Indirect Reciprocity with Contagious Reputation in Large-Scale Small-World Networks

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 20, 000. I also find that selflessness strongly depends on network topology - as heavily clustered small-world societies that resemble tight-knit family or friendship structures promote more cooperation than random networks where connections are more superficial.