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Criminal Justice & Criminology · Criminal Law Foundations

Criminal Defenses

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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

A criminal defense is not one kind of argument. Justifications say the conduct was permissible in the circumstances, so the law does not condemn it: self-defense, necessity, lawful authority. Excuses concede the act was wrongful but deny that this actor deserves blame: duress, insanity, infancy. A third group is not an at all, because it simply denies the prosecution proved an element. Which category a claim belongs to drives who must prove what. This lesson is educational material about United States law, not legal advice.

Why this matters

Sorting defenses into , , and failure of proof is the move that makes the rest of criminal law legible. It tells you what the defendant is conceding, what the prosecution still has to establish, and who carries the burden if the jury ends up unsure. Courses in criminal law, criminal procedure, and courts all assume you can make that sort. It also disciplines public argument: debates about stand-your-ground laws, the insanity defense, and police use of force are usually debates about the boundaries of specific justifications and excuses, and they go badly when the categories are blurred. Knowing the structure lets you evaluate a claim about the law instead of absorbing it.

The college version

Two different claims, not one

Every defense answers the charge, but they do not answer it the same way. A justification asserts that the conduct itself was permissible given the circumstances. The defendant who kills an attacker in lawful self-defense is not asking for mercy; the claim is that the law does not condemn what happened. An excuse concedes the act was wrongful and argues instead that this particular actor should not be blamed for it, because of coercion, immaturity, or a mental condition the law treats as undermining responsibility. Justification looks at the offense; excuse looks at the defendant. The distinction is not merely academic bookkeeping. It shapes whether bystanders may lawfully assist the actor, whether the conduct can be resisted, whether civil liability follows, and how legislatures write the statute. It also explains why two defendants who did exactly the same thing can be acquitted on completely different reasoning, and why the law is comfortable acquitting one while still saying the underlying conduct was wrong.

Failure of proof is not an affirmative defense

A large share of what defendants actually argue is not a defense in the technical sense at all. Under the general rule of American criminal procedure the prosecution must prove every element of the charged offense beyond a reasonable doubt, a requirement the Supreme Court stated in In re Winship (1970) and that the elements-of-a-crime lesson develops. A simply asserts that the prosecution has not done so. Nothing new is introduced; the defendant denies an element and may present evidence, or may present none and argue the state's case is insufficient. An affirmative defense is different in kind, because it raises new matter that goes beyond a denial. Alibi sits on the failure-of-proof side: a claim of having been somewhere else is a denial that the defendant did the act. Even so, procedure can attach obligations to it. Federal Rule of Criminal Procedure 12.1 lets the government demand written notice of an alibi, and a court may exclude an undisclosed alibi witness, though not the defendant's own testimony.

Self-defense: reasonable belief, imminence, proportionality

Self-defense and defense of others are the paradigm justifications, and every American formulation is built from a few recurring requirements. There must be a belief that force is necessary, and that belief is measured against a reasonableness standard rather than sincerity alone. The threatened harm must be imminent, not past and not merely expected someday; a fear of future violence, however well founded, does not by itself justify present force. The force used must be proportionate, so deadly force is generally reserved for threats of death or serious bodily harm. Most jurisdictions also deny or limit the defense to an initial aggressor. New York Penal Law section 35.15 illustrates the drafting: it authorizes defensive physical force where the actor reasonably believes it necessary against what he reasonably believes to be the imminent use of unlawful physical force, and then imposes tighter conditions on deadly force. Where the requirements are met only partially, some jurisdictions recognize an imperfect defense that reduces the offense rather than producing an acquittal.

Retreat, standing your ground, and the castle doctrine

The sharpest state-to-state split concerns whether a person must avoid deadly force by retreating when retreat is safely available. Duty-to-retreat jurisdictions say yes, with exceptions. New York Penal Law section 35.15(2)(a) bars deadly physical force where the actor knows he can avoid the necessity with complete personal safety by retreating, then carves out the actor's own dwelling when he is not the initial aggressor, and officers acting under section 35.30. Stand-your-ground jurisdictions reject the duty outright. Florida Statutes section 776.012(2) provides that a person using or threatening deadly force in the circumstances the statute describes does not have a and has the right to stand his or her ground, so long as he is not engaged in criminal activity and is in a place where he has a right to be. The is a third, older idea about the home specifically. California Penal Code section 198.5 goes further than removing a retreat duty: it presumes a resident held a reasonable fear of imminent peril of death or great bodily injury against someone who unlawfully and forcibly entered the residence, where the resident knew or had reason to believe the entry occurred and the intruder is not a household or family member. These are three distinct rules, and which one applies depends entirely on the jurisdiction.

