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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.