Paul Seabright (Toulouse School of Economics)
Abstract. This paper models the co-evolution by natural selection of an individually costly social behavior trait together with a preference for mating partners who bear that trait. It is well known that costly cooperative behavior can be favored by natural selection if cooperators are likely to associate preferentially with other cooperators; but the mechanisms by which such preferential association may come about have typically been considered as exogenously determined. In this paper we use insights from the theory of sexual selection to endogenize the presence of mechanisms of preferential association, as well as generalizing the argument from cooperative behavior to other kinds of individually costly social behavior. We show that the existence of equilibria with individually costly social behavior depends on general properties of the matching functions that determine probabilities of association given preferences as a function of the relative frequencies of behavioral traits in the population. We derive conditions for the existence of multiple stable monomorphic equilibria and also for balanced polymorphic equilibria in which different behavioral traits coexist.