Handedness

Handedness, a preference for the use of either the right hand or the left hand. Although many animals have a preferred paw or hand, only humans have a species-typical preference for the right hand. Recent studies of handedness in certain toad species suggest that handedness was acquired early in vertebrate evolution. Scientists had previously believed that only animals that routinely use their paws, claws, or feet, had developed a preferential handedness. Approximately 75 percent of the human population is strongly right-handed, and approximately 90 percent is predominantly right-handed. Among the remaining 10 percent, a great deal of variability exists. Some people are strongly left-handed, and others, called ambidextrous, are left-handed for some activities and right-handed for others.

Scientific studies conducted during the 1970s and early '80s suggest that differences in left- and right-handers in patterns of brain organization may be associated with differences in skills, aptitudes, and perhaps even personalities. In the large majority of right-handers (98 or 99 percent), speech is controlled by the left side of the brain. The right hemisphere of the brain is usually specialized for recognizing and remembering faces and understanding relationships in space. In left-handers, the pattern of brain organization is unpredictable. About 65 to 70 percent of left-handers have speech controlled by the left hemisphere of the brain, as is the case for right-handers, but in 30 to 35 percent of left-handers speech is controlled by the right hemisphere. In some left-handers, both hemispheres of the brain are capable of controlling speech.

The hand an individual comes to prefer is determined, in part, genetically, but this does not mean, for example, that two right-handed parents cannot have a left-handed child, or the reverse. The precise mechanisms by which genes affect handedness are still unknown. A physical injury may also be involved. During the birth process, the region of the brain controlling the hand is sometimes damaged, so that a child who would have been right-handed without such damage becomes left-handed.

Social pressures have had a considerable effect on handedness. A few decades ago in the U.S., using the left hand for writing was strongly discouraged, and only 2 percent of the population wrote with the left hand. Today in Taiwan, only 1 percent of the population writes with the left hand because left-handedness is socially condemned. Indeed, only in recent years has society become sufficiently tolerant of differences among people to accept left-handedness as a benign trait, and language still reflects the negative view that used to be held of left-handers. The word sinister, meaning “evil,” also means “left-handed,” and the French word for left (gauche) is also commonly used to mean “awkward.” Left-handers are, of course, no more awkward than right-handers; any awkwardness or psychological disturbances observed in left-handers in the past could well be attributed to the problems that were created for them by an intolerant society. Many famous persons, including Leonardo da Vinci, Benjamin Franklin, and Lewis Carroll, were left-handed.