No easy answers for hand asymmetry

In rare cases, an organ may turn up on the “wrong” side with no untoward effects. But there is one form of asymmetry that is common and, until quite recently, stigmatised: handedness.

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, four of last five US presidents—Reagan, the elder Bush, Clinton, Obama—have been left-handed. But the riddle of why about 10 per cent of children are born with this essentially human asymmetry remains.

“This is really still mysterious,” said Clyde Francks, the lead author of a 2007 study in which Oxford University researchers identified a genetic variant linked to left-handedness.

Hand asymmetry is related to brain asymmetry. “We’re at the very beginning of understanding what makes the brain asymmetrical and what goes wrong—we don’t understand that process in the normal case,” Dr  Francks said.

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. Lateralisation, the distribution of function into right and left
hemispheres, is crucial for understanding language, thought and even creativity. For many years, handedness has been seen as a possible proxy, an external clue to the balance in the brain between left and right. Handedness clearly runs in families. The 2007 paper by the group at Oxford identified a gene, LRRTM1, that they discovered in the course of studying children with dyslexia, and which turned out to be associated with the development of left-handedness.

Dr Francks recalls that the discovery attracted attention because this gene was also found disproportionately in people with schizophrenia, even though none of these connections are well understood. “We’re not looking for a gene for handedness or a gene for schizophrenia. We’re looking for subtle relationships. The gene affects the ways that neurons communicate with one another,” he said.

Dr Daniel Geschwind, a professor of human genetics, neurology and psychiatry at UCLA School of Medicine, is interested in the link between language and handedness. “Handedness has a genetic basis, but like other complex traits —height, weight—it is complex. It’s not a single gene that leads to it. There’s a strong environmental component, too,” he said.

As with other traits that we are tempted to classify as either/or, handedness is probably  better viewed as a spectrum encompassing the very 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 nondominant hand.

In general,  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,” Dr Geschwind said. “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 such an easily measurable “external” trait, and as a key to the complex anatomy of the brain, researchers continue to  debate —links to many other conditions, including immune disorders, learning disabilities and dyslexia, reduced life expectancy and schizophrenia.

The idea of links to schizophrenia have been particularly persistent, but last year, a large study found no increased risk with non-right-handedness for schizophrenia or poorer neurocognition.

The New York Times