General principles - Justification and excuse

Learning Outcomes

This article explains key Criminal Law justification and excuse doctrines for the MBE, including:

• Clarifying the conceptual difference between justification (conduct treated as socially permissible) and excuse (actor treated as not blameworthy) and the implications for accomplice liability.
• Identifying the elements of self-defense for both deadly and non-deadly force, with emphasis on imminence, proportionality, retreat and castle doctrines, and the aggressor rule, including imperfect self-defense.
• Applying rules governing defense of others, defense of dwelling, and defense of other property, with attention to when deadly force is categorically prohibited.
• Analyzing the necessity (choice of evils) defense—its elements, interaction with legislative judgments, and typical homicide limitations tested on the bar.
• Explaining duress, contrasting it with necessity, and evaluating when threats can excuse underlying felonies yet fail as a defense to intentional homicide or felony-murder.
• Evaluating when mistake of fact, mistake of law, and both voluntary and involuntary intoxication can negate required mental states, differentiating specific-intent, general-intent, malice, and strict-liability offenses.
• Integrating these doctrines into MBE-style fact patterns, spotting common distractors, and prioritizing which issues to analyze first under time pressure.

MBE Syllabus

For the MBE, you are required to understand general principles of justification and excuse, with a focus on the following syllabus points:

• The conceptual difference between justification and excuse and their effect on liability of accomplices
• Self-defense: reasonable belief, imminence, proportionality, aggressor rules, imperfect self-defense, and retreat/castle doctrines
• Defense of others and how it tracks the rules of self-defense (majority “reasonable appearance” view)
• Defense of dwelling and defense of other property, including the ban on using deadly force solely to protect property
• Necessity (choice of evils): elements, reasonable belief requirement, “no fault” requirement, and homicide limitation
• Duress: threat requirements, immediacy, reasonable escape, interaction with homicide and felony-murder
• Mistake of fact: how it operates differently for specific intent, general intent/malice, and strict liability crimes
• Mistake of law: general rule, narrow exceptions, and interaction with specific-intent elements such as “willfully”
• Voluntary and involuntary intoxication: mental states affected, crimes to which they apply, and how examiners frame these issues
• Allocation of burdens: when the prosecution must disprove a defense once there is evidence to support it

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 defenses, if successful, results in an acquittal because the defendant's conduct, although criminal on its face, was socially acceptable under the circumstances?
   1. Insanity
   2. Duress
   3. Necessity
   4. Mistake of fact
2. A defendant uses non-deadly force against another person. Under the principles of self-defense, the defendant's actions are justified if:
   1. The defendant was the initial aggressor but later withdrew.
   2. The defendant reasonably believed such force was necessary to protect himself from imminent unlawful force.
   3. The other person had previously threatened the defendant.
   4. The defendant honestly but unreasonably believed force was necessary.
3. Voluntary intoxication may serve as a defense to which type of crime?
   1. Strict liability offenses
   2. General intent crimes
   3. Specific intent crimes
   4. Malice crimes

Introduction

Criminal liability requires both an unlawful act and the required mental state.

Key Term: Actus Reus
The voluntary physical act (or omission when there is a legal duty) that constitutes the external component of a crime.

Key Term: Mens Rea
The mental state required for a crime (such as purpose, knowledge, recklessness, negligence, malice, or strict liability).

Even when the prosecution proves both actus reus and mens rea beyond a reasonable doubt, the defendant may avoid conviction if a justification or excuse applies.

Key Term: Justification Defenses
Defenses where the defendant accepts responsibility for the act but argues the act was permissible or socially acceptable under the circumstances (e.g., self-defense, necessity).

Key Term: Excuse Defenses
Defenses where the defendant admits the act was wrong but argues that they are not personally blameworthy due to a condition such as insanity, duress, or involuntary intoxication.

A justified act is treated as lawful: anyone in the same situation would be entitled to act similarly, and accomplices can share the defense. An excused actor is forgiven, but the act remains wrongful; others who assisted may still be guilty.

On the MBE, justification and excuse questions commonly appear in homicide and theft fact patterns. Pay attention to:

• Imminence and proportionality of force
• Whether the defendant was at fault in creating the situation
• Whether the defense can apply to homicide

Once there is “some evidence” supporting a defense like self-defense, most jurisdictions require the prosecution to disprove that defense beyond a reasonable doubt. A few treat certain defenses (especially insanity) as affirmative, placing the burden on the defendant by a preponderance. Unless a question says otherwise, assume the prosecution bears the burden of disproving a raised defense.

Justification Defenses

Justification defenses render otherwise criminal conduct lawful and socially acceptable.

