Evolutionary developmental psychology proposes that human infants have evolved psychological structures that are optimized for age-appropriate learning in a co-evolved milieu and that family relationships (e.g., parent-child, sibling, kin) are also products of evolution. From the perspective of parental genes, not all offspring have an equal chance of further propagation, necessitating differential investment of resources. From the perspective of genes of siblings, competition for such resources for the individual’s survival and reproduction is of paramount importance. Parental investment theory explains the evolution of sex differences in behavior. The amount of investment of resources (both psychological and biological) of mammals for seeking a mate versus parenting differs according to sex, which is seen to underlie such behavioral differences as what is desirable in a mate (for males, the female’s genetic fitness; for females, the male’s ability to provide resources). Women are more emotionally expressive than men but can also inhibit or control emotional expression better than men. This is attributed to the fact that females who could control or hide their sexual arousal to males other than their spouses and/or who could inhibit their aggressive emotions toward their infants may have had an evolutionary advantage. Parental investment theory also explains the greater likelihood (two to 10 times) of parental abuse of children with congenital defects. There is competition for resources between children and parents as well; therefore, the rate of infanticide is higher among younger than older women because the chances for later reproduction are greater at a younger age. Stepparents naturally invest fewer resources in stepchildren than in biological offspring and tend to abuse their stepchildren more often both in fact and in fairy tales (e.g., Cinderella). Parental investment theory also explains risk-taking and risk-aversive behavior in males—males who are more involved in child care would be risk aversive, and females who have evolved to be caregivers are, in general, risk aversive. The grandmother hypothesis posits that there is survival advantage for older females to participate in nurturing the grandchildren, which will result in their daughters being able to reproduce more, which also selects for longevity among women.