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Differentiated integration has been the subject of political discussion and academic thought for a long time. It has also become an important feature of European integration since the 1990s. By contrast, it is astonishing how poor our research and knowledge about the phenomenon is. Whereas there is an abundance of conceptual work and some normative analysis, positive theories on the causes or effects of differentiated integration are rare. Empirical analysis has concentrated on a few important cases of treaty law (such as EMU and Schengen) while there is no systematic knowledge about differentiated integration in secondary law. The aim of this article is therefore twofold: to review the existing typological and theory-oriented research and to outline a research agenda striving for systematic empirical and explanatory knowledge.
Holzinger et al. (Thu,) studied this question.