Self-Defense

Key Term: Self-Defense
The right to use reasonable force to protect oneself from the imminent use or threat of unlawful force by another, when the person reasonably believes such force is necessary.

Core elements:

• An actual, honest belief that force is necessary to repel
• A reasonable basis for that belief (objective component)
• Imminent unlawful force (not just past threats)
• Proportionality between the threat and the force used
• No disqualifying status as an aggressor, unless the right is regained

Non-Deadly Force

Key Term: Non-Deadly Force
Force that is not likely to cause death or serious bodily harm.

An individual who is not the aggressor may use such non-deadly force as they reasonably believe is necessary to protect against the imminent use of unlawful force. There is no duty to retreat before using non-deadly force.

Examples of non-deadly force include pushing someone away, grabbing an arm to stop a punch, or using mild objects to block an attack.

Deadly Force

Key Term: Deadly Force
Force that is intended or likely to cause death or serious bodily injury.

Deadly force in self-defense is more restricted. A person may use deadly force only if they:

• Actually believe deadly force is necessary to prevent imminent death or serious bodily harm; and
• That belief is reasonable under the circumstances.

Imminence means the threat is about to occur—future threats (“I’ll get you next month”) and retaliation for past harm are not enough.

Key Term: Retreat Doctrine
The rule in some jurisdictions that requires a person to retreat, if safely possible, before using deadly force in self-defense.

Key Term: Castle Doctrine
The principle that there is no duty to retreat from one’s own home before using deadly force, if otherwise justified.

Retreat rules:

• Majority rule: No duty to retreat before using deadly force, even in public, as long as the defendant is otherwise entitled to use deadly force.
• Minority rule (retreat jurisdictions): A person must retreat before using deadly force if they can do so safely, except:
  • When in their own dwelling under the castle doctrine
  • When making a lawful arrest
  • When a victim of a violent felony (varies by statute)

The MBE usually identifies a “retreat jurisdiction” expressly. If it does not, assume the majority no-retreat rule, subject to the castle doctrine.

The Aggressor Rule

Key Term: Aggressor Rule
The rule that an initial aggressor in a confrontation generally loses the right to claim self-defense until they effectively withdraw or the victim escalates the conflict.

An initial aggressor is one who first uses or threatens force. Words alone are usually not enough; there must be an overt threat or physical act.

Initial aggressors cannot use self-defense unless:

1. Complete withdrawal: They in good faith withdraw from the conflict and clearly communicate that withdrawal, and the other party continues or resumes the confrontation; or
2. Escalation by the victim: The aggressor started a non-deadly confrontation, and the other party suddenly responds with disproportionate deadly force, leaving the aggressor no chance to withdraw.

If the aggressor began with deadly force, they must withdraw and communicate that withdrawal before regaining the right to self-defense.

Reasonable Belief and Imperfect Self-Defense

A defendant is entitled to self-defense if their belief in the need to use force is both honest and reasonable.

• A reasonable mistake of fact about the existence of danger does not defeat self-defense.
• An unreasonable but honest belief in the need for deadly force does defeat full self-defense in most jurisdictions, but some recognize:

Key Term: Imperfect Self-Defense
A partial defense where the defendant honestly but unreasonably believes deadly force is necessary; it usually reduces murder to voluntary manslaughter.

Imperfect self-defense does not lead to an acquittal but mitigates the offense by eliminating malice.

Special Situations in Self-Defense

• Battered partner cases: Some jurisdictions admit expert testimony on battered spouse or intimate partner syndrome to evaluate the reasonableness of the defendant’s perception of imminent danger. The MBE still expects an imminence requirement; “eventual harm” is insufficient.
• Resisting unlawful arrest: Traditionally, a person could use reasonable non-deadly force to resist an unlawful arrest. Modern trend (and likely MBE approach) limits this sharply; do not assume a broad right to resist unless the question highlights it.

Worked Example 1.1

A, in a bar, loudly insults B. B stands up, clenches his fists, and moves toward A saying, “I’m going to smash your face right now.” A, who is sitting, quickly pushes B away, causing B to fall and suffer a broken wrist. A is charged with battery and claims self-defense.

Answer:
A can successfully claim self-defense. A reasonably believed that B was about to use imminent unlawful force: the clenched fists, proximity, and explicit threat support this. A used non-deadly force (a push) that was proportionate to the threatened harm. A was not the initial aggressor; insults alone do not make someone an aggressor. No duty to retreat applied before using non-deadly force.

Defense of Others

Key Term: Defense of Others
A justification allowing a person to use force to protect another when the defender reasonably believes the other person is entitled to use self-defense.

