain asymmetry. And that, Francks says, "is not at all understood; we're really at the very beginning of understanding what makes the brain asymmetrical".

Though brain asymmetries exist in our closest primate relatives, there seems to be general consensus that the human brain is more profoundly asymmetric and that understanding that asymmetry will show us much about who we are and how our brains work.

Brain lateralisation, the distribution of function into right and left hemispheres, is crucial for understanding language, thought, memory and perhaps even creativity. For many years, handedness has been seen as a possible external clue to the balance in the brain between left and right. For right-handed people, language activity is predominantly on the left side of the brain. Many left-handers also have left-side language dominance but a significant number have language either more evenly distributed in both hemispheres or predominantly on the right side of the brain.

Handedness clearly runs in families. The 2007 paper by the group at Oxford identified a gene, LRRTM1, which they discovered in the course of studying children with dyslexia and which turned out to be associated with the development of left-handedness. Francks, who is now at the Max Planck Institute for Psycholinguistics in the Netherlands, recalls that the discovery attracted a great deal of attention, the more so because this gene was also found disproportionately in people with schizophrenia, even though none of these connections are simple or well understood.

"We're not looking for a gene for handedness or a gene for schizophrenia," he says. "We're looking for subtle relationships." The gene affects the ways neurons communicate with one another, he says, but its mechanisms still need to be studied.

Dr Daniel Geschwind, a professor of human genetics, neurology and psychiatry at the University of California, Los Angeles, School of Medicine, has studied the connections between language and handedness and the ways handedness can help us understand the evolution of the human brain. "Handedness has a genetic basis but like other complex traits - height, weight - it is complex," he says. "It's not a single gene that leads to it. There's a strong environmental component, too. It's a very tricky problem."

As with other traits that we are tempted to classify as either/or, handedness is probably better viewed as a spectrum that encompasses the strongly right-handed or left-handed and a range of those who prefer to use one hand or the other but have different degrees of comfort and competency with the non-dominant hand.

In general, Geschwind says, left-handers have less-asymmetric brains, with more even distribution over the two hemispheres. "Perhaps a more accurate conceptual way to think about them is as non-right-handers," he says. "Many of them are much more likely to be ambidextrous and have fine motor abilities with their right hands."

Because left-handedness has been seen as a key to the complex anatomy of the human brain, researchers continue to look for - and debate - links to many other conditions, including immune disorders, learning disabilities and dyslexia, reduced life expectancy and schizophrenia. None of it turns out to be simple. The idea of links to schizophrenia has been particularly persistent but schizophrenia is a complicated and probably heterogeneous disorder and studies of different populations show different patterns; last year, a study found no increased risk with non-right-handedness for schizophrenia or poorer neurocognition.

In paediatrics, we sometimes worry about children who manifest handedness before their first birthday. The concern is that if a very young child seems to strongly prefer one hand, there may be some problem - perhaps some kind of neurological damage - on the other side. Well into last century, left-handed children were often forced by teachers and parents to convert and use their right hands.

Historical cave paintings show that left-handers have been among us since the time of early man. Ancient figures such as Caesar are also recorded as being left-handed, so the trait does not appear to have been "weeded out".
So though there has been prejudice against left-handers, and though there may be some developmental risks, Geschwind says, "there clearly must be advantages as well. The reason why it maintains that way, nobody knows what it is.