Social Norms of Cooperation in Small-Scale Societies

Indirect reciprocity, besides providing a convenient framework to address the evolution of moral systems, offers a simple and plausible explanation for the prevalence of cooperation among unrelated individuals. By helping someone, an individual may increase her/his reputation, which may change the pre-disposition of others to help her/him in the future. This, however, depends on what is reckoned as a good or a bad action, i.e., on the adopted social norm responsible for raising or damaging a reputation. In particular, it remains an open question which social norms are able to foster cooperation in small-scale societies, while enduring the wide plethora of stochastic affects inherent to finite populations. Here we address this problem by studying the stochastic dynamics of cooperation under distinct social norms, showing that the leading norms capable of promoting cooperation depend on the community size. However, only a single norm systematically leads to the highest cooperative standards in small communities. That simple norm dictates that only whoever cooperates with good individuals, and defects against bad ones, deserves a good reputation, a pattern that proves robust to errors, mutations and variations in the intensity of selection.Author Summary: The prevalence of cooperation among human societies is a puzzle that has caught the eye of researchers from multiple fields. Why is that people are selfless and often incur costs to aid others? Reputations are intimately linked with the answer to this question, and so are the social norms that dictate what is reckoned as a good or a bad action. Here we present a mathematical framework to analyze the relationship between different social norms and the sustainability of cooperation, in populations of arbitrary sizes. Indeed, it is known that cooperation, norms, reciprocity and the art of managing reputations, are features that go along with humans from their pre-historic existence in small-scale societies to the contemporary times, when technology supports the interaction with a large number of people. We show that population size is relevant when evaluating the merits of each social norm and conclude that there is a social norm especially effective in leveraging cooperation in small populations. That simple norm dictates that only whoever cooperates with good individuals, and defects against bad ones, deserves a good reputation.