Theorizing EU enlargement: research focus, hypotheses, and the state of research

Abstract Despite its indisputable political relevance, the enlargement of the EU has suffered from a theoretical neglect in studies of European integration. While theoretically informed studies have emerged recently, this literature suffers from a predominant focus on single cases and from not being linked to the more general study of international organizations in the social sciences. This article aims to structure the emerging debate in order to generate more generalizable and cumulative insights. First, we define enlargement as a process of gradual and formal horizontal institutionalization. We identify key dependent variables of a so-defined enlargement, for which we propose comparative research strategies. Second, we draw on two basic approaches to the analysis of international organizations - rationalist and sociological or constructivist institutionalism - to derive core hypotheses on the conditions of enlargement. Finally, we demonstrate the usefulness of these theoretical approaches in structuring the debate by giving an overview of the state of research on EU enlargement. Keywords: ConstructivismEnlargementEuropean UnionIntegration TheoryRationalist InstitutionalismRegional Organizations