Left-handers have different brains

People who grow up left-handed have a different, more flexible brain structure than those born to take life by the right hand, say researchers at the University of California, Los Angeles, who used twins to study heredity.

The reason is that right-handers have genes that force their brains into a slightly more one- sided structure, according to research published yesterday in the Proceedings of the National Academy of Sciences. Left-handers appear to be missing those genes.

"There really is a difference in brains that results in a more symmetric brain in left-handers, where the two sides are more equal," said UCLA neurogeneticist Daniel Geschwind, who led the research team. "There is more flexibility, and that is under genetic control."

In the effort to understand how the brain shapes the mind, researchers have been striving to document the way genes and environment affect intelligence and mental abilities. The human insistence on preferring one hand over the other poses a particularly nagging question that touches on both anatomy and behavior.

"There is clearly something fundamental here we need to comprehend if we are to understand what makes us uniquely human," Geschwind said.

Of all the primates, only human beings display such a strong predisposition to right-handedness. Right-handers make up about 90 percent of the population. The left and right halves of the brain are different in both their anatomy and their functions, related in part to hand preference.

But until now, no one could document the connection.

The UCLA study is the strongest evidence yet that heredity shapes the brains of left-handed and right-handed people differently, Dartmouth neuroscientist Michael Gazzaniga said.

The UCLA researchers conducted brain scans on 72 pairs of male identical twins between 75 and 85 years old.

Identical twins, who share the same genes, offer a unique lens through which to study the relative effects of heredity on human nature.

Right- and left-handedness is partially determined by genetics. If a person inherits the gene for right-handedness, that person will be right-handed. People who do not have that gene, however, can be either left- or right-handed. There is no specific gene for left-handedness.

Right-handers typically have a larger left brain hemisphere, where their language abilities are concentrated.

Conversely, left-handers have more balanced brains, with both sides relatively symmetrical. The language abilities of left-handers more often are concentrated on the right side.

If identical twins carry the gene for hand preference, both must be right-handed. If they lack the gene, one twin can develop right-handed while the other develops left- handed.

The researchers found that the brains of identical right-handed twins were very similar in size and structure. But when a left-hander was part of the twin set, the brains were different. The conclusion, researchers said, is that the absence of the gene for hand preference allows the brain to develop differently as the individual grows up.

A similar pattern did not appear in 67 sets of fraternal twins used as a control group