In-group favoritism (e.g. in the context of nationalism or ethnocentrism) has been receiving considerable attention. To explain it, many researches have proposed theoretical models using a tag which was assumed as a variable one. Following the perspective that an ethnicity is invariable, we assume a fixed tag. We also assume that the reputations of others are created and shared within a group. On the basis of these assumptions, we introduce strategies for selecting friends using tags and within-group reputations. We conducted evolutionary simulations and found the emergence of in-group favoritism. The tag in our model is not minimal but highly neutral because it is defined not to cause a direct payoff. In the beginning of a simulation, the identification of a tag is independent of any payoff, but the tag ultimately becomes closely correlated to cooperation and a payoff.
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