Humans often favor members of their own group over members of outgroups, a preference that drives prejudices, discrimination and intergroup conflicts. Across cultures and using various behavioral and attitude measures, such ingroup bias has been documented. However, whether the level of ingroup bias is comparable across different natural groups and whether it interacts with the type of outgroup one faces are largely unknown. Here, we present field evidence of ingroup bias in antisocial group decision-making from an economic experiment with 192 male members of three natural groups in Ethiopia. Natural variance in intergroup relations (enmity versus neutral) and the strength of group identity (strong versus weak) allows us to test for group-level drivers of antisocial behavior. In our sample, lack of outgroup concerns is universal: Almost all subjects chose the antisocial option when targeting an outgroup member, irrespective of friendly or hostile intergroup relations. Members of the two groups with strong ethnic identities exhibited less antisocial behavior towards their own group members. There is substantial individual heterogeneity in antisocial choices; about half of our subjects never chose the antisocial option when the target was an ingroup member, and about one fourth of subjects were antisocial regardless of the group composition. A simple model of ingroup bias organizes the choices of more than 90% of subjects. Our results imply that previous work using Western samples grossly underestimates the extent of antisocial behavior targeting outgroup members.