Property, necessity, and lawful authority

Beyond defense of persons, the justification family includes several narrower doctrines. Defense of property permits reasonable non-deadly force, but the Model Penal Code and most states do not authorize deadly force to protect property other than the home; the home is handled by the separate habitation doctrines described above. Necessity, often called choice of evils, justifies breaking a law to avert a greater harm, and requires that the harms be rankable, that the avoided harm be imminent, and that no lawful alternative exist. Courts apply it sparingly and generally refuse it where a life is on the lower side of the ledger. Law enforcement authority justifies force officers could not otherwise use, but the authority is bounded. In Tennessee v. Garner (1985) the Supreme Court held that deadly force may not be used against an apparently unarmed, nondangerous fleeing suspect unless it is necessary to prevent escape and the officer has probable cause to believe the suspect poses a significant threat of death or serious physical injury.

Duress: the excuse of coerced choice

Duress is the clearest excuse. The defendant admits doing the act with the required mental state and argues that a coercive threat left no genuine choice. The conventional elements are a threat of imminent serious bodily injury or death to the defendant or another, an objectively reasonable belief that committing the offense was the only way to avoid it, and — in most jurisdictions — that the offense charged is not criminal homicide. California Penal Code section 26 codifies a version of this, excluding capital crimes. Duress is often confused with necessity, and the traditional line, traced in United States v. Bailey (1980), is the source of the pressure: duress responds to a human threat, necessity to circumstances or natural forces. Bailey noted that modern cases blur the distinction, but identified one principle that survives under any definition: if a reasonable legal alternative to violating the law existed, the defense fails. In the prison-escape context, Bailey required evidence of a bona fide effort to surrender once the coercion lost its force.

Insanity: four tests, one federal statute, and Kahler

No defense is more misunderstood. American jurisdictions have used four main formulations. M'Naghten's Case (1843) supplies two alternative prongs: cognitive incapacity, meaning the defendant did not know the nature and quality of the act, and moral incapacity, meaning he did not know it was wrong. The irresistible impulse or volitional test was added by some states to cover a defendant who knew the act was wrong but could not restrain himself. The Model Penal Code substantial-capacity test asks whether mental disease or defect left the defendant lacking substantial capacity either to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of law; Hawaii Revised Statutes section 704-400(1) is a verbatim example. The Durham or product test, from Durham v. United States (1954), asked only whether the act was the product of mental disease or defect; the D.C. Circuit abandoned it in 1972, and New Hampshire is now the only state using a product formulation. Federally, the standard is set by statute. Chapter IV of Title II of Public Law 98-473, enacted October 12, 1984, may be cited as the Insanity Defense Reform Act of 1984; it created what is now 18 U.S.C. 17 and amended Federal Rule of Criminal Procedure 12.2. Under section 17 it is an affirmative defense that a severe mental disease or defect left the defendant unable to appreciate the nature and quality or the wrongfulness of his acts; mental disease or defect is not otherwise a defense, and the defendant must prove insanity by clear and convincing evidence. A handful of states go further. In Kahler v. Kansas (2020) the Court held 6-3 that due process does not require a state to adopt a test turning on the ability to recognize that the crime was morally wrong. Kansas lets mental illness show the defendant lacked the culpable mental state that is an element, and lets him raise mental illness at sentencing, but recognizes no separate moral-incapacity acquittal. A majority footnote identifies four other states with a comparable approach: Alaska, Idaho, Montana, and Utah. Justice Breyer's dissent, with a state-by-state appendix, counted 45 states, the federal government, and the District of Columbia as retaining a blameworthiness inquiry as of 2020.

Infancy, intoxication, and the two kinds of mistake

The remaining excuses are narrower than students expect. Infancy at common law made a child under seven incapable of crime and created a rebuttable presumption between seven and fourteen; modern statutes vary widely, and California Penal Code section 26 still lists children under fourteen as incapable absent clear proof they knew the act's wrongfulness. Involuntary intoxication, where a substance was taken unknowingly or under force or fraud, can operate as an excuse. Voluntary intoxication is far weaker. In Montana v. Egelhoff (1996) the Supreme Court reversed a state ruling that due process entitled a defendant to have the jury weigh voluntary intoxication when determining a mental-state element, the plurality relying on the firmly established common-law rule that voluntary intoxication was neither excuse nor justification. Mistake divides in two. Mistake of fact can negate a required mental state, which makes it a failure-of-proof argument rather than an excuse, and it cannot help against a strict liability element. Mistake of law is generally no defense at all. The exceptions are narrow: reasonable reliance on an official statement of the law later held invalid; statutes whose own terms make knowledge an element, as in Cheek v. United States (1991), where a good-faith misunderstanding of the tax code negated willfulness whether or not it was reasonable, though a belief that the tax laws are unconstitutional was held irrelevant; and the notice-based limit of Lambert v. California (1957), which set aside a felon-registration conviction as applied to someone with no actual knowledge of the duty where no probability of knowledge was shown.

