The dynamics of cooperation, power, and inequality in a group-structured society

Most human societies are characterized by the presence of groups which cooperate through joint actions but also compete for resources and power. The processes of within- and between- group cooperation and competition have shaped human history over the last several millennia. To deepen our understanding of the underlying social dynamics, we model a society subdivided into groups with constant sizes and dynamically changing powers. Both individuals within groups and groups themselves participate in collective actions. The groups are also engaged in political contests over power which determines how resources are distributed. Using analytical approximations and agent-based simulations, we show that the model exhibits rich behavior characterized by multiple stable equilibria and, under some conditions, non-equilibrium dynamics. The strength of democratic institutions plays a key role: increasing it promotes cooperation, reduces variation in power, and mitigates inequality among groups. We show that increasing potential benefits of between-group cooperation promotes it only in societies with strong democratic institutions. We show that small groups are successful in competition if the jointly-produced goods are rivalrous and the potential benefit of cooperation is small. Otherwise large groups dominate. Overall our model contributes towards a better understanding of the causes of variation between societies in terms of the economic and political inequality within them.