Differentiated integration in the European Union: institutional effects, public opinion, and alternative flexibility arrangements

Research on differentiated integration (DI) in the European Union (EU) has focused on the causes, conditions, and patterns of differentiation in European integration. By contrast, we know less about its effects on institutional outcomes and public support; moreover, alternatives to de jure DI in providing flexibility are still rarely accounted for. This introduction to the special issue takes stock of, and discusses omissions, in the current literature on DI. We propose an analytical framework, centering on efficiency and legitimacy, to study the effects of different types of DI. We use this framework to motivate the choice and assess the contributions of the articles selected for this special issue.