Our paper models the co-evolution by natural selection of an individually costly cooperative behavior trait together with a preference for social partners who bear that trait. Under a sexual reproduction population structure - the non-random association of individuals within a population - co-evolves with behavioral traits that are not individually adaptive but whose bearers face enhanced reproductive prospects due to the structure. It is well known that cooperation can be favored by natural selection if cooperators are likely to associate with other cooperators. In this paper we use insights from sexual selection to endogenize the mechanisms of association, characterized as general properties of matching functions. We derive conditions for existence of multiple stable monomorphic equilibria and also for balanced polymorphic equilibria in which different behavioral traits coexist. Such equilibria an support not only mutual cooperation, but also hierarchies in which cooperators are exploited by others.
This repository contains a collection of Jupyter notebooks that demonstrate the key results from our paper. Play around with our model on Binder!