Paper short abstract:

Explanations for the rise of stratified societies propose various benefits, from management of irrigation and agricultural surplus through superior warfare. Our demographic simulations show that inequality spreads by creating instability that drives migration and conflict, and may select for selfish traits.

Paper long abstract:

Why did socioeconomic stratification—institutionalized inequality—spread and take over during the past 5,000+ years? The origins of inequality have been debated from the time of Plato, through Rousseau, Marx and Engels, and the Social Darwinists. Twentieth-century explanations for the spread of hierarchical societies posited a variety of benefits and efficiencies, from management of irrigation and agricultural surplus through superior outcomes in warfare. However, our demographic simulations show that inequality in access to resources is inherently destabilizing, driving spread of stratified populations via migration. In other words, inequality did not spread from group to group because it is a beneficial cultural trait that imparts efficiencies and motivates innovation, but instead because it creates demographic instability that drives migration and conflict, leading to the cultural—or physical—extinction of egalitarian societies. Our current research investigates the very real possibility that natural selection itself operates differently under regimes of equality versus inequality. Egalitarian societies may foster group-level selection for low fertility, cooperation, and altruism, while inequality might exacerbate individual-level selection, thus leading to high fertility, competition, aggression, social climbing, and other selfish traits.