Existing studies suggest that differentiation, as determined in the primary law of the European Union (EU), creates stable boundaries between insiders and outsiders, with member states rarely altering their formal status. However, instances in which outsiders participate in insider policies or insiders pursue outsider strategies suggest that the empirical reality is more complex. This article develops and applies a fine-tuned notion of an additional type of differentiation in the EU, referred to as secondary differentiation. Applying this complementary perspective, the article sheds light on the grey areas of insiderness and outsiderness. Discrepancies between primary and secondary differentiation are explained through variations in the dependencies of and politicisation in individual EU member states. Findings from the Economic and Monetary Union and the Schengen Area show that EU member states regularly refer to secondary differentiation to respond to issue-specific incentives without changing their formal status.
