Addressing the global challenges arising from climate change requires international environmental cooperation. Previous work on the design of international institutions highlights the role of reciprocity and burden sharing for the evolution of lasting cooperation between countries. While scholarship acknowledges that in democratic systems domestic support for international cooperation eventually determines its long-term prospects, we know very little about how the design of international agreements affects individual support for establishing and joining such institutions. The project's comparative research starts filling this gap by exploring how reciprocity and the distribution of costs arising from climate change mitigation efforts stipulated in international climate agreements affect mass support for these institutions. Empirically, the project examines the determinants of preferences for international environmental agreements using randomized experiments embedded in representative surveys in four economically important democracies (United States, United Kingdom, France, and Germany). The findings speak to the literatures on the design of international institutions and cooperation in environmental policy and will provide policymakers with important knowledge about which types of international environmental cooperation are likely to have long-term prospects in democracies and which will not.