Modern majority view: A person may use the same degree of force that would be justified in self-defense if:

• They actually and reasonably believe the person aided faces imminent unlawful force; and
• They use no more force than would be justified for that person.

Key Term: Defense of Dwelling
The limited right to use force to prevent or terminate an unlawful entry or attack on one’s dwelling; deadly force is only permitted in narrow circumstances.

Key Term: Defense of Property
The right to use reasonable, non-deadly force to protect property from imminent unlawful interference; deadly force may not be used solely to protect property.

Key points:

• No special relationship is required with the person aided.
• A reasonable mistake about who is the aggressor does not defeat the defense under the majority “reasonable appearance” rule.
• A minority “step into the shoes” rule allows defense only if the person aided actually had a valid self-defense claim, regardless of the defender’s reasonable mistake. The MBE typically uses the majority rule unless the minority is highlighted.

Worked Example 1.2

C sees D apparently punching E in the face. Believing E is being attacked, C tackles D to stop the fight, breaking D’s arm. In fact, D was lawfully acting in self-defense against E, who had attacked first. C is charged with battery.

Answer:
Under the majority “reasonable appearance” rule, C can claim defense of others if his belief that E was the victim was reasonable. The facts (C saw D punching E and nothing more) support a reasonable belief that E was being unlawfully attacked. Even though C was mistaken about who was the true aggressor, the mistake is reasonable, so the defense applies. Under the minority “step into the shoes” rule, the defense would fail because E did not actually have a right to self-defense.

Defense of Dwelling and Defense of Property

For dwelling:

• Non-deadly force may be used to prevent or terminate an unlawful entry or attack on the home if reasonably necessary.
• Deadly force may be used to prevent a violent entry when the resident reasonably believes:
  • The intruder intends to commit a felony inside, and
  • The felony poses a threat to occupants (e.g., burglary with potential violence), or
  • The intruder threatens occupants with deadly force.

Merely entering to commit a non-violent property offense is generally not enough for deadly force.

For other property:

• Reasonable non-deadly force can be used to prevent imminent unlawful interference (theft, vandalism, trespass).
• Deadly force may never be used solely to protect property.
• A request to desist is usually required if practical.
• Force to recapture property is limited: it is allowed only in immediate pursuit of someone who has just dispossessed the owner.

Mechanical devices:

• Setting deadly traps (e.g., a spring gun in an empty building) is improper because the owner could not lawfully use such deadly force if present.

Worked Example 1.3

Owner of a warehouse that has been repeatedly burglarized rigs a shotgun to fire at anyone opening a side door. At night, Thief breaks in and is shot. Owner is charged with aggravated assault and asserts defense of property.

Answer:
Owner cannot rely on defense of property. Deadly force may not be used solely to protect property, and the use of a mechanical trap is treated as if the owner personally used the force. Even if the property has been repeatedly burglarized, there was no immediate threat to human life at the moment the trap was triggered. The defense fails.

Necessity (Choice of Evils)

Key Term: Necessity
A justification defense where the defendant commits an otherwise criminal act to avoid a greater harm, reasonably believing the conduct is necessary and choosing the lesser of two evils.

Elements (common law formulation):

• The defendant reasonably believed their conduct was necessary to avoid an imminent harm;
• The harm avoided was greater than the harm caused by violating the law;
• No adequate lawful alternative existed; and
• The defendant was not at fault in creating the situation.

Necessity is sometimes called the “choice of evils” defense. Historically, it applied mainly to natural forces (storms, fires), but modern doctrines often extend it to a range of emergencies, subject to statutes.

Limitations:

• Not available if the legislature has weighed the competing harms and clearly chosen to forbid the conduct regardless (e.g., statutory schemes making certain conduct absolutely prohibited).
• Generally not a defense to intentional homicide at common law: one may not kill an innocent person to save oneself or others. The Model Penal Code is more permissive, but the MBE follows common law unless the question specifies otherwise.
• The harm must be imminent—not merely speculative or long-term.

Worked Example 1.4

David is hiking when a sudden blizzard traps him in the mountains. Freezing and lost, he breaks into an unoccupied cabin to take shelter and finds emergency supplies, which he consumes. The cabin owner later prosecutes David for burglary and larceny. David asserts the defense of necessity. Is David likely to succeed?

Answer:
Yes. David reasonably believed that breaking into the cabin and consuming supplies was necessary to avoid a greater harm—death from exposure and starvation. The harm to property is less than the threat to human life. David did not create the blizzard or put himself recklessly in harm’s way. Necessity justifies his conduct, though he may still be liable civilly for damage to the property.

