The architecture of the project has rested on five building blocks. Block one consisted of three work packages-the philosophical foundations of a fair system of DI (WP1), legal feasibility and constitutional acceptability of DI (WP2) and what citizens, governments and parties want from DI (WP3). The work packages have brought together political theory analysis, legal analysis and the analysis of public opinion data, interviews with party representatives, member state reports and data on the governmental salience and positions regarding DI. Block 2 has focused on the patterns, causes and effects of DI (WP4). Work performed in this block consisted in updating, expanding, and analysing datasets on primary-law and secondary-law differentiation (EUDIFF1 and EUDIFF2) up until the end of 2020. Moreover, WP4 has focused on describing and explaining patterns, trends, and developments in DI. In Block 3, the work packages have zoomed in on internal differentiation (WP5) and external differentiation (WP6). Research in WP5 examined DI in the domain of core state powers (such as EMU, Schengen and CFSP). It has expanded the DI datasets by including data on differentiation regarding material contirbutions and on the partial and informal reintegration of opt-out countries. WP6 has focused on a series of case studies on the relationship between the EU and its neighbours, notably the European Free Trade Association (EFTA) states, Turkey and the UK. Block 4 has assessed alternatives to DI. WP 7 has built a dataset on flexible implementation based on a selection of directives from the EUDIFF2 dataset. WP8 has conducted case studies of experimentalist governance on EU energy regulation, genetically modified organisms (GMOs) and EU financial regulations. Finally, Block 5 has synthesized the findings and developed scenarios for DI.
The main output of the project consists in the following contributions. First, InDivEU has formulated criteria for DI as a legitimate and fair scheme of international cooperation and assessed DI in terms of representation, transparency as well as legal and political accountability. Thereby, InDivEU provides guidelines for assessing the legitimacy of DI and for correcting current procedural and substantive deficits. The studies find that DI can be considered to meet standards of justice and democracy in principle, but also point at practical deficits. Second, InDivEU has surveyed the legal and political instruments of internal and external differentiation and their application in a variety of policy domains and EU-third country relations, also contrasting legal differentiation with alternative instruments of providing flexibility in EU governance. InDivEU thereby offers a catalogue of flexibility instruments and provides information on their usage. Third, InDivEU has collected comprehensive and novel data on public opinion, party views, and governmental salience and preferences; the use and development of DI in EU treaty and secondary law as well as in the EEA; the flexible implementation of EU legislation and differentiated compliance; and the practical cooperation of insiders and outsiders beyond legal differentiation. InDivEU thus provides valuable resources for future research and analysis. These main results have been and will further be published in academic journals and books as well as working papers and policy briefs, and they have been disseminated at academic conferences and meetings with stakeholders and policymakers.