Overview

In humans, sex differentiation in body and behavior, although in evidence throughout ontogeny (Bjorklund & Pellegrini, 2002), becomes most marked at puberty. Before puberty, the sexes are relatively similar (Willner & Martin, 1985). After puberty, hardly anything can be said about adolescents that applies equally to boys and girls.

Yet the amount of sex differentiation in adult humans is relatively small. Human sexual size dimorphism is comparatively modest, suggesting that behavioral sex differences are likewise relatively small. Indeed, most studies of behavioral sex differences reveal a great deal of overlap between the distributions of males and females. Individual differences tend to be far greater than sex differences in behavior (Schlegel & Barry, 1991).

Why is sex differentiation, to the extent that it exists, so pronounced at puberty? Puberty constitutes sexual maturation. It prepares males and females to fulfill their specialized reproductive roles, like the complementarity of sperm and egg. Primatologists define adolescence as the period from the onset of puberty to the attainment of fertility (Pereira & Altmann, 1985). Likewise, adolescence is recognized as a life stage in all cultures and is usually delimited by the observable changes of sexual maturation

(Schlegel & Barry, 1991). Before maturity, the child depends heavily on parents and others for assistance—so much so that Bogin (1999) has asserted that humans are the only primate with childhood, a stage of feeding by the mother after weaning. After maturity, adolescents themselves become, potentially, the parents of dependents (see Charlesworth, 1988; Schlegel, 1995). Given the great amount of parental care exhibited by our species, this is indeed a radical transformation. Accordingly, puberty entails dramatic changes in body and behavior.

Sex differences emerge or intensify at puberty in libido, spatial skills, arithmetic skills, verbal skills, strength, nurturance, and dominance aggression, among other behaviors (Hoyenga & Hoyenga, 1979, 1993; Kimura, 1999). Gonadal hormones contribute to all of these sex differences, as indicated by research on prenatal and adult hormonal exposure, sometimes using assays of amniotic fluid, umbilical cord blood, serum, or saliva; individuals with abnormal levels of endogenous and exogenous gonadal hormones; prepubertal, adult, pregnant, lactating, and postmenopausal individuals; and variation across menstrual, diurnal, and circannual hormonal fluctuations (Hampson, 2002). Some cognitive as well as motivational sex differences have been confirmed in studies on other species (Kimura, 1999; Mitchell, 1981; Patterson, Holts, & Saphire, 1991) and cultures (Christiansen, 1993). Hormones can affect the adolescent's behavior directly, not just by altering her body and thus changing others' reactions to her (e.g., Nottelmann et al., 1990). Furthermore, others' reactions to adolescents' bodily changes may themselves have an evolved basis.

Some sex differences emerge before puberty, but doubtless have implications for adolescence. Prenatal androgen exposure during the second trimester, when the brain is sex differentiating, is related to young girls' interest in play fighting versus doll play and in motherhood versus a career, employment in male-dominated fields, and some personality measures (Hampson, 2002; Mealey, 2000; Udry, Morris, & Kovenock, 1995). Prenatal androgen levels can also play a role in the development of sexual orientation, as can genes and adult experience (L. Ellis, 1996). Methodological objections that prenatally masculinized girls are treated differently by their parents, and that control groups and measures have been inadequate, have been addressed by subsequent research (e.g., Hines & Kaufman, 1994). However, this research does not gainsay the likelihood that socialization forces also contribute to these sex differences.

The functions of sex differences with evolved bases can often be understood by considering their possible advantages for our hominid ancestors. The tentative functional analysis proposed here will begin with the changes of puberty.

Pregnancy And Childbirth

Pregnancy And Childbirth

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