The Legal Space for Structural Differentiation in the EU: Reciprocity, Interconnectedness and Effectiveness as Sources of Constitutional Rigidity

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Table of Contents: I. Introduction. – II. Sources of rigidity. – II.1. Omnidirectional reciprocity as a source of rigidity. – II.2. Interconnectedness as a source of rigidity. – II.3. Effectiveness as a source of rigidity. – II.4. The clash between rigidity and the dynamics of differentiation. – III. Brexit insights for future attempts at structural differentiation. – III.1. The transition period as sheer rigidity. – III.2. The Northern-Ireland Protocol: Flexible borders and rigid law. – III.3. The TCA as the residual space of rigidity. – IV. Conclusion: Rigidity roadblocks to future structural integration.

Abstract: This Article argues that the legal space for truly structural forms of differentiation in the EU is limited by several sources of rigidity. Sources of rigidity are thereby understood as legal rules and principles that, either by themselves or in interaction with other rules, principles and material facts, limit the scope for structurally significant differentiation in the EU’s legal and constitutional set-up. A source of rigidity can therefore be broader than a single legal rule or principle because the rigidity may stem from a combination of or interaction between multiple principles, rules and facts. Using Brexit as a prism, this Article identifies at least three such sources of legal rigidity, being reciprocity, interconnectedness and effectiveness. These sources of rigidity also place significant limits on the possible dynamics of differentiation. It is demonstrated how Brexit brought these sources of rigidity to the surface, and how legal rigidity can and likely will collide with an increasing political desire for more structural differentiation in the future.

Keywords: differentiation – legal rigidity – Brexit – differentiated integration – future of EU integration – general principles of EU law.

European Papers, Vol. 7, 2022, No 3, pp. 1165-1190
2499-8249 - doi: 10.15166/2499-8249/606

* Professor of EU law, Europa Instituut Leiden Law School,


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