About 10 percent of people are left-handed: writing throwing, chopping, any activity in which one hand does most or all of the work, they naturally use their left hand where most proper use the right. It’s not clear why this happens, or why it isn’t more evenly distributed. It may be down to genetics. Some scientists believe that handedness is determined at an early stage of fetal development, when the embryo transitions from being a round ball of cells to something with a definite left and right. It is around this point that the groundwork is laid to distinguish left from right, and place organs and other structures on their proper sides. The gene responsible for that, the researchers suggest, is also responsible for other asymmetries, such as one hand being stronger.
Another group of scientists believes that, rather than genetic factors, left-handedness is an indicator and commemoration of maternal distress. A recent study cast doubt on the idea that there are genes for handedness, finding that twins, who are genetically identical to each other, are no more likely to have the same handedness than unrelated people are. Meanwhile, fetal motion on the left side has been found when the mother is undergoing stress. However, this may only apply to the fetus, rather then the child once born.
Another hypothesis that has been offered is that the position of the fetus in the final weeks of pregnancy has significant bearing on whether the child will end up right- or left-handed. This affects the shape of a brain region called the vestibular cortex. This is an asymmetrical region, and the direction of the asymmetry is connected with hand preference. Hormone exposure has also been cited as a possible reason, including some fertility treatments.
It was formerly believed that left-handed people were more prone to stuttering and had shorter lives. The stuttering was determined to largely be due to efforts to make left-handed elementary school students use their right hands. These efforts similarly confounded the longevity question. In the most famous study, obituaries identifying the deceased as left-handed tended to be for younger people; however, this was the result not of left-handed people dying younger, but of older people, born earlier, not being willing to be identify as left-handed. However, let-handed people are prone to schizophrenia and other psychiatric disorders.