Stern-Judging: A Simple, Successful Norm Which Promotes Cooperation under Indirect Reciprocity

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Abstract
We study the evolution of cooperation under indirect reciprocity, believed to constitute the biological basis of morality. We employ an evolutionary game theoretical model of multilevel selection, and show that natural selection and mutation lead to the emergence of a robust and simple social norm, which we call stern-judging. Under stern-judging, helping a good individual or refusing help to a bad individual leads to a good reputation, whereas refusing help to a good individual or helping a bad one leads to a bad reputation. Similarly for tit-for-tat and win-stay-lose-shift, the simplest ubiquitous strategies in direct reciprocity, the lack of ambiguity of stern-judging, where implacable punishment is compensated by prompt forgiving, supports the idea that simplicity is often associated with evolutionary success. Humans, unlike other animal species, form large social groups in which cooperation among non-kin is widespread. This contrasts with the general assumption that the strong and selfish individuals are the ones who benefit most from natural selection. Among the different mechanisms invoked to explain the evolution of cooperation, indirect reciprocity is associated with cooperation supported by reputation: I help you and someone else helps me. However, how did reputation evolve and which type of moral is encapsulated in those social norms that are evolutionary successful? Suggesting a simple scenario for the evolution of social norms, Pacheco, Santos, and Chalub propose a reputation-based multilevel selection model, where individual behaviour and moral systems co-evolve, governed by competition and natural selection. Evolution leads to the emergence of a simple and robust social norm, which the authors call stern-judging, where implacable punishment goes side-by-side with prompt forgiving. The low level of complexity of this norm, which is supported by empirical observations in e-trade, conveys the idea that simplicity is often associated with evolutionary success.