Perhaps the most basic fact of human genetics is that females have two X chromosomes in the 23rd position but males have only one. There are, of course, exceptions to this rule, for examples, Turner's and Klinefelter's syndromes, but almost all women are endowed with two X chromosomes in each cell. The question is not whether males have an imbalance of X chromosomes, but rather to what extent certain disorders that are more common in males than females are attributable to the imbalance.
In addition to having more X chromosomes than males, females generally have a lower metabolic rate, a higher brain-to-body weight, and a lower level of testosterone. Could it be that such differences are responsible for behavioral and cognitive differences between the sexes? The possibility that female hormones have a protective effect and that male hormones promote certain disorders has been the subject of continuing research and controversy (Luria et al., 1982; Waldron, 1983). Sex hormones have also been found to be related to cognitive abilities. For example, Hier and Crowley (1982) obtained a positive correlation between spatial ability and secretions of male sex hormones during puberty. In addition, the results of several investigations suggest that testosterone, the most important male hormone, slows the development of the left hemisphere and enhances the development of the right hemisphere of the brain. Note that the right hemisphere is associated with the types of reasoning skills crucial for solving mathematical problems (Christiansen & Knussman, 1987).
Although the effects of testosterone on aggression in humans are not as clear as they are in other animals, a weak correlation between the two variables has been found. However, this finding does not necessarily mean that high testosterone levels cause aggressive behavior; the result can also be interpreted as indicating that repeated aggression increases the level of testosterone in the blood (Archer, 1991). With respect to the female hormone estrogen, Hampson (1990) and Kimura and Hampson (1993) reported that women perform better on tests of motor coordination and verbal facility but poorer on tests of spatial reasoning during times of the month when estrogen levels in the blood are highest.
Although genes, chromosomes, hormones, neural structure, and other biological factors contribute to the determination of sex differences before and after birth, environmental factors such as differential cultural reinforcement of sex-appropriate behavior, the imitation of gender-role models, and other psychosocial variables are at least as important (Wittig & Petersen, 1978). As with all human behavior, explanations of gender differences must take into account both the biology of the individual and the environment in which he or she has to function.
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