Mainstream theories of international cooperation posit that states build and operate within multilateral institutions to overcome problems of collective action. Taking a contrarian view, Rathbun argues that cooperation is better seen as a reflection of the beliefs people have about the trustworthiness of others. Borrowing findings from social psychology, Rathbun notes that people on the left tend to have a more benign view of human nature and see the world as less threatening than people on the right, and he hypothesizes that significant partisan differences exist on the issue of international cooperation. Rathbun applies his theory to the United States’ great moments of institutional order building: the League of Nations, the United Nations, and NATO. In these episodes, Rathbun argues, Democrats were more trusting than Republicans, leading them to be more supportive of multilateral security cooperation. But it is hardly surprising that partisan differences exist regarding such issues, and that insight itself is not very useful in explaining the dramatic sweep of security cooperation in the twentieth century unless it is tied to grander theories of power and order.