[Source: The New York Times]
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 structure.
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 on 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, and for all that, 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 our last seven presidents — Ford, the elder Bush, Clinton, Obama — have been left-handed. (Reagan is sometimes cited as ambidextrous.)
But the riddle of what underlies handedness remains. Its proportions — roughly 90 percent of people are right-handed and 10 percent left-handed — stay consistent over time.
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