The evolution of norms.

We develop a two-level selection model in the framework of evolutionary game theory, in which fitness selection at different levels is related to different games. We consider an archipelago of communities, such that selection operates at an individual level inside each community and at a group level whenever evolution of communities is at stake. We apply this model to the evolution of social norms, an open problem of ubiquitous importance in social science. Extensive statistical analysis of our results lead to the emergence of one common social norm, of which the evolutionary outcomes in different communities are simple by-products. This social norm induces reputation-based cooperative behavior, and reflects the evolutionary propensity to promote simple, unambiguous norms, in which forgiveness and repent are welcome, while punishment is implacable.