Social norms are the unplanned, unexpected result of individuals’ interactions, a kind of grammar of social interaction which codifies what is and is not acceptable in a society or group. As with grammar, social norms are not the product of human design, and so the study of the conditions in which social norms arise is important to our understanding of their distinction from other types of injunction, for example hypothetical imperatives, moral codes or legal rules.
Though norms develop in smaller groups, they often spread beyond the narrow boundaries of the original group: how does this happen? This is a question to which an evolutionary model of social norms might attempt an answer, in terms of the dynamics of propagation from small groups to whole populations.
Norms can die out, but it is unclear how this happens, since corruption of social norms is not by itself powerful enough to generate an overhaul of the system. Thus, efficiency cannot be said to be a necessary and sufficient condition of social norms.
Maybe it’s because social norms, and normativity more generally, are groundless grounds?