Who proves what, and what ends a case without a verdict

Two burdens must be kept apart. The is the obligation to put enough evidence forward to get an issue submitted to the jury at all. The is the obligation to convince the factfinder to a stated standard. Defendants nearly always carry production on an affirmative defense. Persuasion is the contested part, and its allocation varies by defense and by jurisdiction. Patterson v. New York (1977) upheld requiring a defendant to prove extreme emotional disturbance by a preponderance, reasoning that Winship governs elements rather than affirmative defenses that do not negate one. Martin v. Ohio (1987) applied that to self-defense, while confirming the state must still prove every element of aggravated murder beyond a reasonable doubt. Dixon v. United States (2006) placed the duress burden on federal defendants by a preponderance, reasoning from the common-law background and presumed congressional intent rather than announcing a constitutional floor, and repeating that duress does not negate the mental state. Federal insanity carries the heavier clear-and-convincing burden by statute. Separately, procedural bars can end a case without any merits verdict: 18 U.S.C. 3282 gives most non-capital federal offenses a five-year limitations period unless otherwise provided, and double jeopardy analysis for one act charged under two provisions uses the Blockburger v. United States (1932) same-elements test, asking whether each provision requires proof of a fact the other does not. This lesson describes doctrine at a general level and is educational material, not legal advice.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine three different things a person can say when accused of breaking a rule. The first is 'I didn't do it' — that is not really a defense, it is a denial, and the person accusing you still has to prove you did. The second is 'I did it, and doing it was the right call.' The third is 'I did it, it was the wrong thing, but I was not in a position where you can fairly blame me.' Lawyers call the second one a justification and the third one an excuse. Almost every defense in criminal law is one of those three shapes, and figuring out which shape you are looking at tells you who has to convince the jury.

Picture it like this

Think about a door with an alarm marked emergency exit only. Pushing it is normally against the rules. If the building is on fire and you push it to get people out, nobody says you broke the rule and got forgiven — pushing it was the correct thing to do. That is a justification. Now imagine someone sleepwalking who pushes the same door at 3 a.m. Opening it was still the wrong thing, but you would not punish a sleeping person for it. That is an excuse.

Where the picture stops working

The analogy stops at the courthouse door. A fire is obvious to everyone in the building, while real justifications turn on precisely defined elements — reasonable belief, imminence, proportionality — that a jury has to find. There is no alarmed door where the rules differ from state to state, but American defenses genuinely do: the same act can be justified in one state and not in the next. And a building has no rule about who must prove what, whereas in court the allocation of the burden often decides the case.

Worked example

A prosecutor charges Reyes with aggravated assault. Reyes says a neighbor advanced on him with a raised pipe in a parking lot and he swung first. Work the analysis in order. Is this a denial or new matter? It is new matter, so it is an affirmative defense, not failure of proof. Justification or excuse? Justification: the claim is that the force was permissible, not that Reyes was unblameworthy. Now the elements. Did Reyes reasonably believe force was necessary? Was the threat imminent, meaning a raised pipe and not a past insult? Was the force proportionate to a threat of serious bodily harm? Was Reyes the initial aggressor? Then the jurisdictional question that decides many real cases: does this state impose a duty to retreat where withdrawal was completely safe, as New York does, or has it adopted stand-your-ground, as Florida has? Finally, burdens. Reyes must produce enough evidence to get the instruction. Whether he must also persuade the jury by a preponderance, as Martin v. Ohio permits a state to require, or whether the prosecution must disprove self-defense once raised, depends on the state. Change the facts so the pipe was raised a week earlier and imminence fails, and the justification collapses no matter which state you are in.

Key takeaway

Sort the claim before you evaluate it: a failure-of-proof defense denies an element, a justification says the conduct was permissible, and an excuse concedes wrongfulness while denying blame. That sort, plus the jurisdiction, determines both the elements the defendant must satisfy and who bears the burden of persuasion.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

What is the analytic difference between a justification and an excuse?

Choose an answer, then check it.
Question 2 of 3intermediate

A defendant charged with burglary presents witnesses placing her in another city at the time of the break-in. How is this claim best classified?

