Definition
Latin: "against good morals." A common law doctrine, inherited from Roman law, that renders contracts and other legal obligations void or unenforceable when they offend public morality. A transaction is contra bonos mores when its subject matter, purpose, or consideration is immoral, indecent, or so contrary to accepted social norms that courts refuse to give it effect. The doctrine operates as a public policy limit on freedom of contract: no matter how clearly the parties agreed, courts will not enforce an agreement that the law regards as morally corrupting or socially harmful.
The phrase appears in two related but distinct contexts:
1. CONTRACT INVALIDITY: An agreement is void as contra bonos mores when its consideration or performance requires something immoral — such as agreements to commit crimes, to facilitate prostitution, to suppress criminal prosecutions for payment, or to engage in conduct grossly offensive to public decency.
2. GENERAL LEGAL PRINCIPLE: Beyond contract, the phrase functions as a shorthand for the broader proposition that the law will not lend its enforcement machinery to transactions that violate fundamental moral standards, regardless of the legal form those transactions take.
Common Language
This is pure Latin legal terminology with no meaningful counterpart in ordinary English usage. Common speakers do not use the phrase. The closest lay equivalents — "immoral," "against public decency," or "against good morals" — lack the specific doctrinal content the legal phrase carries. Omitted from this section for that reason.
Common Confusion
Contra bonos mores is closely related to, but distinct from, the doctrine of contracts against public policy. The two doctrines frequently overlap and historical sources sometimes treat them interchangeably, but they are not identical. A contract can be void as against public policy for reasons that have nothing to do with morality — restraint of trade, for instance, or interference with government functions. Contra bonos mores is the narrower, morality-specific strand of the larger public policy invalidation principle. Researchers should not assume that every case citing one doctrine is citing both.
Why It Matters in Research
The doctrine is older than most of the common law frameworks researchers are accustomed to working with, and its contours shifted significantly over the nineteenth and twentieth centuries. Several research traps deserve attention.
First, what counted as contra bonos mores was not fixed. Victorian-era courts applied the doctrine expansively to agreements touching Sunday trading, gambling, cohabitation outside marriage, and transactions seen as encouraging vice. Many of those applications would not survive modern scrutiny. Researchers reading nineteenth-century decisions should treat the doctrine's scope as historically contingent, not as a stable rule that can be read forward or backward without adjustment.
Second, the phrase appears frequently in equity as well as common law sources, and the doctrinal work it does can differ between those traditions. In equity, the moral character of a party's conduct could affect the availability of relief even when strict contract invalidity was not at issue.
Third, modern courts rarely use the Latin phrase. Contemporary opinions typically speak of contracts "against public policy" or "void for illegality" rather than invoking contra bonos mores by name. Corpus searches using the Latin term will undercount relevant modern authority. Researchers should run parallel searches using the English equivalents when surveying post-1950 material.
Fourth, the doctrine connects directly to the in pari delicto defense: when both parties to a void contra bonos mores agreement are equally at fault, courts historically refused relief to either. That downstream consequence matters for understanding why the characterization of a contract as contra bonos mores had practical stakes beyond mere invalidity.
Historical Dictionary Support
The three shelf sources agree on the core definition — "against good morals" — and on the primary consequence: contracts so characterized are void. They diverge in depth and usefulness.
Bouvier's is the most practically informative, offering a functional description of the categories of agreements that attracted the doctrine: those that incentivize crime, impose improper obligations prejudicial to third parties, are offensive to decency, or carry a tendency toward mischievous consequences. This four-part characterization reflects the actual range of the doctrine in English equity and common law courts of the eighteenth and early nineteenth centuries and remains useful for identifying relevant primary sources.
Black's entry is characteristically spare — essentially a translation plus the legal consequence — and offers nothing for the researcher trying to understand the doctrine's scope or application.
Burrill's notes the Roman law origin, citing the Codex, which is accurate and important. The doctrine does have genuine Roman roots and passed into English law through civil law influence, particularly in ecclesiastical and admiralty contexts before being absorbed into common law contract doctrine. Burrill's cross-reference to Hobart 167 points toward early seventeenth-century English authority, situating the doctrine's English pedigree earlier than the Bouvier citations, which are predominantly late eighteenth and early nineteenth century.
What all three sources miss: the doctrine's relationship to in pari delicto, its differing application in equity versus common law, and its gradual subsumption into the broader "public policy" vocabulary of modern courts.
Jurisdictional Note
The doctrine originated in English common law and was received into American law through that channel. Its specific applications varied significantly by state, particularly in areas like gambling contracts, Sunday contracts, and agreements touching sexual conduct, where state statutory frameworks often displaced common law doctrine. Researchers working on pre-twentieth-century American sources should not assume uniform application across jurisdictions.