Trivers' (1972) theory of differential parental investment finally explained why males court females in most species. In biology, the differentiation of sexes is based on the relative size of their biological contribution to offspring. Those who contribute the greater portion are considered female, those who contribute the lesser portion are considered male. In humans, the mother provides the cytoplasm, pregnancy and breast milk.
The increase in brain size experienced by our ancestors also resulted in a more difficult childbirth due to the enlargement of the skull. In order to compensate for this difficulty, childbirth occurred earlier in development, and human infants were born relatively immature as compared to other animals. While other animals became more and more independent within a matter of months, humans remained dependent on their parents at least until their teenage years (Fisher, 1992). Since women breast-fed infants, they were the likely candidate to care for small children, and became increasingly more burdened by child care as the span of development elongated. In humans, females do not only make the greater pre-natal contribution of resources, they make the greater post-natal contribution of resources as well.
The basic female strategy is one of intensive care for each member of a relatively small number of young. Since females have a much lower reproductive ceiling than males, the female strategy is quality over quantity. Males vary more in their reproductive success than females. By expending a relatively large part of their reproductive effort on mating, males of most species devote rather little in comparison to parental care (Daly & Wilson, 1978).
The Trivers-Willard hypothesis (1973) about the allocation of parental investment states that if variance in reproductive success of males exceeded that of females, if relative health and dominance of mothers is passed on to their progeny, and if healthy or dominant males attract more females than males lacking these attributes, then females will be selected to allocate resources in progeny as a function of the progeny's health or dominance. This is easily seen in red tailed deer; sons born to mothers of above median rank were more reproductively successful than their sisters, whereas daughters born to subordinate mothers were more successful than their brothers.
Each sex probably evolved a multitude of flexible strategies based on their current circumstances. Primates, especially hominids, are extremely k-selected taxa, meaning that we have slower development, larger bodies, fewer offspring, higher survival rate and longer lifespans than r-selected taxa such as insects, fish or rodents. This increases the relative importance of sexual selection to natural selection in the development and modification of characteristics (Miller, 1998).