Unexplored variation in European integration research: Mapping and discussing vertical differentiation in the EU’s area of freedom, security and justice

This paper is explicitly devoted to the question of vertical differentiation in the European Union. Most literature on differentiated integration has exclusively focused on the analysis of horizontal differentiation, and hence on the phenomenon that the validity of EU rules varies across member states. The analysis of vertical differentiation, in other words why the EU’s authority varies across policies, has so far been neglected. Taking the EU’s Area of Freedom, Security and Justice as an empirical laboratory, this paper maps and discusses vertical differentiation and hence so far unexplored variation in this policy area.