Many of the early applications of evolutionary theory to human behavior were rejected by progressive opinion because they seemed to biologize existing social arrangements and give them a spurious legitimacy. The relationship between early experience and later social behavior has often been seen as a process by which people's relationship to societal authority is modeled on their childhood relationship to parental authority. In modern societies, where children grow up in a nuclear family environment in which the quality of social relations might be quite different from those in the wider society, the results of early sensitivity can often look counterproductive. Many children are brought up amid great conflict and end up lacking the social skills, such as the ability to trust and cooperate, that are helpful during adult life in modern societies. This chapter provides a brief outline of the biological effects of stress and explains how it conies to exert such a powerful effect on physiological health.