The Subsidiarity of Unjustified Enrichment

The objective of this contribution is, therefore, to assess whether the theory and practice of private law should move away from the subsidiarity of unjustified enrichment altogether.4 To this end, a mapping of civil law and common law jurisdictions where this doctrine or rule has been established will be conducted (section I.B). Furthermore, the vast array of different interpretations and applications of this doctrine or rule will be analysed (section I.C), with a particular focus on the two extremes of the spectrum: the strong (or in abstracto) and weak (or in concreto) versions of subsidiarity. It will then be shown that, in the legal systems considered, there is a general trend towards abandoning (or at least mitigating) the doctrine or rule of subsidiarity of unjustified enrichment, both in theory and in practice. This is due to the perception that this doctrine or rule is inconsistent, superfluous, or at the very least incapable of rationalizing the case law on unjustified enrichment and providing it with a logically satisfactory foundation (section I.D). Subsequently, the chapter will explore the theoretical and practical issues arising from the intersections of unjustified enrichment with the three other core areas of private law, that is, contract (section II), property (section III), and tort (section IV), focusing on the mechanisms that regulate the overlap of remedies within each domain. The conclusion (section V) is that the doctrine or rule of subsidiarity should be abandoned. It persists merely as a historical remnant of the original foundation of unjustified enrichment on purely equitable grounds. Subsidiarity was formulated in contexts where claims in unjustified enrichment as such were established as exceptional corrective measures to the strict application of the law, rather than as occurrences of a necessary structural principle. However, with the integration of unjustified enrichment into codified law, or its incorporation into legal systems in other forms, a subsidiarity- based approach appears outdated, if not entirely untenable. In other words, the fear that the restitutionary claims at stake might undermine existing legal structures is unfounded, as unjust enrichment is now an integral and regulated part of the law. Concurrently, to prevent unjust enrichment from extending beyond its proper scope, it remains crucial precisely to define its essential elements, especially the requirement of ‘at the claimant’s expense’.