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Studies: About 10% of people are left-handed, 20% of twins. Left-handers tend to die about 9 years sooner than right-handers. Left-handedness has been linked to a variety of physical abnormalities, most of which can be traced to development or immune system dysfunction (e.g., asthma, bowel and thyroid complaints, myopia, dyslexia, migraine, stuttering, and allergies). (Carter, Rita, Ed. Mapping the Mind. CA: University of California Press, 1998, pp 46-47)