Left-Handers Are Found to 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 UCLA researchers who use 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 their research published Monday 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 Dr. Daniel Geschwind, who led the research team. “There is more flexibility, and that is under genetic control.”

That hereditary difference between right-handers and left-handers also appears to affect how the brain changes in size throughout a lifetime, the researchers found.

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 humans display such a strong predisposition to right-handedness. Right-handers make up about 90% 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.

“Not only does this study tell us something about the genetic and environmental influences on brain development during youth, it may tell us something about those influences on aging,” said Dr. Mark Jude Tramo, a neuroscientist and neurologist at Harvard University Medical School. “It is very significant and very important.”

To study brain size and structure, the UCLA researchers used a brain-scanning technique called functional magnetic resonance to compare brains in 72 pairs of identical twins, all of them male World War II veterans ages 75 to 85.

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 carry 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.

“Overall, this study shows us that brain structure is highly influenced by genetics, even later in life,” Geschwind said. “This implies that aging-related changes to the brain also possess a strong genetic basis. That is kind of wild.”