Tracing the development of altruism and group social control over 6 million years, Boehm argues that our moral sense is a sophisticated defense mechanism that enables individuals to survive and thrive in groups. One of the biggest risks of group living is the possibility of being punished for our misdeeds by those around us. Bullies, thieves, free-riders, and especially psychopaths--those who make it difficult for others to go about their lives--are the most likely to suffer this fate. Getting by requires getting along, and this social type of selection, Boehm shows, singles out altruists for survival. This selection pressure has been unique in shaping human nature, and it bred the first stirrings of conscience in the human species. Ultimately, it led to the fully developed sense of virtue and shame that we know today.