Score contribution per author:
α: calibrated so average coauthorship-adjusted count equals average raw count
We provide an evolutionary explanation for the well-established evidence of the existence of in-group favoritism in intergroup conflict. Using a model of group contest for a (local) public good, we show that the larger the number of groups competing against one another, or the larger the degree of complementarity between individual efforts, the more likely are group members altruistic towards their teammates under preference evolution.