Social institutions are the laws, informal rules, and conventions that give durable structure to social interactions within a population. Such institutions are typically not designed consciously, are heritable at the population level, are frequently but not always group benefi cial, and are often symbolically marked. Conceptualizing social institutions as one of multiple possible stable cultural equilibrium allows a straightforward explanation of their properties. The evolution of institutions is partly driven by both the deliberate and intuitive decisions of individuals and collectivities. The innate components of human psychology coevolved in response to a culturally evolved, institutional environment and refl ect a prosocial tendency of choices we make about institutional forms.