What makes you right-handed or left-handed?

Why some 90 percent of people posses a greater degree of strength and coordination in their right hands than in their left - and why the opposite is true for the other 10 percent - is something that remains unexplained.

There are many theories. The Greek philosopher Plato believed that human were by nature ambidextrous, and the "folly of our nurses and mothers" imparted the "bad habit" of right-handedness. Today, theories on why individuals have a dominant right or left hand tend to place more emphasis on inheritance and less on learning. Even infants seem to demonstrate, by the direction in which they turn their heads and by their strength of grip, a preference for one side over the other.

If your parents are left-handed, the probability that you will be, too, is 23 times higher than if both parents are right-handed. British researcher Marian Annett thinks that most people are born with a gene inclining them to right-handedness. The 18 percent without this gene, she suggests, become lefties or righties depending on childhood influences.

Other theories propose that fetal or birth events may be a cause of left handedness. Among twins - who are somewhat more prone than single babies to fetal injuries - the incidence of left-handedness is more than double the average. However, there is not conclusive evidence that left-handed people have experienced any from of brain damage.