Abstract-There is wide variety in dominance hierarchies in both animal and human societies, with major implications for health and well-being. Understanding which conditions favour egalitarianism—a social setting characterized by low levels of aggression, muted hierarchies, and relatively equal distributions of resources and fitness outcomes—is thus of great theoretical and societal interest. Previous work has highlighted the role of low economic defensibility of resources, high costs of competition, levelling coalitions, and high gains to cooperation. However, there is a lack of formal theoretical models that combine these conditions and generalize well across species and contexts. Here we provide a simple evolutionary model that incorporates economic defensibility, costs of competition and gains to cooperation, and shows for the first time that gains to cooperation alone can drive the evolution of egalitarianism. The model combines the well-known Hawk-Dove and Prisoner’s Dilemma games, which model dominance and cooperation, respectively. We show that when the gains to repeated cooperation are high relative to the benefits of hawkish social dominance, a ‘Leveller’ strategy—which punishes Hawks with non-cooperation—can evolve and drive Hawks out of the population. We find empirical support for the model among human foragers, in that groups with a greater reliance on hunting, which requires cooperation, are more likely to be egalitarian. We suspect that the model can also explain observed egalitarian outcomes in a number of other species relying on within-group cooperation. Unlike previous theoretical models of egalitarianism our model does not depend on coalitions or sophisticated cognitive abilities, and highlights a small number of ecological parameters to explain variation in dominance hierarchies and inequality across groups.

Bio- Paul Hooper is an evolutionary anthropologist with broad training in quantitative social sciences and complex dynamical systems. His theoretical work has developed the evolutionary theory of family formation among mammals and humans, as well as social dynamics of cooperation, leadership, territoriality, and hierarchy formation. This mathematical and computational modeling is complemented by analyses of high-resolution field data collected in lowland Bolivia, Inner Asia and elsewhere. He has served as Assistant Professor of Biological Anthropology at Emory University and Director of Education at the Santa Fe Institute. He is now based in Denver as the head of Systems Science, a company specializing in systems modeling and prediction.