Although theory suggests individuals are more willing to incur a personal cost to benefit ingroup members, compared to outgroup members, there is inconsistent evidence in support of this perspective. Applying meta-analytic techniques, we harness a relatively recent explosion of research on intergroup discrimination in cooperative decision making to address several fundamental unresolved issues. First, summarizing evidence across studies, we find a small to medium effect size indicating that people are more cooperative with ingroup, compared to outgroup, members (d = 0.32). Second, we forward and test predictions about the conditions that moderate ingroup favoritism from 2 influential perspectives: a social identity approach and a bounded generalized reciprocity perspective. Although we find evidence for a slight tendency for ingroup favoritism through categorization with no mutual interdependence between group members (e.g., dictator games; d = 0.19), situations that contain interdependence result in stronger ingroup favoritism (e.g., social dilemmas; d = 0.42). We also find that ingroup favoritism is stronger when there is common (vs. unilateral) knowledge of group membership, and stronger during simultaneous (vs. sequential) exchanges. Third, we find support for the hypothesis that intergroup discrimination in cooperation is the result of ingroup favoritism rather than outgroup derogation. Finally, we test for additional moderators of ingroup favoritism, such as the percentage of men in the sample, experimental versus natural groups, and the country of participants. We discuss the implications of these findings for theoretical perspectives on ingroup favoritism, address implications for the methodologies used to study this phenomenon, and suggest directions for future research.
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