It will be a universal feature of sexual populations that individuals prefer mates with typical rather than rare characteristics--essentially because most mutations reduce fitness. This is termed koinophilia. Koinophilia will also apply to behaviour. In particular, individuals will prefer mates that behave in social interactions that follow whatever rules are common in that population. Suppose that individuals interact in situations which can be represented by the Iterated Prisoner's Dilemma Game (IPD). If koinophilia is ignored, previous authors have shown that it is hard to find an evolutionarily stable strategy, and that strategies cycle indefinitely. However, if koinophilia is included, it has the effect of increasing the fitness of whatever happens to be the common strategy. This, in turn, has the effect of stabilizing almost any strategy (that has, for whatever reason, become the local norm) in the IPD. Different, partially isolated groups will thus become evolutionarily trapped in different behaviours, which are defended against alternative strategies originating through mutation or immigration. Groups that happen, by chance, to reach a cooperative strategy will be fitter, as groups, than those that reach defection (even though, in one-to-one encounters, it is the selfish individual who always wins). The ultimate result will be the replacement of selfish groups by cooperative groups.