One for all and all for one? Altruistic behavior, population size and network topology

Working paper available upon request.

The question of why acts of selflessness occur in a Hobbesian self-help world has fascinated scholars for decades, if not centuries. Utilizing simulations, previous research has shown that altruism can be evolutionarily stable in small-scale societies under a narrow set of circumstances. However, when expanding such models to populations of anything larger than a few hundred people, they generally break down. In this paper, I modify the widely used image-score mechanism to include contagion-based reputation and demonstrate how altruism can survive in populations of up to 10,000. I also find that selflessness strongly depends on network topology - with important implications for issues such as social capital and the structure of political organizations.