The writings of the American economist Jack Hirshleifer (1925–2005) are considered some of the most influential contributions to the economics of conflict. This article examines how Hirshleifer transformed the behavioral assumptions of Homo economicus (the self-interest model and the concept of rationality) to offer economically grounded explanations for conflicts. We explain how, through his interactions with sociobiologists and game theorists, he reduced self-interest to mere selfishness and redefined rationality to encompass emotions and feelings. The final result was a disciplinary reclassification of conflict as a subvariety of selfish, rational behavior amenable to economic analysis.