 Social norms, the informal rules that govern behavior in groups and societies, have been extensively studied in the social sciences. Anthropologists have described how social norms function in different cultures (Geertz 1973), sociologists have focused on their social functions and how they motivate people to act (Durkheim 1895 [1982], 1950 [1957]; Parsons 1937; Parsons &amp; Shils 1951; James Coleman 1990; Hechter &amp; Opp 2001), and economists have explored how adherence to norms influences market behavior (Akerlof 1976; Young 1998a). More recently, also legal scholars have touted social norms as efficient alternatives to legal rules, as they may internalize negative externalities and provide signaling mechanisms at little or no cost (Ellickson 1991; Posner 2000).