Almost forty years ago, a small group of scholars drew on cognitive psychology to explain anomalous patterns of behavior by leaders on issues of international security. Although it made significant contributions to theory and research, that scholarship did not diffuse broadly into the field. Drawing on concepts in psychology and behavioral economics, research that uses new methods is now producing a wave of scholarship in international relations exemplified by the work in this special issue. Analysis of the use of prospect theory over the last three decades identifies the scope conditions that enable the predictions of rational choice and psychological theories. These scope conditions motivate the focus on the heterogeneity of decision makers that is at the core of current contributions. Future research will move beyond the now-sterile debate between rational choice and psychology.