Chapter 2 develops the theory of moral psychology that is used in later chapters to explain the evolution of the laws of war. It begins by briefly explaining why it is necessary to develop a theory of moral pscyhology to account for the evolution of international norms. It argues that existing theories have difficulty explaining why some norms repeatedly emerge in human societies. It then outlines a theory of moral psychology that holds that some norms are based on evolved cognitive heuristics and emotions. It argues that empathy and perspective-taking encourage prosocial behavior and that when these emotional capacities are engaged, they lead people to create norms that protect civilians from deliberate killing. However, it also shows that moral thinking is governed by subtle asymmetries in the way that people evaluate intentional action: intended harms are judged as morally worse than unintended harms. The rest of the book shows that this "intention/side-effect distinction" underlies the key principles of the civilian protection regime. Finally, the chapter develops a methodology for examining how moral intuitions shape international norms.