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. 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.
Each sex probably evolved a multitude of flexible strategies based on their current circumstances.