Self-interested behavior need not earn higher material payoffs than norm-guided behavior in strategic environments in which a recognized adherence to social norms can help solve the commitment problem. This insight can provide the basis for an evolutionary theory of social norms. If the composition of behaviors evolves under pressure of differential payoffs, a population consisting exclusively of optimizers may be unstable, and there may exist a multiplicity of attractors representing a variety of stable norms. Societies facing similar material conditions may therefore come to adopt distinct norms as a result of chance events or differences in initial conditions.