 As Boyd and Richerson showed, though, in many contexts it’s not essential that everybody be willing to punish bad behavior. What is essential is that enough individuals are willing to do this. However, this generates a pair of puzzles. First, in many cases it’s costly to police norms. In order to sustain cooperation, then, groups often need some of their members to be prepared to engage in altruistic punishment. That is, some individuals need to be willing to incur individual costs associated with punishment for the benefit of others. Second, the willingness of some individuals to police norms does not just allow the evolution of cooperation and other prosocial norms. Instead it potentially allows any norm to emerge and become stable. So, although we’ve seen how game theory can help explain why a disposition to comply with and police certain norms may be important features of morality, important questions remain concerning whether this is necessarily rational from the perspective of the individual. We’ll turn to those questions now. And in section 5 we’ll return to the question of how to distinguish good from bad norms.