Left-handedness is due to asymmetry of the brain. It is not a disorder and is more common in males than in females. Actually, in the foetus, the left and right hemispheres of the brain are symmetrical.
Only after about six months of its birth, babies show a preference for either the right or left side of the body. This is due to functional asymmetry of two sides of the brain. It is said that the left hemisphere of the brain controls the right side of the rest of the body and vice versa.
As a result, it seems the left hemisphere is more active than the right, in a majority of the population. Though this theory is not accepted by neurologists in humans it has been proved in the case of dolphins.
Due to lack of symmetry in the brain, in 90 per cent of the population, the right leg, right arm and muscles on the right side are slightly larger and heavier than those on the left. It is almost the reverse in the rest of the population.
Left handedness is generally associated with special talents and also with certain disorders such as stuttering, dyslexia, depression and emotional withdrawal.
Nearly 75 per cent of the population is strongly right- handed and about 90 per cent is predominantly right-handed. Among the rest, a great deal of variability exists. Some are strongly left-handed, and others, called ambidextrous, are left-handed for some activities and right-handed for others.
Handedness is defined as a preference for the use of either the right hand or the left hand. Although most animals have a preferred paw or hand, only people have a species-typical preference for the right hand. Researchers suggest that differences in left and right-handers in patterns of brain organization may be associated with differences in skills, aptitudes, and perhaps even personalities. In the large majority of right-handers (98 or 99 per cent), speech is controlled by the left side of the brain.
The right hemisphere of the brain is usually specialized for recognizing and remembering faces and understanding spatial relationships. In left-handers, the brain organization is unpredictable. In about 70 per cent of the left-handers speech is controlled by the left hemisphere of the brain, as is the case for right-handers, but in the remaining left-handers speech is controlled by the right hemisphere. In some left-handers, both the hemispheres are capable of controlling, speech.
The hand an individual comes to prefer is determined, in part, genetically, but this does not mean, for example, that two right-handed parents cannot have a left-handed child, or the reverse.
The precise mechanisms by which genes affect handedness are still unknown. A physical injury may also be involved.
During the birth process, the region of the brain controlling the hand is sometimes damaged, so that a child who would have been right-handed without such damage becomes left-handed.
Social pressures have had a considerable effect on handedness. Using the left hand to pass on or receive something from others is strongly discouraged even now in many countries including India.
Only in recent years has society become tolerant of differences among people to accept left-handedness as a benign trait.
Many famous persons, including Leonardo da Vinci, Benjamin Franklin, and Lewis Carroll, were left-handed.
