Explaining Sex Differences in Reproductive Behavior

Why do these particular sex differences in social behavior occur? Why are females more nurturant, and males more aggressive and competitive? Why are males more interested in multiple sex partners and more prone to sexual jealousy? Why are men the sexual initiators and women the main choosers cross-culturally (Stephens, 1963)? Why do adolescent girls avoid being nude around males even in a sexually permissive society (Spiro, 1979)?

Explanations are to be found by recognizing that this pattern of sex differences is not confined to our culture or even our species (Daly & Wilson, 1983; Schlegel & Barry, 1991), but occurs in almost all sexually reproducing species (Bjorklund & Shackelford, 1999; Trivers, 1972). The female of a species, by definition, produces the larger and less mobile gamete. After fertilization, this gamete usually develops near or within her body, so the female is typically better situated to care for the offspring. In mammals, the female is always present when the young are born, but the male may not be. He may enhance his reproductive fitness more efficiently by seeking other females than by caring for offspring that may not even be his own. Therefore natural selection has favored mammalian females that are successful in bearing and raising offspring, and males that are efficient at attracting mates and repelling rivals. Providing most of the parental care, the female would benefit from choosing a mate carefully so as not to waste her parental effort on sickly offspring. On the other hand, the male would waste little effort on an unfertile mating, and would therefore benefit from being promiscuous.

Now, in those few mammalian species in which the young are so helpless that they need the efforts of both parents to survive, males tend to pursue a mixed or variable strategy of caring for their putative offspring but also seeking additional sex partners. Thus, even in mammals with paternal care, the male always provides less care than the female. Males can pass on their genes with a minimum of effort, if the female is able to bear and nurture their common offspring. Therefore male mammals are more inclined to seek extra-pair copulations, to seek sexual variety, whereas females have less to gain repro-ductively by pursuing multiple sexual liaisons. They can only have one litter at a time.

However, males in paternal species are wary of being cuckolded, of caring for a rival's offspring, and so they usually resort to mate guarding. Men's sexual jealousy is readily aroused by the prospect of their mate having sex with a rival male (Buss, 1994). Women may gain a fitness advantage by being fertilized by a man with better genes than their husband's, and so female marital infidelity occurs with some frequency (Baker & Bellis, 1995). Women's sexual jealousy, on the other hand, is most strongly aroused by the image of her mate deserting her for another woman and withdrawing his paternal support. It is important not to exaggerate these sex differences. Men are parentally inclined, and women are competitive. Sexual competition is intense for adolescent girls, since most of them are vying for the same few boys and under time pressure to marry. As in males, female assault and homicide rates peak in the reproductive years (Daly & Wilson, 1988), although females compete less violently than males, such as by insulting a rival. Even though women will invest more care in the children than will men, both sexes invest mightily and hence exercise care in choosing a mate. Both sexes sustain great costs in order to reproduce.

Pregnancy And Childbirth

Pregnancy And Childbirth

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