This Chapter charts the intellectual history of unjust enrichment, showing that many legal systems in the Western tradition identify unjust enrichment as a source of personal obligation separate from contract or tort. In the United States, unjust enrichment developed in law and equity, suffered through a period of instability in the post-fusion legal landscape, and has experienced a recent resurgence. The idea of “unjust enrichment” researched for this Chapter includes any treatment of an unequal transfer of value that operates as a source of obligation separate from obligations arising from consent or wrongdoing. This separate source of obligation can be identified as far back as the Roman Empire. The definition of “unjust enrichment” that seems to best fit this source of obligation is that of the Third Restatement: any unequal transfer of value without an adequate legal basis.1×1. See Restatement (Third) of Restitution and Unjust Enrichment § 1 cmt. b (Am. Law Inst. 2011).
Given that both contract and tort are sources of obligation recognized within the common law tradition, one might think that unjust enrichment would fall squarely within the common law, but the story is more complex. The American legal tradition inherited the English distinction between “common law” and “equity.”2×2. See Samuel L. Bray, Equity: Notes on the American Reception, in Equity and Law 31, 36–37 (John C.P. Goldberg et al. eds., 2019). This Chapter uses the term “equity” to describe the legal reasoning employed by the courts of equity in the Anglo-American tradition. In contrast, the rules of law employed by common law courts are termed “common law” rules. Although separate common law and equity jurisdictions were mostly abolished by the early twentieth century, American courts have struggled to determine how much to “fuse” them.3×3. Id. at 38–39. Unjust enrichment developed as a common law source of obligation and as an equitable principle,4×4. See infra pp. 2081–82; see also, e.g., cases cited infra note 71. and it now occupies an uncomfortable space in American jurisprudence. In some ways this is surprising, since unjust enrichment would seem to be the prime candidate for perfect fusion. However, the half steps taken by the American fusion of law and equity, and the particularities of how this fusion occurred, have contributed to uncertainty about how to employ unjust enrichment.
The confusion over characterization has impacted plaintiffs who might be in a position to bring an unjust enrichment claim. After the fusion of law and equity, unjust enrichment was predominantly categorized as “equitable.”5×5. See, e.g., cases cited infra note 161. This label has caused unjust enrichment to become unpopular and misunderstood in the United States, in contrast to the vibrant unjust enrichment scholarship in other countries.6×6. See Chaim Saiman, Restitution in America: Why the US Refuses to Join the Global Restitution Party, 28 Oxford J. Legal Stud. 99, 99–100 (2008). Some state courts, misinterpreting unjust enrichment as a purely equitable claim, have put up barriers, such as the rule that equity does not step in if there is an adequate remedy “at law.”7×7. See Eric J. Konopka, Hey That’s Cheating! The Misuse of the Irreparable Injury Rule as a Shortcut to Preclude Unjust-Enrichment Claims, 114 Colum. L. Rev. 2045, 2045–47 (2014). This Chapter will highlight better theoretical approaches to unjust enrichment in a post-fusion world.
Section A explores the early common law and equity roots of unjust enrichment in the Western tradition. Section B demonstrates that pre-1900 American jurisprudence employed the doctrine in common law and equity courts. Section C explores attempts to fuse the law and equity sides of unjust enrichment and misapplication of these attempts after the fusion of law and equity in the United States. Finally, section D addresses recent developments here and abroad, as well as different options for theorizing unjust enrichment in the post-fusion landscape.