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Differentiated Integration and European Union Politics

Differentiated integration has become a core feature of the European Union. Whereas in uniform integration, all member states (and only member states) equally participate in all integrated policies, in differentiated integration, member and non-member states participate in EU policies selectively. At its core, differentiated integration is formally codified in EU treaties and legislation.
 The study of differentiated integration has long remained limited to policy-oriented conceptual debate. “Multi-speed integration,” “core Europe,” and “Europe à la carte” are prominent labels that have resulted from this debate. Theoretical and systematic empirical analysis of differentiated integration is a more recent phenomenon.
 Demand for differentiated integration is theorized to be rooted in international diversity of country size, wealth, and national identity, which result in heterogeneity of integration preferences, interdependence, and state capacities. In addition, agreement on differentiated integration depends on the size and bargaining power of the insider and outsider groups, the externalities that differentiation produces, and the institutional context in which negotiations take place. Finally, differentiated integration is subject to centrifugal and centripetal dynamics of path dependence and institutional practice.
 Evaluations of differentiated integration vary between negative assessments based on the principles of legal unity, European democracy, and solidarity and positive assessments based on demoi-cratic standards and the facilitation of integration. More research is needed on the relationship of differentiated integration with other forms of flexibility in the EU, citizen attitudes, and party positions on differentiated integration and the effects of differentiation.