describes a new social theory of norms to provide a compelling explanation why people punish. Identifying mechanisms that link interdependence with norm enforcement, it reveals how social relationships lead individuals to enforce norms, even when doing so makes little sense. This groundbreaking book tells the whole story, from ideas, to experiments, to real-world applications. In addition to addressing longstanding theoretical puzzles—such as why harmful behavior is not always punished, why individuals enforce norms in ways that actually hurt the group, why people enforce norms that benefit others rather than themselves, why groups punish behavior that has only trivial effects, and why atypical behaviors are sometimes punished and sometimes not—it explores the implications of the theory for substantive issues, including norms regulating sex, crime, and international human rights.