Ethnocentrism in Unstructured Populations: Revisiting the Relationship between Stability and Scalability

Ingroup favoritism on the basis of cultural markers could provide another mechanism for cooperation in human societies. Since existing models of ethnocentrism mostly rely on spatial reciprocity, it still remains to be seen how to explain relatively stable and large-scale cooperation frequently observed among non-kin strangers in increasingly differentiated and inviscid societies. We mathematically analyze a model consisting of independently encoded tags and strategies under weak selection to identify the conditions under which cooperation is more dominant over defection on average in finite well-mixed populations. Consistent with previous findings, the evolution of cooperation driven by ethnocentrism alone requires a large benefit-to-cost ratio in a Prisoner's dilemma game for the case of a small rate of mutation unless cultural diversity is sufficiently high. Although even ethnocentrism is inherently susceptible to the invasion by indistinguishable defectors, cooperation could be dynamically stable and robust with relatively smaller benefits through a more fine-grained distinction due to increased heterogeneity, but at the sacrifice of society-wide integration. This appears to imply that ethnocentrism in highly mobile contemporary societies breeds global cooperation to the extent which multiculturalism is suppressed.