Effects of reproductive skew on the evolution of ethnocentrism in structured populations with variable size

Abstract

Large-scale cooperation is arguably one of the most astonishing achievements of humankind. In spite of its global abundance, members of many species, including humans, tend to direct their generosity exclusively towards phenotypically similar group-mates, thereby elevating the levels of intergroup tension and ingroup biased ethnocentrism. Recent studies have evidenced, however, that stable dominance of ethnocentric cooperation is endangered in the presence of memory, mobility, conformity, or link undirectedness in structured populations. In the present paper, we investigated the heterogeneity of reproductive time scales as a possible mechanism for the reduction of ethnocentric behavior and the promotion of global unconditional cooperation in phenotypically diverse populations with demographic fluctuations. To this effect, the regular reproduction stage that typically follows the interaction of agents in the evolutionary process of the ethnocentrism model is discarded. Instead, agents reproduce in accordance with their assigned reproduction times that obey a power-law distribution. Unlike the standard model of ethnocentric cooperation with proportional selection, we found that reproductive skew can elevate cooperation also by promoting non-ethnocentric strategies such as global altruism and conditional extra-group cooperation, while simultaneously reducing ethnocentric behavior. However, we found that this ethnocentrism-controlling potency of reproductive heterogeneity depends on the underlying population structure, with more pronounced effects on Barabási–Albert scale-free networks than on regular square lattices. With an increasing power-law exponent of the reproduction-time distribution, we identified a remarkable coexistence of strategies that was not reported in previous models of tag-based cooperation. Our further simulations revealed that this strategy coexistence varied with the fluctuating population size which in turn depended on the correlation between the node connectivity and the reproduction rate of individual agents. Our findings highlight the importance of reproductive skew and the relationship between node degree and reproduction rate for the stability of intra-group generosity in structured populations, potentially explaining the widespread coexistence of strategies in phenotypically diverse societies.