How do cognitive biases relevant to foreign policy decision making aggregate in groups? Many tendencies identified in the behavioral decision-making literature—such as reactive devaluation, the intentionality bias, and risk seeking in the domain of losses—have been linked to hawkishness in foreign policy choices, potentially increasing the risk of conflict, but how these “hawkish biases” operate in the small-group contexts in which foreign policy decisions are often made is unknown. We field three large-scale group experiments to test how these biases aggregate in groups. We find that groups are just as susceptible as individuals to these canonical biases, with neither hierarchical nor horizontal group decision-making structures significantly attenuating the magnitude of bias. Moreover, diverse groups perform similarly to more homogeneous ones, exhibiting similar degrees of bias and marginally increased risk of dissension. These results suggest that at least with these types of biases, the “aggregation problem” may be less problematic for psychological theories in international relations than some critics have argued. This has important implications for understanding foreign policy decision making, the role of group processes, and the behavioral revolution in international relations.