The prominent psychological accounts of how people become moral derived from social learning, cognitive-developmental, and dual-process theories explain important aspects of the process but leave other questions unanswered. Why do children copy the moral ideas and behaviors of models, and why are children selective about the models they imitate? How did moral norms originate? Why do people sometimes derive moral decisions from sophisticated forms of moral reasoning, but other times derive them in other ways? Why do people sometimes deliberate about moral issues, but other times derive moral decisions automatically and intuitively? Evolutionary accounts supply a basis for answering these questions and refining the explanations of morality advanced by psychologists. Viewed from the perspective of evolution, the key to explaining how humans became a moral animal lies in identifying the functions that moral traits evolved to serve in archaic environments. The main answer offered by evolutionary psychologists is that moral traits evolved to help early humans living in relatively small groups resolve conflicts of interest that constrained their ability to reap the biological benefits of cooperation. Attending to the biological functions of morality may supply a basis for resolving philosophical disputes about ultimate principles of morality.