Anatomically, our left and right hands are mirror images. Nevertheless, the vast majority of people perform tasks requiring skillfulness with their right hand exclusively. Only about eight percent of the population is left-handed. Remarkably, this percentage has been consistent for over fifty centuries. Even Stone Age cave drawings and tools indicate that our prehistoric ancestors were predominantly right-handed. Like humans, nonhuman primates also show a strong tendency to be right-handed on a variety of tasks.
The brains of almost all right-handed people display left-hemisphere dominance for speech and language functions. However, neurosurgeons have discovered a surprising pattern of differences in brain organization among left-handed people. Using a special technique to study the left and right hemispheres of the brain separately, scientists found that about seventy percent of left-handed people show the same pattern as right-handed people, with the left hemisphere dominant for language. About fifteen percent of left-handed individuals show the opposite pattern, with language function localized in the right hemisphere. The remaining fifteen percent of left-handed individuals process language in both hemispheres.
Scientists have also discovered that in left-handed and ambidextrous (using both hands with equal skill) individuals the corpus callosum, the thick band of nerve fibers that connects the two hemispheres of the brain and acts as a communication link between them, is about eleven percent larger than it is in right-handed individuals. This finding suggests that left-handed people may experience more efficient communication between the two hemispheres and less hemispheric specialization. It might also help explain why some left-handed people process speech and language in both hemispheres.
The variability in brain organization among left-handed people may also account for the differences in degree of hand preference. Right-handed people consistently perform virtually all tasks requiring dexterity with right hands. But many left-handed people show a pattern of mixed handedness, preferring to use their right hand for some tasks.