Definition
Mala in se (singular: malum in se; Latin, "wrongs in themselves") are acts that are inherently and morally wrong, independent of any positive law prohibiting them. An act is malum in se when its wrongfulness derives from its own nature — from natural law, moral principle, or the universal conscience of civilized society — rather than from the fact that a legislature has chosen to forbid it.
The classic examples are murder, rape, theft, and fraud: acts that would be recognized as grievously wrong even in the complete absence of a statute declaring them illegal. The concept is fundamentally a philosophical one, rooted in natural law theory, and its legal significance arises from the contrast with its counterpart, mala prohibita.
Common Confusion
MALA IN SE vs. MALA PROHIBITA: These two Latin phrases are complementary and must be understood together. Mala prohibita (wrongs because prohibited) describes acts that are illegal solely because a governing authority has enacted a rule against them — acts that carry no inherent moral stain. Driving on the left side of the road, failing to file a tax form by a deadline, or importing a regulated item without a license are mala prohibita: there is nothing morally corrupt about the act itself; it is simply against the rules. The distinction matters in several doctrinal contexts: criminal intent (mens rea requirements are often relaxed or eliminated for mala prohibita offenses), the common law treatment of per se negligence, and the historical enforcement of laws in the conflict-of-laws context. Researchers conflating the two will misread historical sources that invoke one term to explain why the other standard does or does not apply.
Why It Matters in Research
The mala in se / mala prohibita distinction is one of the most enduring organizing principles in Anglo-American legal thought, but its doctrinal weight has shifted considerably over time, and researchers must navigate that shift carefully.
In earlier common law sources — and in Blackstone, who is the authority all three historical dictionaries cite — the distinction carried significant practical consequences. Courts asked whether an act was malum in se to determine whether a contract touching on it was void, whether criminal liability could attach without proof of intent, and whether a civil remedy existed independent of statute. The distinction also did real work in conflict-of-laws analysis: courts would enforce foreign law creating rights out of mala prohibita violations but often refused to assist plaintiffs whose claims arose from acts deemed mala in se.
Modern criminal law has substantially diluted the doctrinal sharpness of the distinction. The rise of regulatory offenses and strict liability statutes has produced a large body of law treating conduct as criminal without regard to moral culpability, which blurs the line between the two categories. Researchers working in nineteenth-century criminal law treatises or colonial-era court records will find the distinction invoked far more confidently and consequentially than in twentieth-century sources.
The phrase also appears — sometimes without acknowledgment of its Latin origin — in negligence per se analysis. Some jurisdictions and commentators, when discussing whether violation of a statute creates negligence as a matter of law, distinguish between statutes codifying mala in se conduct (where courts are more willing to infer a civil duty) and purely regulatory statutes (mala prohibita), where the negligence per se inference is more contested. Researchers moving between criminal and tort sources should watch for this crossover usage.
One navigational trap: older sources occasionally use malum in se (the singular form) when speaking of a class of conduct, not a single act. The Latin grammar can mislead a researcher into thinking a source is discussing a single incident rather than a category.
Historical Dictionary Support
All three historical dictionaries — Black's, Bouvier's, and Burrill's — converge on a single, spare definition: wrongs in themselves; acts morally wrong; offenses against conscience. All three anchor the definition to the same passage in Blackstone's Commentaries (1 Bl. Comm. 57-58; 4 Bl. Comm. 8), which reflects how thoroughly Blackstone set the terms for Anglo-American understanding of this concept.
Bouvier adds the most substance among the three. He includes a cross-reference to mala prohibita (essential) and quotes the Blackstone passage that distinguishes the two categories in terms of conscience: that for mala in se, the conscience is bound "because we are bound by superior laws" — a phrasing that reflects the natural law foundation of the concept. The entry in Bouvier is incomplete as reproduced here (Blackstone's reasoning about mala prohibita is cut off), but the fragment present confirms that Bouvier understood the distinction as a matter of the source of obligation, not merely the presence or absence of a statute.
What the historical dictionaries do not address — because they could not — is the erosion of the distinction in modern regulatory criminal law, or its migration into tort doctrine. Researchers relying solely on these sources will get a sound foundation for understanding nineteenth-century usage but will need supplementary sources to trace how courts and commentators deployed or abandoned the distinction in later periods.
Jurisdictional Note
The mala in se / mala prohibita distinction is a common law concept operative across all common law jurisdictions, but its practical legal consequences vary. Some states have codified the distinction in statutes addressing criminal intent or negligence per se; others treat it as a background interpretive principle without statutory grounding. Civil law jurisdictions do not use this framework.
Encyclopedia Cross-Reference
The Law Mind Torts & Personal Injury Encyclopedia: Negligence Per Se (Violation of Statute as Negligence) [torts_8] — for how the mala in se / mala prohibita distinction informs whether statutory violation generates civil liability.
The Law Mind Torts & Personal Injury Encyclopedia: Defamation Per Se Categories [torts_123] — for a parallel per se structure in tort law where the nature of the act, rather than proof of harm, drives legal consequence.