>Unlike men, women cannot generally enhance their reproductive success by acquiring wealth or accumulating mates, and in some cases it appears that women undermine their reproductive success by acquiring political status. As primatologist Barbara Smuts has suggested, competition among females is at a low level because "the outcome of a single interaction rarely leads to large variations in reproductive success because female reproductive performance depends mainly on the ability to sustain investment in offspring over long periods of time." In contrast to men, then, women have increased their reproductive success by devoting the bulk of their energies to investment in children, through provision of milk and other forms of direct caretaking, rather than through acquisition of resources.
There is reason to believe that the closer connection of women to their infants is an evolved response, as a general tendency in an mammalian mother to be indifferent to be indifferent to separation from her infant would have been highly disadvantageous. An intriguing line of evidence implying sex differences in the factors that activate parental feelings and behaviors are studies finding that among non-genetic parents, women's attachment may suffer more than men's. For example, stepfathers tend to have better relationships with their stepchildren than stepmothers do and are more likely to report parental feelings. Similarly, the relationship of adoptive mothers to their adoptive children seems to resemble the genetic mother-child relationship less than the relationship of adoptive fathers resembles the genetic father-child relationship. A study by psychologist Irwin Silverman and colleagues examined perception of parental solicitude among adults who had been raised either by birth parents or adoptive parents. Not surprisingly, birth children tended to perceive their mothers as the more solicitous parent. However, adoptees reported substantially less parental solicitude from their mothers than birth children did, but there was no such decline in solicitude between birth and adoptive fathers. Indeed, adoptive fathers tended to be rated higher in solicitude than adoptive mothers. These findings can plausibly be interpreted to mean that experience of carrying and giving birth to a child predisposes women toward later nurturing feelings and behavior toward their children, while for fathers other factors trigger these feelings. Psychologist Geoffrey Miller has suggested that in our ancestral environment women commonly had children by successive males and that an evolved willingness to invest somewhat in stepchildren (although less than in their own biological offspring) may has been selected for as mating tactic.
Substantial reproductive tradeoffs for female competition and aggressiveness may limit the development of dominance in females. Among baboons, for example, high-ranking females obtain some clear reproductive benefits as a consequence of enhanced access to nutritional resources: they have higher infant survival, shorter interbirth intervals, and daughters who usually give birth at a younger age. Their lifetime reproductive success, however, may not substantially exceed that of less-dominant females. Dominant females have greater miscarriage rates and may show signs of reduced fertility. Thus, the same causal factors that lead to high dominance may also carry reproductive costs that have acted as a constraint on selection for competitiveness among females. That common factor is likely to be testosterone.
The same effect may occur in humans. It is often reported that female executives have fewer children than male executives and fewer than the average woman. A study of law school graduates found that 40 percent of women remained childless fifteen years after graduation. The usual implication of these findings is that women must choose between work and family and that these women chose work. There is another possible explanation, however. Women who succeed in business tend to be relatively high in testosterone, which can result in lower female fertility, whether because of ovulatory irregularities or reduced interest in having children. Thus, rather than the high-powered career being responsible for the high rate of childlessness, it may be that high testosterone levels be responsible for both.<
Kingsley R Browne; 2002
Biology at Work- Rethinking Sexual Equality