Sex differences in the division of labor in mating relationships are addressed by adaptionist theory. It is thought that the sexual division of labor during hominid evolution produced the diverging characteristics evident today. 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. 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.
Adaptionist theory produces a portrait of nurturing women and competitive men.
In most species, mating efforts are most likely to be male, parenting efforts female. Mating effort has a high fixed cost, typically, a male has to establish himself as successful in order to mate. Once a male has been established as a successful resource provider, his fitness in sexual selection multiplies. This dichotomy creates a strong bias in polygynous species such as our own. Because of the higher variability in reproductive success for males, maturation is delayed to achieve larger size and competitive ability.