 Now that we have examined norm emergence, we must examine what happens when a population is exposed to more than one social norm. In this instance, social norms must compete with each other for adherents. This lends itself to investigations about the competitive dynamics of norms over long time horizons. In particular, we can investigate the features of norms and of their environments, such as the populations themselves, which help facilitate one norm becoming dominant over others, or becoming prone to elimination by its competitors. An evolutionary model provides a description of the conditions under which social norms may spread. One may think of several environments to start with. A population can be represented as entirely homogeneous, in the sense that everybody is adopting the same type of behavior, or heterogeneous to various degrees. In the former case, it is important to know whether the commonly adopted behavior is stable against mutations. The relevant concept here is that of an evolutionarily stable strategy (ESS; Maynard Smith &amp; Price 1973; Taylor &amp; Jonker 1978): when a population of individuals adopts such a strategy, it cannot be successfully invaded by isolated mutants, since the mutants will be at a disadvantage with respect to reproductive success. An evolutionarily stable strategy is a refinement of the Nash equilibrium in game theory. Unlike standard Nash equilibria, evolutionarily stable strategies must either be strict equilibria, or have an advantage when playing against mutant strategies. Since strict equilibria are always superior to any unilateral deviations, and the second condition requires that the ESS have an advantage in playing against mutants, the strategy will remain resistant to any mutant invasion. This is a difficult criterion to meet, however. For example, a classic Tit-For-Tat strategy in the prisoner’s dilemma is not an ESS. Many strategies perform equally well against it, including the very simple “Always Cooperate” strategy, let alone Tit-For-Two-Tats, and any number of variations. Tit-For-Tat is merely an evolutionarily neutral strategy relative to these others. If we only consider strategies that are defection-oriented, then Tit-For-Tat is an ESS, since it will do better against itself, and no worse than defection strategies when paired with them.