EU Differentiated Integration (DI) is a long-lasting feature of core state powers (CSP). But how “real” is it? Does a formal opt-out prevent a member state to uphold cooperation in the area it opted out from? Through a novel dataset, EUDIFF-RES, we compare legal DI to states´ material involvement in CSP, by means of financial and human resources, between 1998 and 2019. We find differences across policy areas. In CFSP, states consistently opt-out both formally and materially. In JHA and economic policy, by contrast, formal opt-outs are often followed by increased material cooperation. We explain these differences by the dynamics of mass politics producing formal DI, vis-à-vis the functional benefits that states may reap from a deeper involvement in CSP.
