Polymorphic Equilibrium in Advertising

This article is concerned with the possibility that natural selection can lead to an evolutionarily stable equilibrium where otherwise identical profit maximizing firms follow distinctly different strategies. In biology such occurrences are called polymorphic equilibrium. We develop a model of nonprice competition and from this model two classes of polymorphic equilibria arise. In the first class, advertising by expanding market demand can create a niche large enough to sustain entry by nonadvertising firms. Thus, otherwise identical firms following advertising and no advertising strategies can coexist with equal profits in a polymorphic equilibrium The second class of polymorphic equilibria includes the case where advertising does not expand market demand and instead only affects market shares. The article concludes with a discussion of the implications that polymorphism has for empirical work in economics.