Left- and right-handedness refer to the tendency of most people to use the same hand for certain tasks requiring skill. These tasks include writing, throwing a ball, and using a knife or hammer. Most people use the right hand for such tasks, but others use the left hand. A person who almost always uses the right hand for such activities is referred to as right-handed. A person who almost always uses the left hand for these tasks is called left-handed. About 87 percent of all people are right-handed. About 12 percent are considered left-handed. Males are more likely to be left-handed than are females. Less than 1 percent of all people can perform tasks equally well with either hand. These people are said to be ambidextrous.
Researchers studying handedness classify an individual even further, according to how strongly the person prefers a certain hand. People with a strong preference for one hand—that is, those who use the same hand, left or right, for almost all tasks—are distinguished from mixed-handers, who use their nonpreferred hand for some tasks. About 50 percent of people are strongly right-handed, while about 37 percent are mixed right-handed. Only about 10 percent of people are mixed left-handed, and 2 percent are strongly left-handed.
Causes.
No one knows for certain why some people are right-handed and others are left-handed. Scientists know that the left side of the brain controls movements on the right side of the body, and vice versa. However, they are not sure how this relates to hand preference.
Handedness appears to be at least partly genetic. That is, people seem to be predisposed by their genes to prefer one hand or the other. Genes are parts of a cell that determine which features living things inherit from their parents. Nevertheless, having a left-handed parent has only a small influence on which hand a person favors.
According to one theory, there is a particular variety of gene that causes one to be right-handed. A person inherits two copies of every gene, one from each parent. If a person inherits two copies of this particular gene, they will be strongly right-handed. If a person gets only one copy, they will be weakly right-handed. If a person inherits no copies of this particular gene variety, their handedness will be mixed. In addition, researchers believe that a stressful birth or difficult pregnancy may disturb the natural development of right-handedness. Research shows that there are more left-handers among people who had a difficult birth.
An important difference between the brains of left- and right-handers can be seen in the corpus callosum, a structure that forms the main connection between the left and right sides of the brain. The corpus callosum is larger in left-handers. To researchers, this suggests that the two sides of the brain cooperate more readily in left-handers.
Social consequences of left-handedness.
Many cultures have traditionally discouraged left-handedness. In English, the word left comes from the Anglo-Saxon word lyft, which means weak or broken. The word sinister comes from the Latin word for left. Many phrases in English demonstrate a traditionally unfavorable view of left-handedness. For instance, the phrase left-handed compliment actually implies an insult. When sailors say a ship is left-handed, they mean that the ship is unlucky or defective. The practice of wearing a wedding ring on the left hand started with the ancient Romans. They believed it helped fight off the evil aspects of the left hand.
Christianity traditionally viewed the devil as being left-handed, and so Christians once claimed that left-handed children were “doing Satan’s work.” Members of many Asian and Middle Eastern societies consider using the left hand for eating, writing, or other activities as insulting or a sign of a poor upbringing.
Parents or teachers have sometimes tried to convert left-handers to right-handers. Some of these attempts involve punishment for use of the left hand. Most attempts to change a person’s handedness fail. Only about 2 out of 5 individuals who try to change do so successfully. And even for those who do change, the switch is only for activities that are singled out for change. Thus, left-handed children who learn to eat or write with their right hand may still throw a ball or brush their teeth with their left hand. In addition, research indicates that left-handed children who are forced to switch their handedness are more likely to develop learning disabilities. In the United States and many other countries, the practice of trying to convert handedness in children has largely ended.
There is no overall difference in intelligence between left- and right-handers. But left-handedness is more common among people of both extremely high and extremely low intelligence.
Because of cultural bias toward right-handedness, left-handedness can pose challenges. Many manufacturers design products for the convenience of the right-handed majority. For instance, many buttons and controls are placed where they can be most easily reached by a right-hander. Operating these same items may be awkward for a left-hander. Because living in a right-handed world can present certain difficulties and dangers to left-handers, some scientific studies have suggested that left-handers die at an earlier age than right-handers. However, more recent research shows that left-handers and right-handers live the same length of time. One of the few common tools that actually favors left-handers is the computer keyboard, on which most commonly used letters tend to be typed by the left hand.
There are also a few activities in which left-handedness provides a benefit. For example, left-handers have more success than right-handers at mentally visualizing three-dimensional objects. This could explain why many graphic artists, architects, and chess masters are left-handed. There is also evidence that mixed-handers have better memories, whereas strong-handers are better at doing two different things at the same time.
Other types of side dominance.
Handedness is just one of many dominant-side behaviors. People tend to prefer a certain foot in much the same way as a hand. A person who always uses the right foot for such tasks as kicking a ball would be classified as right-footed. About 80 percent of all people are right-footed.
Most people also prefer a certain eye when performing such tasks as looking through a telescope or through a small hole. About 70 percent of all people are right-eyed. Similarly, people prefer to use one ear when performing such activities as listening to the faint ticking of a clock or talking on the phone. About 60 percent of all people are right-eared. Finally, nostril dominance describes which side of the nose is more open to the flow of air. Sixty percent of left-handers are left nostril dominant and 60 percent of right-handers are right nostril dominant.
Side dominance in animals.
Research indicates that, like people, almost all cats, rats, and monkeys prefer to use either their right forelimb (arm or front leg) or their left forelimb. These animals differ from human beings, however, in the proportion of left and right dominance. In most animals, half of the individuals are right side dominant and half are left side dominant. However, the great apes—chimpanzees, gorillas, and orangutans—are an important exception. About 70 percent of these apes show a right hand preference.
