Why Are Some People Left- handed? Why is a mere 4% of the population left-handed? Most authorities agree, to the relief of many a mother of a left-handed child, that if the child prefers using the left hand, and functions well with it, there is no need to correct this “condition”.One theory centers on the two halves of the brain, each of which functions differently. Medical science believes that the opposite sides of the body are supplied by the opposite sides of the brain.The predominant left half of the brain, which controls the right half of the body, makes it more skillful in reading, writing, and speaking, and makes most people right-handed. "Lefties," however, are the product of an inversion. They also work best with the left side of their bodies.There is no doubt that all exist in a "right-handed society," which manufactures most basics for the 96%. But don’t forget many of the greatest geniuses are left-handed, including Leonardo da Vinci.