The European Union is without question the most densely institutionalised international organization in the world, with a welter of intergovernmental and supranational institutions and a rapidly growing body of primary and secondary legislation. This paper examines the new
institutionalisms in rational choice and historical analysis and their contributions to EU studies,
briefly summarizing the core assumptions of each approach before discussing specific applications to the study of the EU and the question of enlargement. The rational-choice
institutionalist study of the EU, I argue, has advanced considerably over the past two decades, with increasingly sophisticated elaboration and testing of theories relating to executive, legislative and judicial politics as well as public opinion, enlargement and other vital topics.
Historical institutionalist accounts, by contrast, have been slower to move beyond the concepts of lock-in and path-dependence as broad metaphors for the integration process, but here too recent work has begun to refine the theory into distinct, testable hypotheses. In that context, the primary challenge for rational-choice institutionalists consists in the specification of new and
more accurate models of EU institutions, and the testing of those models through a range of empirical methods; while for historical institutionalists, the primary challenge is to specify and test more precise hypotheses about types of institutions likely to generate positive or negative feedbacks, the mechanisms of path-dependence, and the impact of temporal factors on the path of European integration.