Lord Mansfield, via the 1750 English case of Moses v. Macferlan, had injected unjust enrichment doctrine into the English legal tradition. Long before 1937, which was when the U.S. Supreme Court expressly applied the doctrine in Stone v. White, the doctrine had become woven into the fabric of our legal tradition as well.

Whether at law or in equity, unjust enrichment is the principle on this side of the Atlantic that now underlies the substantive equitable remedy of restitution. Restitution as a remedy for a trustee's unauthorized self-dealing is covered in §7.2.3.3 of Loring and Rounds: A Trustee’s Handbook (2024), which section is reproduced in the appendix below.

The quasi contract, a creature of the law, was all about unjust enrichment. On the equity side, the concept of unjust enrichment evolved as a corollary to both the fiduciary principle and constructive-trust jurisprudence. By the end of the nineteenth century, American legal scholars were busy developing a unified theory of unjust enrichment that would come to straddle and transcend the traditional law-equity divide. The Restatement of Restitution (1936) is the culmination of those efforts. Sad to say, on this side of the Atlantic there are now few lawyers left who are equipped, by formal legal training at least, to appreciate the boldness of the efforts of those realists to colonize the “vast terra incognita occupied by the set of legal actions grouped under the impenetrable name of ‘quasi-contract’ and a miscellaneous set of equitable remedies (principally constructive trust)…[M]any American lawyers would be hard pressed even to say what equity is (or was).” Andrew Kull, Restitution and Reform, 32 S. Ill. U. L.J. 83, 87 (2007). The Uniform Trust Code, just an aggregation of legislative tweaks to certain aspects of core trust principles, stays out of the Restatement’s bailiwick altogether. The Restitution’s authors are memorialized in the appendix below.

As to the intersection of agency fiduciary law (not trust fiduciary law) and unjust-enrichment jurisprudence, consider the litigation complaint in X Corp [formerly Twitter] v. Wachtell, Lipton, Rosen & Katz, Sup. Ct. State of Calif., CGC-23-60746, filed July 5, 2023. X Corp seeks restitution for the defendants’ alleged unjust enrichment. See generally Restatement of Restitution (1936), specifically Chapter 12 (Acquisition of Property by a Fiduciary).