Early students of morality, such as Jean Piaget (1896-1980) and Lawrence Kohlberg (1927-1987), focused on the intellectual aspect of moral development. They studied the role of reason in moral judgments. Carol Gilligan protested this narrow focus and emphasized the importance of compassion and caring. More recent psychologists, such as Steven Pinker, emphasize an evolutionary approach to human morality. In a 2008 article, Pinker noted that some of our most strongly held moral positions may not have any basis in reasoning or compassion. For example, many people react in horror when hearing scenarios in which an adult brother and sister engage in mutually consensual incest, a dog owner eats the family dog after it has died from natural causes, or a homeowner cuts up the American flag to use as a dust rag. (These scenarios were first described by the psychologist Jonathan Haidt.)

From research such as this, psychologists have concluded that evolution has inscribed in us the tendency to react with emotional disgust or horror at certain classes of situations. Such situations have evolutionary significance and involve behaviors that, over time, are destructive to our species. For example, the incest taboo is observed in many animal species and serves to protect variability in the gene pool. An aversion to eating members of our own family (and our pets become part of our family) has obvious benefits for kinship survival. In effect, we have evolved certain moral instincts.