Right and Left-handedness in Humans Why do humans, virtually alone among all animal species, display a distinct left-or right-handedness? Not even our closest relatives among the apes possess such decided lateral asymmetry (不对称), as psychologists call it. Yet about 90 percent of every human population that has ever lived appears to have been right-handed. Professor Bryan Turner at Deakin University has studied the research literature on left-handedness and found that handedness goes with sidedness. So nine out of ten people are right-handed and eight are right-footed. He noted that this distinctive asymmetry in the human population is itself systematic. Humans think in categories: black and white, up and down, left and fight. It's a system of signs that enables us to categorize phenomena that are essentially ambiguous (含糊不清的). Research has shown that there is genetic or inherited element to handedness. But while left- handedness tends to run in families, neither left nor fight handers will automatically produce off- spring with the same handedness in fact about 6 percent of children with two fight-handed parents will be left-handed. However, among two left-handed parents, perhaps 40 percent of the children will also be left-handed. With one fight and one left-handed parent, 15 to 20 percent of the offspring will be left-handed. Even among identical twins who have exactly the same genes, one in six pairs will differ in their handedness. What then makes people left-handed if it is not simply genetic? Other factors must be at work and researchers have turned to the brain for clues. In .the 1860s the French surgeon and anthropologist (人类学家), Dr. Paul Broca, made the remarkable finding that patients who had lost their powers of speech as a result of a stroke (a blood clot in the brain) had paralysis (瘫痪) of the right half of their body. He noted that since the left hemisphere of the brain controls the right half of the body, and vice versa, the brain damage must have been in the brain's left hemisphere, Psychologists now believe that among right handed people, probably 95 percent have their language centre in the left hemisphere, while 5 percent have right-sided language. Left-handers, however, do not show the reverse pattern but instead a majority also have left hemisphere language. Dr. Brinkman, a brain researcher at the Australian National University in Canberra, has suggested that evolution of speech went with right-handed preference. According to Brinkman, as the brain evolved, one side became specialized for free control of movement (necessary for producing speech) and along with this evolution came right-hand preference. According to Brinkman, most left-handers have left hemisphere dominance but also some capacity in the right hemisphere. She has observed that if a left-handed person is brain-damaged in the left hemisphere, the recovery of speech is quite often better and this is explained by the fact that left-handers have a more bilateral (双边的) speech function. In her studies of macaque (猕猴) monkeys, Brinkman has noticed that primates (monkeys) seem to learn a hand preference from their mother in the first year of life but this could be one hand or the other. In humans, however, the specialization in function of the two hemispheres results in anatomical (人体的) differences areas that are involved with the production of speech are usually larger on the left side than on the right. Since monkeys have not acquired the art of speech, one would not expect to see such a variation but Brinkman claims to have discovered a trend in monkeys towards the asymmetry that is evident in the human brain, Two American researchers, Geschwind and Galaburda, studied the brains of human embryos and discovered that the left-right asymmetry exists before birth. But as the brain develops, a number of things can affect it. Every brain is initially female in its organiz