Correction to This Article
This article said that Ronald Reagan was left-handed. Although many lists of left-handed presidents include him, Reagan was right-handed, according to people who knew him. He performed some activities that require dexterity with his left hand, but he wrote with his right, which is the conventional determiner of handedness. Joanne Drake, who served as Reagan's chief of staff after his presidency, said yesterday that she and others had "heard the president say he was born left-handed and was forced to learn to write with his right hand as a young child."

Why Righties and Lefties? Scientists Have Hands Full.

By David Brown
Washington Post Staff Writer
Monday, March 23, 2009

Five of the last seven presidents have been left-handed. Ford, Reagan, Bush the elder, Clinton and now Obama (but not Carter or Bush the younger).

So what does this mean in a world where only one out of every 10 people, roughly speaking, is a lefty?

The answer is . . . nobody knows. It may be a fluke. But even if it isn't, exactly what left-handedness has to do with political skill, intelligence, popularity, family connections, wealth and luck -- all at play in our selection of national leaders -- is almost certainly a matter of subtle advantage, not one of dramatic benefit.

What is clear is that "handedness" runs all through the animal world.

Once thought to be uniquely human, some version of this attribute has been seen in chimpanzees, marmosets, cats, chickens, toads, mice, rats and almost certainly thousands of other species. It is present in animals that don't have hands (fish) and in some that don't have backbones (honeybees).

In biology, this phenomenon is known as "lateralization." It is the preference for doing or perceiving things more with one side of the body than the other. It appears to be an important -- although perhaps not necessary -- consequence of having a brain.

Like many structures in the body, the brain is "bilaterally symmetrical." It is made up of two halves -- called "hemispheres" -- divided by a plane that makes one the mirror image of the other.

Lateralization saves space and, therefore, working capacity, by not requiring that both hemispheres do the same thing. It diminishes the chance of interference and confusion, which might arise if each side of the brain independently analyzed the same input from the environment and came up with its own decisions about what to do about it. It also allows the brain to sometimes do two things at once.

"Any brain seems to lateralize if it can," said Lesley J. Rogers, a longtime researcher in the field who is an emerita professor at the University of New England in Australia.

The brain's asymmetry is primarily in function, not structure (although careful measurement shows that certain regions are bigger on one side than the other in nearly everyone). The most dramatic example involves language.

The ability to produce and comprehend language emanates from the left side of the brain in more than 95 percent of right-handed people and in about 70 percent of left-handed ones. That difference is a big clue that the neural wiring -- and perhaps more subtle things -- may be different in lefties.

Curiously, there is no anatomical "home" for handedness the way there is for language. For most voluntary movement, however, each side of the brain controls the opposite side of the body. A part of the right side of the brain drives the left arm and leg, and vice versa. Similarly, sensation, including vision, that is perceived on one side of the body is projected to the opposite side of the brain for processing.

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