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HANDEDNESS

The term "handedness" typically refers to a person's preference for the use of a particular hand in familiar, unimanual tasks such as handwriting and throwing a ball. Depending on the criteria used, 65 to 90 percent of adults are right-handed, about 4 percent are left-handed, and the rest are mixed-handed, preferring the right hand for some tasks and the left for others. The tendency to prefer the right hand exists across cultures, and fossil evidence suggests that this preference dates to prehistoric times. Some bias toward the use of the right hand is evident even before birth, but within individual infants, hand preference often varies across time and tasks. It is not until sometime in the second year after birth that handedness becomes clearly established for the majority of children. Because the preferred hand is controlled by the opposite cerebral hemisphere, handedness brings up important questions about how functional differences develop between the two sides of the brain.