Why no twin-track Europe? Unity, discontent, and differentiation in European integration

European integration has grown increasingly differentiated. EU member countries now integrate at different speeds and frequently resort to opt-out clauses, while occasionally voicing deep discontent with the direction of the integration process. Nevertheless, European integration essentially remains a single-track enterprise, whereby member countries move in the same direction under the same set of EU institutions. Offering a novel perspective on EU integration, we argue that the real puzzle is not why integration has become differentiated but rather why it has not become more differentiated. Using a simple formal model to illustrate and deepen our argument, we throw new light on the bewildering coexistence of unity, discontent, and differentiation in the European project. We show that a twin-track Europe would likely leave more EU members discontented.