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Description: Identify fusion theory has become a popular psychological explanation of costly self-sacrifice, with recent work positing that an interaction between negative outgroup relations and fusion with one’s ingroup which would lead to sacrificial behavior that benefits the ingroup. We test this hypothesis using a behavioral economic experiment designed to detect biased, self-interested favoritism among eight different populations ranging from foragers and horticulturalists to the fully market-integrated. We find that while individuals favor themselves on average, those with higher ingroup fusion sacrifice more money to others. However, the posited negative interaction between ingroup and outgroup relations shows no consistent effects at the individual or population levels because outgroup fusion also predicts sacrificing an opportunity to take more money. We conclude by suggesting that the fusion scale measures generalized sociability and/or is not necessarily able to precisely capture outgroup hostility.