A broader theory of cooperation can better explain "purity".

Self-control provides one cooperative explanation for "purity." Other types of cooperation provide additional explanations. For example, individuals compete for status by displaying high-value social and sexual traits, which are moralised because they reduce the mutual costs of conflict. As this theory predicts, sexually unattractive traits are perceived as morally bad, aside from self-control. Moral psychology will advance more quickly by drawing on all theories of cooperation.