Even though differentiation has become a core feature of the EU, the grand theories have focused almost exclusively on uniform integration. In this article, we derive hypotheses about differentiated integration from liberal intergovernmentalism, neofunctionalism, and postfunctionalism. In an analysis of EU treaty making between 1992 and 2016, we find evidence that heterogeneity of both wealth and identity, integration in the area of core state powers, and pre-existing differentiation drive differentiated integration. A comparison of the explanatory power of the grand theories shows that neo- and post-functionalism explain the differentiations that member states obtain in EU reform treaties more convincingly than liberal intergovernmentalism. A synthetic model performs best, however. The grand theories also leave noteworthy variation unexplained.
