Mistake of fact is a doctrine across criminal, civil, and international law that allows a defendant to argue that an honest (and often reasonable) misapprehension of a factual situation prevented them from forming the mens rea, or guilty mind, required for an offense. It is generally contrasted with mistake of law, which is rarely a defense under the maxim ignorantia juris non excusat.
In common-law criminal systems, the standard varies by offense:
- For specific intent crimes, even an unreasonable but honest mistake may negate intent.
- For general intent crimes, the mistake must usually be both honest and reasonable.
- For strict liability offenses (e.g., many statutory rape and regulatory statutes), mistake of fact is typically unavailable.
The U.S. Model Penal Code §2.04 codifies the defense, providing that ignorance or mistake is a defense when it negates the purpose, knowledge, belief, recklessness, or negligence required to establish a material element. English law treats the doctrine similarly, with leading authority in DPP v Morgan [1976] AC 182 on honest belief, later modified for sexual offenses by the Sexual Offences Act 2003, which imposed a reasonableness requirement.
In international criminal law, Article 32 of the Rome Statute of the International Criminal Court explicitly recognizes mistake of fact as a ground for excluding criminal responsibility if it negates the mental element of the crime. This has been raised in proceedings concerning command responsibility and targeting decisions, where commanders argued they reasonably believed a target was a lawful military objective under international humanitarian law.
In contract law, mistake of fact—particularly mutual mistake about a basic assumption—can render an agreement voidable, as illustrated by Sherwood v. Walker (Mich. 1887) involving the sale of a cow believed to be barren.
The defense's force depends heavily on the offense's mental-state requirement and on whether the mistake bears on an element the prosecution must prove.
Example
In the 2015 U.S. airstrike on a Médecins Sans Frontières hospital in Kunduz, Afghanistan, U.S. commanders cited a mistake of fact—believing the building was a Taliban-controlled compound—as a key factor in the Pentagon's determination that the strike was not a war crime.
Frequently asked questions
Mistake of fact concerns misunderstanding a factual circumstance and is often a valid defense; mistake of law concerns ignorance of legal rules and is generally not excused, with narrow exceptions.
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