Political theorists and philosophers have recently directed their attention to understanding how individuals may become motivated to act as ethical cosmopolitans. A prominent theory – termed "thick cosmopolitanism" – argues that the realization one’s ingroup is responsible for causing harm to people in distant nations will increase cosmopolitan helping behavior. Additionally, thick cosmopolitanism suggests that guilt may explain this effect. This article presents the first experimental tests of these claims, and is the first research to use experiments to investigate cosmopolitan helping. Results demonstrate a substantial, but previously unrecognized, limitation to thick cosmopolitanism. Specifically, reminders of ingroup responsibility for causing harm not only increased individuals’ acceptance of responsibility and collective guilt, which indirectly enhanced cosmopolitan helping (Studies 1 and 2), but simultaneously increased dehumanization of the harmed outgroup, which indirectly diminished helping (Study 2). These conflicting processes resulted in no overall increase in cosmopolitan helping, contrary to the predictions of thick cosmopolitanism.