Exam Tip: Necessity vs. Self-Defense vs. Duress

• Self-defense: Responds to unlawful force by another person directed at the defendant or others.
• Necessity: Responds to circumstances (often natural forces) and involves balancing harms.
• Duress: Responds to unlawful human threats compelling the defendant to commit a crime.

The same facts may suggest more than one defense; on the MBE, choose the doctrine that fits the question’s emphasis.

Excuse Defenses

Excuse defenses concede that the act was wrongful but argue that the defendant should not be held criminally responsible.

Duress

Key Term: Duress
An excuse defense where the defendant commits a crime because of a threat by another person of imminent death or serious bodily harm, and the defendant reasonably believes the threat will be carried out and has no reasonable escape.

Typical elements:

• An immediate or imminent threat of death or serious bodily harm
• Threat directed at the defendant or a close family member; many jurisdictions extend to threats against any person
• A well-grounded, reasonable fear that the threat will be carried out
• No reasonable opportunity to escape or seek help
• The defendant did not recklessly place themselves in the situation where duress became likely

Key limitation:

• At common law, duress is not a defense to intentional homicide. A defendant cannot kill an innocent third party to save their own life. Some jurisdictions allow duress to mitigate murder to manslaughter or to operate as a defense to felony-murder, but the standard MBE position is that duress cannot excuse intentional killing.

Necessity vs. duress:

• Source of pressure: Duress involves human threats; necessity typically involves circumstances.
• Moral structure: Necessity involves choosing the lesser evil; duress involves coercion where a third party dictates the wrongful act.

Worked Example 1.5

F, a gang leader, threatens to kill G immediately unless G drives a getaway car for an armed robbery. Believing F will carry out the threat, G drives the car. During the robbery, F shoots and kills the store owner. G is charged with robbery and felony-murder. G raises duress.

Answer:
Duress is a complete defense to the robbery charge if G reasonably believed he faced an imminent threat of death and had no reasonable escape. However, duress is not a defense to intentional homicide at common law. Most jurisdictions also do not allow duress as a defense to felony-murder, though some might treat it as mitigating. For MBE purposes, G likely remains liable for felony-murder despite duress.

Mistake of Fact

Key Term: Mistake of Fact
A misunderstanding or ignorance about a factual circumstance that negates the mental state required for the offense.

How mistake of fact operates depends on the type of crime.

Key Term: Specific Intent Crime
A crime that requires not only doing the act, but doing it with an additional, particular purpose (e.g., intent to permanently deprive, intent to kill, intent to commit a felony once inside a dwelling).

Key Term: General Intent Crime
A crime that requires only the intent to do the prohibited act (or recklessness or negligence as defined), without any further special purpose.

Key Term: Strict Liability Crime
A crime that does not require proof of any mental state regarding at least one element; mistake defenses cannot negate liability as to that element.

Rules:

• Specific intent crimes: Any honest mistake of fact, reasonable or unreasonable, is a defense if it negates the specific intent. Examples: larceny, burglary, attempt, conspiracy, robbery, embezzlement, solicitation.
• General intent or malice crimes: Only a reasonable mistake of fact is a defense. Malice crimes (e.g., common-law murder, arson) require a reckless or depraved mental state; unreasonable mistakes generally do not excuse them.
• Strict liability crimes: Mistake of fact is never a defense, whether reasonable or not, because no mental state is required on the relevant element (e.g., statutory rape, certain regulatory offenses).

Worked Example 1.6

Clara takes an umbrella from a restaurant stand, honestly but unreasonably believing it is hers. The umbrella actually belongs to Ben. Clara is charged with larceny (a specific intent crime requiring intent to permanently deprive another of their property). Is mistake of fact a defense?

Answer:
Yes. Larceny requires specific intent to permanently deprive another of their property. Clara’s honest belief that the umbrella was hers, though unreasonable, negates the intent to take “another’s” property. For specific intent crimes, any mistake—reasonable or not—is a defense if it negates the required specific intent.

Mistake of Law

Key Term: Mistake of Law
A misunderstanding or ignorance about what the law prohibits or requires; generally not a defense.

General rule: Ignorance of the law is no excuse. People are presumed to know the criminal law.

Narrow exceptions (often tested):

• Statute not reasonably available: If the law was not published or made reasonably available, some courts allow a defense.
• Reasonable reliance on an official statement: Reliance on a statute or judicial decision later declared invalid, or on an official interpretation by someone charged with enforcing the law (such as the Attorney General) can excuse. Reliance on private counsel’s advice does not qualify.
• Knowledge of the law as an element: When a crime’s definition requires that the defendant know the law (e.g., “willfully” failing to file a tax return), a mistake of law may negate that specific intent.

