The treaty of Amsterdam included an innovative provision called ‘closer cooperation’ that allowed a subgroup of member states to move forward and deepen integration in policy areas where consensus could not be found. While scholarly interest in understanding differentiated integration (DI) has grown recently, a gap in our understanding of why member states join enhanced cooperation initiatives or not remains. Enhanced cooperation has been used to deepen integration in the areas of divorce law, the European unitary patent, property regimes of international couples, the European Public Prosecutor’s Office (EPPO), and Permanent Structured Cooperation (PESCO) in the area of defence and security. Across the instances of enhanced cooperation, participation patterns remain puzzling. This report tests the explanatory power of DI theories when it comes to the case of the EPPO establishment, in which 22 member states participate to date. Using crisp-set qualitative comparative analysis, this study finds that high interdependence and relative power of member states are sufficient explanations of the decision to join when combined with nonEurosceptic governing parties and non-Eurosceptic public attitudes. Better governance standards or Eurosceptic governing parties explain the decision not to join the enhanced cooperation to establish the EPPO. Puzzling results about the role of exclusive national identities raise further questions about the relevance of the constructivist explanations and the accuracy of the measurements commonly used in this type of research. The study concludes that understanding the motivations of member states for deciding to join differentiated integration projects or not is complex and thus, requires causality to be understood as multiple and conjunctural.
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ARENA Report 9/22 (pdf) (110 pages)
To join or not to join? An analysis of the enhanced cooperation to establish the European Public Prosecutor’s Office
Raquel Ugarte Díez
November 2022