Abstract
Parochialism occurs when an individual mentally codes the population into in-group and out-group members and cooperates only with in-group members. Conditional cooperation of this kind is distinct from strategies such as tit-for-tat where the decision to cooperate is conditioned on others’ previous actions. Parochialists practice in-group favoritism by cooperating with others, conditional on their spatial proximity or cultural similarity. We consider an evolutionary model of local interaction with three types of strategies: altruists “always cooperate”; egoists “always defect”; and parochialists cooperate only with neighbors within a certain radius on a spatial network. In the model, we provide a new explanation for why parochialism is durable and can stably remain prevalent in human populations. The main driving force is the homophily effect. Interestingly, the homophily effect leads to the prevalence of parochialists but not altruists who benefit more from homophilic association, because altruists are invaded by parochialists. The two groups can coexist only if egoists buffer their direct interaction. Accordingly, the proportion of egoists can be greater than that of altruists in our model, contrary to the result of Eshel et al. (
1998
). Simulations show, for most parameter values, that the socially optimal cooperation radius (achieving the greatest mean fitness across the entire population) is two and that narrow in-group parochialism is prevalent regardless of frequency of interaction within a society.