Choose an answer, then check it.
Question 3 of 3foundational

Under 18 U.S.C. 17, the federal insanity provision enacted in 1984, what must a federal defendant establish and to what standard?

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Practice all 5

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Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Distinguish a justification from an excuse, and explain why the distinction changes what the defendant is conceding.
  • Explain why a failure-of-proof defense and an alibi are not affirmative defenses.
  • Analyze a self-defense claim using reasonable belief, imminence, proportionality, and the jurisdiction's retreat rule.
  • Compare the M'Naghten, irresistible impulse, Model Penal Code, and Durham insanity tests, and state the federal standard under 18 U.S.C. 17.
  • Evaluate how the burden of production and the burden of persuasion are allocated, and why the allocation varies by defense and by jurisdiction.
  • Assess claims about how often the insanity defense is raised and how often it succeeds against published data.

Common mistakes

  • Treating every defense as an admission plus a story, so that raising any defense concedes the act.

    Failure-of-proof defenses and alibi concede nothing; they deny that the prosecution has proved an element. Only affirmative defenses introduce new matter beyond a denial, and only excuses actually concede wrongfulness.

  • Believing the insanity defense is a common and reliable route to acquittal.

    An eight-state study by Callahan and colleagues, published in 1991, found the defense was raised in about one percent of felony cases and that roughly 26 percent of those pleas resulted in acquittal. The figures are dated and cover eight states, but the pattern — rarely raised, and usually unsuccessful when raised — is what the data support.

  • Using stand-your-ground, castle doctrine, and duty to retreat as if they were the same rule, or as if one of them were the national rule.

    They are three distinct rules and there is no national rule. Compare New York's retreat requirement, Florida's stand-your-ground provision, and California's residential presumption of reasonable fear: each answers a different question, and which applies depends on the jurisdiction.

  • Assuming the defendant always bears the burden of persuasion on a defense, or that the prosecution always must disprove it.

    Allocation varies by defense and by jurisdiction. Patterson and Martin permit states to put persuasion on the defendant for some affirmative defenses; Dixon put duress on federal defendants by a preponderance; 18 U.S.C. 17 requires clear and convincing evidence for federal insanity, and many states place the burden differently.

  • Reasoning that because a mistake of fact can defeat a charge, a mistake about the law should too.

    Mistake of fact works by negating a required mental state, which is why it fails against strict liability elements. Mistake of law is generally no defense at all, with narrow exceptions such as reasonable reliance on an official statement of law, statutes that make knowledge of illegality an element as in Cheek, and the notice-based limit recognized in Lambert.

Easily confused

Justification vs. Excuse

A justification says the conduct itself was permissible, so there is nothing for the law to condemn. An excuse concedes the conduct was wrongful and argues only that this actor is not to blame. Justification evaluates the offense; excuse evaluates the defendant.

Affirmative defense vs. Failure-of-proof defense

An affirmative defense raises new matter beyond a denial and typically carries a production burden and sometimes a persuasion burden for the defendant. A failure-of-proof defense negates an element and rides entirely on the prosecution's own burden.

Duress vs. Necessity (choice of evils)

Traditionally duress responds to a human threat and necessity to circumstances or natural forces, as United States v. Bailey traced. Modern cases blur the line, but both fail where a reasonable legal alternative to breaking the law existed.

Duty to retreat vs. Stand your ground

New York bars deadly force where the actor knows he can retreat in complete personal safety, subject to a dwelling exception. Florida provides there is no duty to retreat for a person not engaged in criminal activity who is where he has a right to be. Same facts, different outcomes.

Insanity as an affirmative defense vs. Mental illness offered only to negate mens rea

Under 18 U.S.C. 17 and in most states, insanity excuses a defendant who could not appreciate wrongfulness even though the mental state element was satisfied. Kansas and four other states allow mental illness only to show the culpable mental state was absent; Kahler v. Kansas held due process does not forbid that choice.

Mistake of fact vs. Mistake of law

Mistake of fact can negate a required mental state and therefore functions as a failure-of-proof argument, though it cannot defeat a strict liability element. Mistake of law is generally no defense, with narrow exceptions turning on official statements, statutory knowledge elements, or fair notice.

