Chapter 8. Social Norms Social norms are customary or ideal forms of behavior to which individuals in a group try to conform. From an analytical standpoint the key feature of social norms is that they induce a positive feedback loop between individual and group behavior: the more widely that a norm is practiced by members of a group the more strongly others are motivated to practice it too. In this chapter we show how to model this type of process using evolutionary game theory. The theory suggests that norm dynamics have several distinctive features. First behavior within a group will be more uniform than if people optimized solely according to their personal preferences that is individual choices will be shifted in the direction of the average choice ( conformity warp ); second there will be greater variability between groups than within groups ( local conformity global diversity ); third norm dynamics tend to be characterized by long periods of inertia punctuated by occasional large changes ( punctuated equilibrium ). We study these and other effects in the context of three examples: contractual norms in agriculture norms of medical practice and body weight norms.