Intoxication

Intoxication interacts with mens rea rather than directly justifying or excusing an act.

Key Term: Voluntary Intoxication
Intoxication produced by the defendant’s knowing and purposeful consumption of an intoxicant, without duress or medical necessity.

Key Term: Involuntary Intoxication
Intoxication produced without the defendant’s knowledge of the intoxicant’s nature, under duress, or pursuant to medical advice without awareness of the intoxicating effect.

Voluntary Intoxication

Voluntary intoxication is not a defense to:

• General intent crimes
• Malice crimes (e.g., murder, arson)
• Strict liability offenses

It may be a defense to specific intent crimes if the intoxication is so extreme that the defendant could not form the required specific intent (e.g., intent to permanently deprive, intent to kill with premeditation). Mere drunkenness is insufficient; the intoxication must actually negate the mental state.

It is not a defense if the defendant became intoxicated to commit the crime (e.g., “I got drunk to work up the courage to kill him”).

Involuntary Intoxication

Involuntary intoxication is treated like insanity:

• It is a defense to all crimes if, due to the intoxication, the defendant meets the jurisdiction’s insanity standard (such as being unable to appreciate the nature or wrongfulness of their acts).
• Common examples include being drugged without knowledge or coerced into taking intoxicants.

Worked Example 1.7

H, after voluntarily consuming a large amount of alcohol, breaks into a store at night intending to sleep inside because it is warm. He takes nothing and is charged with burglary (breaking and entering a dwelling at night with intent to commit a felony inside). He argues he was too drunk to form intent.

Answer:
Voluntary intoxication may be a defense to burglary if H’s intoxication prevented him from forming the specific intent to commit a felony inside. If the jury believes he only intended to sleep and not to commit a felony, burglary is not established. He may still be guilty of a lesser offense such as criminal trespass, which requires only general intent.

Exam Warning

Distinguish carefully between Necessity and Duress:

• Necessity:

  • Usually arises from circumstances (often natural forces).
  • Involves choosing the lesser of two evils.
  • Is a justification—the act is treated as correct under the circumstances.
  • Generally unavailable for homicide.

• Duress:

  • Arises from human threats.
  • Excuses the defendant’s conduct because their will was overborne.
  • Is an excuse—the act remains wrongful, but the actor is not blameworthy.
  • Generally not a defense to intentional homicide.

On the MBE, when both might plausibly apply, pay attention to whether the question mentions threats by a person (duress) or an external emergency (necessity).

Key Point Checklist

This article has covered the following key knowledge points:

• Justification defenses (e.g., self-defense, necessity) treat the act as permissible under the circumstances; excuse defenses (e.g., duress, insanity) treat the actor as not blameworthy.
• Self-defense requires an honest and reasonable belief in imminent unlawful force; deadly force is limited to threats of death or serious bodily harm and is subject to retreat rules and the castle doctrine.
• Initial aggressors lose the right to self-defense but may regain it by complete withdrawal or where the victim escalates to deadly force.
• Defense of others generally uses the same rules as self-defense, with the majority adopting a reasonable appearance standard for mistakes.
• Defense of dwelling allows deadly force only to stop violent entry accompanied by threats to occupants; defense of property never permits deadly force solely to protect property.
• Necessity requires a reasonable belief that the criminal act avoids a greater imminent harm, that no lawful alternative exists, and that the defendant is not at fault in creating the situation; it is generally unavailable for intentional homicide.
• Duress requires a threat of imminent death or serious bodily harm, a well-grounded fear, and no reasonable escape; it excuses many crimes but not intentional homicide.
• Mistake of fact is a defense when it negates the required mental state: any mistake for specific intent crimes; only reasonable mistakes for general intent/malice; never for strict liability.
• Mistake of law is almost never a defense, except for reliance on official statements or when knowledge of the law is itself an element.
• Voluntary intoxication can negate specific intent but not general intent, malice, or strict liability; involuntary intoxication can operate like insanity and is a defense to all crimes if the standard is met.

Key Terms and Concepts

• Actus Reus
• Mens Rea
• Justification Defenses
• Excuse Defenses
• Self-Defense
• Deadly Force
• Non-Deadly Force
• Retreat Doctrine
• Castle Doctrine
• Aggressor Rule
• Imperfect Self-Defense
• Defense of Others
• Defense of Dwelling
• Defense of Property
• Necessity
• Duress
• Mistake of Fact
• Specific Intent Crime
• General Intent Crime
• Strict Liability Crime
• Mistake of Law
• Voluntary Intoxication
• Involuntary Intoxication