Key vocabulary

Justification
A defense asserting that conduct the law normally forbids was permissible in the circumstances, so the legal system does not condemn what the actor did.
Excuse
A defense conceding that the conduct was wrongful while denying that this actor is blameworthy for it, because of coercion, immaturity, or a qualifying mental condition.
Affirmative defense
A claim that introduces new matter beyond a denial of the charge, which the defendant must raise and, in many jurisdictions, prove to a specified standard.
Failure-of-proof defense
An argument that the prosecution has not established some element of the offense, so it negates the charge rather than adding a separate justifying or excusing claim.
Burden of production
The obligation to put enough evidence before the court that an issue is submitted to the jury at all, distinct from convincing anyone of it.
Burden of persuasion
The obligation to convince the factfinder of a proposition to a stated standard, such as preponderance of the evidence, clear and convincing evidence, or beyond a reasonable doubt.
Imminence
The requirement that a threatened harm be about to occur rather than already finished or merely anticipated at some indefinite future point.
Duty to retreat
A rule requiring a person who can withdraw in complete safety to do so before using deadly defensive force, subject to exceptions such as the person's own dwelling.
Castle doctrine
A set of rules treating the home as a place where an occupant need not withdraw, and which in some states presumes reasonable fear against a forcible unlawful intruder.
M'Naghten test
An insanity standard asking whether mental disease left the defendant unable to know the nature and quality of the act, or unable to know that the act was wrong.

Sources & references

  1. Kahler v. Kansas, 589 U.S. 271 (2020) (slip opinion, No. 18-6135) — Supreme Court of the United States
  2. 18 U.S.C. 17 - Insanity defense — Office of the Law Revision Counsel, U.S. House of Representatives
  3. 18 U.S.C. 4241 - Determination of mental competency to stand trial (with the short-title note for the Insanity Defense Reform Act of 1984) — Office of the Law Revision Counsel, U.S. House of Representatives
  4. Patterson v. New York, 432 U.S. 197 (1977) — U.S. Government Publishing Office (govinfo), United States Reports
  5. Martin v. Ohio, 480 U.S. 228 (1987) — U.S. Government Publishing Office (govinfo), United States Reports
  6. Dixon v. United States, 548 U.S. 1 (2006) — U.S. Government Publishing Office (govinfo), United States Reports
  7. United States v. Bailey, 444 U.S. 394 (1980) — Library of Congress (United States Reports, volume 444)
  8. In re Winship, 397 U.S. 358 (1970) — U.S. Government Publishing Office, United States Reports volume 397 (govinfo)
  9. Cheek v. United States, 498 U.S. 192 (1991) — U.S. Government Publishing Office (govinfo), United States Reports
  10. 26 U.S.C. 7203 - Willful failure to file return, supply information, or pay tax — Office of the Law Revision Counsel, U.S. House of Representatives
  11. Lambert v. California, 355 U.S. 225 (1957) — U.S. Government Publishing Office (govinfo), United States Reports
  12. Montana v. Egelhoff, 518 U.S. 37 (1996) — U.S. Government Publishing Office (govinfo), United States Reports
  13. Tennessee v. Garner, 471 U.S. 1 (1985) — U.S. Government Publishing Office (govinfo), United States Reports
  14. Blockburger v. United States, 284 U.S. 299 (1932) — U.S. Government Publishing Office (govinfo), United States Reports
  15. 18 U.S.C. 3282 - Offenses not capital — Office of the Law Revision Counsel, U.S. House of Representatives
  16. California Penal Code section 26 - Persons capable of committing crimes — California Legislative Counsel (leginfo)
  17. California Penal Code section 198.5 - Residential presumption of reasonable fear — California Legislative Counsel (leginfo)
  18. Florida Statutes section 776.012 - Use or threatened use of force in defense of person — Florida Legislature, Online Sunshine
  19. New York Penal Law section 35.15 - Justification; use of physical force in defense of a person — New York State Senate
  20. Federal Rule of Criminal Procedure 12.1 - Notice of an Alibi Defense — Cornell Legal Information Institute (mirror of the Federal Rules of Criminal Procedure)
  21. Federal Rule of Criminal Procedure 12.2 - Notice of an Insanity Defense; Mental Examination — Cornell Legal Information Institute (mirror of the Federal Rules of Criminal Procedure)
  22. The volume and characteristics of insanity defense pleas: an eight-state study, Bulletin of the American Academy of Psychiatry and the Law 19(4):331-338 (1991) — Callahan LA, Steadman HJ, McGreevy MA, Robbins PC; record via PubMed (National Library of Medicine)
  23. Introduction to Criminal Law, 5.1: Criminal Defenses — LibreTexts
  24. Introduction to Criminal Law, 5.3: Other Use-of-Force Defenses — LibreTexts
  25. Introduction to Criminal Law, 5.4: Defenses Based on Choice — LibreTexts
  26. Introduction to Criminal Law, 6.1: The Insanity Defense — LibreTexts
  27. Introduction to Criminal Law, 6.2: Infancy, Intoxication, Ignorance, and Mistake — LibreTexts

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Researched 2026-08-18

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