Pairwise competition and the replicator equation

Spite in Hamilton's sense is defined as the willingness to harm oneself in order to harm another more. The standard replicator dynamic predicts that evolutionarily stable strategies are payoff-maximizing equilibria of the underlying game, and hence rules out the evolution of spiteful behavior. We propose a modified replicator dynamic, where selection is based on local outcomes, rather than on the population 'state', as in standard models. We show that under this new model spite can evolve readily. The new dynamic suggests conditions under which spite in animals might be found.