Individual and Cultural Differences

Adolescents within each sex also differ. Although many intrasexual differences are due to experiential factors, some are due to differences in hormone levels. Individual differences in testosterone level are associated with the strength of libido in adolescents of both sexes (Udry, 1988). Moreover, experience can sometimes affect hormonal levels that alter behavior. Youths who live in violent neighborhoods tend to have higher testosterone levels than those living in peaceful ones, controlling for various factors (Mazur & Booth, 1998). In mammals generally, testosterone rises in competitive situations to mobilize the individual for aggression. Thus living in a dangerous environment can potentiate aggressiveness and competitiveness through a rise in testosterone. Similarly, when men marry, their testosterone levels tend to fall as they withdraw from mating competition, and to rise again if they divorce.

Evolutionary analysis helps to explain individual differences in maturation rate. Reproductive maturation in mammals is accelerated by the presence of potential mates. It is adaptive to mature more rapidly when potential mates are available. In the 19th-century Oneida Community in New York State, prepubertal girls practiced frequent sex and reached puberty about 2 years earlier than girls in the surrounding area (Jones, 1991). In addition, although most stressors slow reproductive maturation, mild stress sometimes speeds maturation in mammals, probably so that the organism is assured of reaching maturity and reproducing under adverse conditions (Worthman, 1993). A cross-cultural comparison indicated that painful treatment during infancy can accelerate menarche (J. W. M. Whiting, 1965). Girls also tend to reach menarche early if they suffer from paternal absence or neglect, or from other family stresses (Coall & Chisholm, 1999; B. J. Ellis, McFadyen-Ketchum, Dodge, Pettit, & Bates, 1999; Rowe, 1999; Surbey, 1990). The presence of a potential mate (a stepfather or the mother's boyfriend) can accelerate menarche independently from the effect of father absence; in a recent study, total number of such males was the best predictor of the girl's tendency to engage in early sexual behavior (B. J. Ellis & Garber, 2000).

Thus, evolutionary hypotheses are being proposed for some individual and population differences, not just for universals. It makes adaptive sense for organisms to vary their development to meet different environmental contingencies. Such evolved pluripotentiality has been demonstrated even in insects, so it would be surprising if it were absent from our genetically more complex species. This variability is likely to be adaptive, to enhance the individual's reproductive interests. For example, father absence is common in societies with polygyny and frequent warfare (Chisholm, 1999), and girls tend to marry early in polygynous societies; early maturity may be advantageous under these conditions.

As these examples show, current evolutionary analyses do not discount the role of environment in human diversity. In fact, functional analysis can sometimes explain a pattern of cross-cultural diversity. To take another illustration: adult status is conferred on young men at different average ages in different societies, but a pattern exists. Adult status tends to be conferred on a youth when he marries (van den Berghe, 1980). In societies in which men require many years to accumulate sufficient wealth or economic skills to afford marriage, adult status usually comes relatively late (G. E. Weisfeld, 1999). Thus, cultural differences can sometimes be explained in functional terms, rather than as historical accidents or consequences of conceptual, linguistic, or other cultural features. Cultural practices, like genes, are subject to selection pressure even if historical and other factors also shape them. Practices that enhance reproductive success under extant ecological conditions will be passed on across generations.

Pregnancy And Childbirth

Pregnancy And Childbirth

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