Restitution and unjust enrichment

Restitution and unjust enrichment is the field of law relating to gains-based recovery. In contrast with damages (the law of compensation), restitution is a claim or remedy requiring a defendant to give up benefits wrongfully obtained. Liability for restitution is primarily governed by the "principle of unjust enrichment": A person who has been unjustly enriched at the expense of another is required to make restitution.

This principle derives from late Roman law, as stated in the Latin maxim attributed to Sextus Pomponius, Jure naturae aequum est neminem cum alterius detrimentum et injuria fieri locupletiorem ("By natural law it is just that no one should be enriched by another's loss or injury"). In civil law systems, it is also referred to as enrichment without cause or unjustified enrichment.

In pre-modern English common law, restitutionary claims were often brought in an action for assumpsit and later in a claim for money had and received. The seminal case giving a general theory for when restitution would be available is Lord Mansfield's decision in Moses v Macferlan (1760), which imported into the common law notions of conscience from English chancery. Blackstone's Commentaries also endorsed this approach, citing Moses.

Where an individual is unjustly enriched, modern common law imposes an obligation upon the recipient to make restitution, subject to defences such as change of position and the protection of bona fide purchasers from contrary equitable title. Liability for an unjust enrichment arises irrespective of wrongdoing on the part of the recipient, though it may affect available remedies. And restitution can also be ordered for wrongs (also called "waiver of tort" because election of remedies historically occurred when first filing a suit). This may be treated as a distinct basis for restitution, or it may be treated as a subset of unjust enrichment.

Unjust enrichment is not to be confused with illicit enrichment, which is a legal concept referring to the enjoyment of an amount of wealth by a person that is not justified by reference to their lawful income.

In civil law systems, unjust enrichment is often referred to as unjustified enrichment. Its historical foundation of enrichment without cause can be traced back to the Corpus Iuris Civilis. While the concept of enrichment without cause was unknown in classical Roman law, Roman legal compilers eventually enunciated the principle of unjustified enrichment based on two actions of the classical Roman period—the condictio and the actio de in rem verso.

The condictio authorized recovery by the plaintiff of a certain object or money in the hands of the defendant. The defendant was considered a borrower who was charged with returning the object or money. For the actio de in rem verso, the plaintiff bore the burden of specifying the cause for his demand, namely, demanding the restitution of assets that had exited the plaintiff's patrimony and entered the defendant’s patrimony through the acts of the defendant’s servants.

The coherent concept of unjustified enrichment then appeared in the Justinian Code, based on Roman pragmatism with equitable considerations and moral principles of Greek philosophy. In the Justinian Code, condictiones were grouped into categories, such as when the plaintiff had given a thing or money: