General principles - Mistake of fact or law

Learning Outcomes

This article explains mistake of fact and mistake of law in criminal liability, including:

• Distinguishing mistake of fact from mistake of law, accurately labeling each in bar-style fact patterns, and spotting when the exam is testing that classification.
• Identifying when a mistake of fact negates mens rea for specific intent, general intent, malice, and strict liability crimes, and mapping each question to the correct category.
• Determining whether a mistake must be honest, reasonable, or both, and evaluating whether a described belief satisfies those standards on MBE questions.
• Recognizing the narrow circumstances in which a mistake of law may function as a defense, such as reasonable reliance on official statements or when knowledge of the law is an element.
• Comparing common law and Model Penal Code approaches to mistakes and predicting how each is likely to appear in multiple-choice answer choices.
• Applying mistake doctrines in combination with related defenses—especially intoxication—and avoiding traps that confuse mistake analysis with mere lack of moral blameworthiness.
• Using structured, stepwise reasoning to eliminate wrong answer choices that rely on “ignorance of the law” or that misstate when unreasonable mistakes can excuse otherwise criminal conduct.

MBE Syllabus

For the MBE, you are required to understand how mistakes of fact or law affect criminal liability, with a focus on the following syllabus points:

• The distinction between mistake of fact and mistake of law.
• How mistake of fact interacts with different mens rea requirements (specific intent, general intent, malice, strict liability).
• When an unreasonable mistake of fact can still be a defense.
• The general rule that ignorance of the law is no excuse, and its limited exceptions.
• How the Model Penal Code (MPC) and common law approaches are likely to appear on the MBE.
• Relationship of mistake doctrines with other defenses (especially intoxication and insanity).

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. Which of the following is most likely a defense to a general intent crime?
   1. A reasonable mistake of fact.
   2. An unreasonable mistake of fact.
   3. A mistake of law.
   4. A reasonable belief that the law did not apply.
2. For which type of crime is an unreasonable mistake of fact potentially a defense?
   1. Strict liability crimes.
   2. General intent crimes.
   3. Specific intent crimes.
   4. Malice crimes.
3. Which of the following is generally NOT a defense to criminal liability?
   1. Mistake of fact that negates the required mental state.
   2. Mistake of law about whether conduct is criminal.
   3. Reasonable mistake of fact in a general intent crime.
   4. Unreasonable mistake of fact in a specific intent crime.

Introduction

Mistake doctrines sit at the intersection of mens rea and defenses. On the MBE, you will often be asked whether a defendant’s misunderstanding can prevent conviction. To answer correctly, you must:

• Classify the mistake as one of fact or law.
• Identify the type of crime: specific intent, general intent, malice, or strict liability.
• Decide whether the mistake is reasonable or unreasonable and whether it negates the required mental state.

Key Term: Mens Rea
The mental state required for a particular offense (e.g., purposely, knowingly, recklessly, negligently, or “intent” at common law). A mistake matters only if it prevents the defendant from having the required mens rea.

At common law and on the MBE:

• Mistake of fact can sometimes be a defense because it may show the defendant lacked the required mens rea.
• Mistake of law almost never excuses criminal conduct, even if the defendant acted in complete good faith.

Mistake of Fact

A mistake of fact occurs when a person misunderstands a factual circumstance relevant to the alleged crime. The key question is whether the mistake, if true, would mean the defendant lacked the mental state required by the offense.

Key Term: Mistake of Fact
A misunderstanding or ignorance about a factual situation relevant to the alleged crime, which may, in some cases, negate the required mental state for liability.

Key Term: Reasonable Mistake
A mistake that a hypothetical reasonable person in the defendant’s position could have made, judged by an objective standard.

Key Term: Unreasonable Mistake
A mistake that is honestly held by the defendant but that no reasonable person would make under the circumstances.

Two universal points for the exam:

• The mistake must be genuine (actually believed by the defendant).
• The mistake is relevant only if, had the facts been as the defendant believed, the elements of the crime would not be satisfied.

Example: If a defendant honestly believes a wallet on the table is his own, his mistake is about who owns the property, a factual issue.

Mistake of Law

A mistake of law arises when a person is unaware that their conduct is criminal or misunderstands what the law prohibits.

Key Term: Mistake of Law
A misunderstanding or ignorance about whether conduct is prohibited by law, which is almost never a defense to criminal liability except in narrow circumstances.

Key Term: Ignorance of the Law
The defendant’s lack of awareness that their conduct is illegal. As a general rule, ignorance of the law is not a defense.

Classic example: “I didn’t know it was illegal to possess this type of knife.” That is a mistake of law, and on the MBE it almost never helps the defendant.

Types of Crimes and the Effect of Mistake

The effect of a mistake depends heavily on the mental state required by the crime:

• Specific intent crimes: require that the defendant act with a particular purpose or objective (e.g., intent to permanently deprive, intent to commit a felony inside a dwelling).
• General intent crimes: require only an intent to do the act itself, not a further purpose.
• Malice crimes: require at least recklessness, or a conscious disregard of a substantial and unjustifiable risk.
• Strict liability crimes: require no mens rea as to one or more elements; the act alone is enough.

Key Term: Specific Intent Crime
A crime that requires the defendant to act with a particular purpose or objective in mind, beyond merely performing the act itself (e.g., larceny, burglary, robbery, attempt, conspiracy, solicitation).

Key Term: General Intent Crime
A crime that requires only an intent to perform the prohibited act, not to achieve a further result (e.g., battery, false imprisonment, rape at common law).

Key Term: Malice Crime
A crime requiring that the defendant act with malice—typically a reckless disregard of an obvious or high risk (e.g., common law murder with “depraved heart,” common law arson).

Key Term: Strict Liability Crime
A crime for which no mental state is required as to at least one element; liability attaches upon commission of the prohibited act (e.g., statutory rape, many public-welfare offenses such as selling alcohol to a minor).

On the MBE, the classification drives the answer:

• Specific intent → mistake of fact can be reasonable or unreasonable.
• General intent or malice → mistake of fact must be reasonable.
• Strict liability → mistake of fact is never a defense.

Mistake of Fact as a Defense

Core Principle

A mistake of fact is a defense if—and only if—it negates the mens rea required by the particular offense.

• Ask: “If the facts were as the defendant believed, would they be guilty of this crime?”

If the answer is no, the mistake can be a defense, subject to the type-of-crime rules below.

Specific Intent Crimes

For specific intent crimes, any honest mistake of fact is a potential defense, even if it is wildly unreasonable.

• Larceny (intent to permanently deprive).
• Robbery.
• Burglary (intent to commit a felony inside).
• Attempt, conspiracy, solicitation.
• First-degree premeditated murder (intent to kill with premeditation).

If the defendant’s mistaken belief means they did not have that further objective, they cannot be convicted of the specific intent form of the offense.

On the MBE, if the crime is clearly specific intent and the defendant honestly misunderstood a fact, do not worry about whether an ordinary person would have made the same mistake.

Worked Example 1.1

A man picks up a suitcase at an airport, genuinely believing it is his own, but it actually belongs to someone else. He is charged with larceny, a specific intent crime.

Answer:
The man has a defense. Larceny requires the specific intent to permanently deprive another of their property. Because he honestly believed the suitcase was his, he lacked this intent. For specific intent crimes, any genuine mistake of fact—reasonable or not—is a defense if it negates the specific intent.

Worked Example 1.2

A defendant decides to steal a leather jacket from a restaurant coat rack. He sees a jacket identical to his and, without checking, takes it, thinking it is his own. It actually belongs to another patron. He is charged with larceny.

Answer:
He has a defense to larceny. Larceny is a specific intent crime. If the facts were as he believed (the jacket was his), he would not be guilty of larceny. His mistake about ownership, even if careless, negates the specific intent to permanently deprive another of property.

General Intent and Malice Crimes

For general intent and malice crimes, a mistake of fact is a defense only if it is both honest and reasonable.

• The defendant must actually hold the mistaken belief.
• A reasonable person in the same situation must also be able to make that mistake.

If the mistake is unreasonable, the law treats the defendant as having the required mental state.

Examples:

• Battery (general intent).
• Rape (common law, not statutory rape).
• Arson and “depraved heart” murder (malice).

Worked Example 1.3

A woman is charged with battery (a general intent crime) after hitting someone, believing the person was an intruder. In fact, it was her neighbor, who had permission to enter.

Answer:
The woman has a defense only if her mistake was reasonable. Battery requires only the intent to apply unlawful force. For general intent crimes, an honest and reasonable mistake of fact is a defense because it shows she lacked intent to commit an unlawful touching.

Strict Liability Crimes

For strict liability offenses, mens rea is irrelevant as to the strict-liability element. A mistake of fact never helps, even if the defendant’s mistake is entirely reasonable and even if they took precautions.

Typical strict liability exam patterns:

• Selling alcohol to a minor.
• Traffic and regulatory offenses.
• Statutory rape (on the MBE, assume strict liability unless the question clearly says otherwise).

Worked Example 1.4

A defendant is charged with selling alcohol to a minor, a strict liability offense. He reasonably believed the buyer was over 21 based on a realistic fake ID.

Answer:
No defense. For strict liability crimes, mistake of fact is never a defense, even if the mistake is reasonable and the defendant acted in good faith.

Worked Example 1.5

A 22-year-old has consensual intercourse with a 15-year-old who looks older, lies about her age, and shows a fake ID stating she is 19. The jurisdiction treats statutory rape as strict liability. He is charged with statutory rape.

Answer:
He has no defense based on mistake of fact. Statutory rape is treated as strict liability on the MBE: intercourse with someone under the specified age is sufficient, regardless of the defendant’s reasonable and honest belief about age.

Mistake of Law as a Defense

General Rule

The general rule is harsh and frequently tested:

• Ignorance of the law is no excuse.

It is no defense that the defendant:

• Did not know the conduct was illegal, or
• Misunderstood the meaning or scope of the statute.

Example: “I thought marijuana possession had been legalized” is almost always a losing argument, unless one of the narrow exceptions applies.

Key Exceptions

Despite the general rule, there are three classic, exam-relevant exceptions where a mistake of law may be a defense.

1. Law not reasonably available

   • If the law was not reasonably made available or published (e.g., a criminal statute was newly enacted but never promulgated), due process may bar punishment.

2. Reasonable reliance on official statement

   • The defendant reasonably relies on an official statement of law later found to be erroneous, such as:

     • A statute later declared unconstitutional.
     • A judicial decision later overruled.
     • An official interpretation by the body charged with enforcing or interpreting the law.

   • Reliance on a private attorney’s advice is not enough; the source must be official.

3. Knowledge of law as an element of the offense

   • If the crime itself requires that the defendant know the legal status of their conduct (e.g., “know that their conduct is unlawful,” “know that they are unlicensed”), then a mistake about the law can negate that element.

   Example: A statute punishes “knowingly failing to register as a sex offender.” If the defendant reasonably believes, based on official notice, that his duty to register has expired, that mistake may be a defense.

Worked Example 1.6

A defendant reads the current printed version of a state statute that expressly allows carrying a particular type of knife. Unknown to him, the legislature repealed the provision last month, but the updated code has not yet been distributed. Relying on the statute, he carries the knife and is prosecuted under the new law.

Answer:
He may have a defense. This is a rare situation where mistake of law can excuse: he reasonably relied on what appeared to be an official statement of the law (the published statute), and the contrary law was not reasonably available. Due process concerns make punishment improper.

Key Term: Legal Wrong Doctrine
A doctrine under which, if a defendant’s mistake of fact would make them guilty of a lesser offense, some jurisdictions allow conviction for the more serious offense they actually committed. This doctrine is rarely central on the MBE; focus instead on whether the mistake negates the required mens rea.

Classifying Mistakes: Law or Fact

MBE questions often hinge on whether a misunderstanding is truly about law or fact. Use these cues:

• If the defendant misunderstands who, what, where, or when (ownership, age, identity, location), it is almost always a mistake of fact.
• If the defendant misunderstands what the law prohibits, the legal status of their conduct, or the validity of a statute, it is a mistake of law.

Crucially, some issues that sound legal are treated as factual for this purpose:

• Believing property belongs to you is treated as a fact question (ownership), not a mistake about the law of property.
• Believing you have permission from the owner is also a factual question, not a legal one.

When in doubt on the exam, ask:

• “Would we resolve this confusion by finding out what actually happened in the world (fact) or by reading a statute/case (law)?”

Additional Exam Applications

Mistake of Fact, Intoxication, and Mens Rea

Mistake of fact is one of several doctrines that can negate mens rea. Others include voluntary and involuntary intoxication and insanity.

• For specific intent crimes, involuntary or voluntary intoxication and unreasonable mistakes of fact can all be relevant.
• For general intent/malice crimes, only reasonable mistakes of fact and involuntary intoxication are potential defenses.

Do not double-count: if the fact pattern clearly frames the issue as mistake rather than intoxication, analyze under mistake rules.

Worked Example 1.7

A hunter fires into what he honestly believes is an empty field, having carefully checked for people. In fact, a hiker is hidden in tall grass and is killed. He is charged with depraved-heart (reckless) murder.

Answer:
He may have a defense if his mistake was reasonable. Malice murder requires at least reckless disregard of a substantial risk. If a reasonable person would also have believed the field was empty after similar precautions, his honest and reasonable mistake of fact means he did not consciously disregard a substantial risk. Under these facts, he might be guilty of negligence-based manslaughter instead, but not malice murder.

Exam Warning

On the MBE, do not confuse mistake of fact with mistake of law. Only mistake of fact can be a defense to most crimes, and only in the circumstances outlined above. Mistake of law is almost never a defense. If an answer choice says, “He did not know the law,” it is almost always wrong unless one of the narrow exceptions is expressly triggered.

Common pitfalls:

• Treating a specific intent crime as if only reasonable mistakes of fact count.
• Forgetting that statutory rape is strict liability and that no mistake of age defense is available.
• Calling a clearly factual misunderstanding (such as ownership or permission) a mistake of law.

Revision Tip

For specific intent crimes, always consider whether any mistake—reasonable or not—could negate the required intent. For general intent or malice crimes, check if the mistake was reasonable. For strict liability crimes, do not waste time analyzing mistake of fact; it is never a defense.

Key Point Checklist

This article has covered the following key knowledge points:

• Mistake of fact may be a defense to criminal liability only when it negates the required mens rea.
• For specific intent crimes, any genuine mistake of fact (reasonable or unreasonable) is a defense if it negates the specific intent.
• For general intent or malice crimes, only an honest and reasonable mistake of fact is a defense.
• For strict liability crimes, mistake of fact is never a defense.
• Mistake of law is almost never a defense; narrow exceptions exist where the law was not reasonably available, the defendant reasonably relied on an official statement, or knowledge of the law is an element of the offense.
• Correctly classifying a mistake as one of fact or law is essential; many exam traps hinge on this classification.
• Mistake doctrines operate by negating mens rea and must be analyzed alongside the type of crime charged.

Key Terms and Concepts

• Mens Rea
• Mistake of Fact
• Mistake of Law
• Specific Intent Crime
• General Intent Crime
• Malice Crime
• Strict Liability Crime
• Reasonable Mistake
• Unreasonable Mistake
• Ignorance of the Law
• Legal Wrong Doctrine