Pacheco, Jorge M.Santos, Francisco C.Chalub, Fábio Augusto da Costa Carvalho2017-10-302017-10-302006-121553-734XPURE: 357377PURE UUID: 7809b5e3-bc6b-4848-96ef-353f0694c3d8researchoutputwizard: 11767WOS: 000243482000014Scopus: 33845978053PubMed: 17196034PubMedCentral: PMC1761656http://hdl.handle.net/10362/24757JMP acknowledges support from FCT, Portugal, and the Program for Evolutionary Dynamics at Harvard University. FCS acknowledges the support of COMP2SYS, a Marie Curie Early Stage Training Site, funded by the European Community through the HRM activity. FACCC was supported by project POCI/MAT/57546/2004.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.183762engALTRUISTIC PUNISHMENTSOCIAL NORMSEVOLUTIONCOEVOLUTIONjournal article10.1371/journal.pcbi.0020178