The most common criminal law defenses in the United States fall into a short list: self-defense, alibi, duress, necessity, insanity, entrapment, mistake of fact, and involuntary intoxication. Some of these deny that the defendant committed the act at all. Others concede the act but argue it was justified. A third group admits the conduct was wrong but asks the court to excuse the defendant because of extraordinary circumstances. Which defense fits depends entirely on the facts, and the wrong choice can be as damaging as no defense at all.
Self-Defense
Self-defense is the most widely recognized justification defense. A person who uses force to protect against an immediate physical threat can avoid criminal liability if the force was proportional to the danger and the person genuinely believed they were about to be harmed. Both halves matter: a subjective belief that danger is imminent, and an objective standard asking whether a reasonable person in the same situation would have felt the same way.1Legal Information Institute. Self-Defense
Proportionality is where most self-defense claims succeed or fail. Non-deadly force can be used against a non-deadly threat. Deadly force is justified only when the person reasonably believes they face imminent death or serious bodily injury. Pulling a weapon during a shoving match will almost certainly exceed what the situation called for. The person claiming self-defense also cannot have started the confrontation. Initial aggressors lose the defense unless they clearly withdrew from the fight before using force.1Legal Information Institute. Self-Defense
Retreat, Stand Your Ground, and the Castle Doctrine
States split on whether you must try to escape before using force. In duty-to-retreat states, self-defense applies only if you had no safe way to leave the situation first. At least 31 states have gone the other direction with stand-your-ground laws, which let you use force wherever you are legally allowed to be without first trying to flee.2National Conference of State Legislatures. Self-Defense and Stand Your Ground
Even in states that require retreat, the castle doctrine creates an exception for your own home. If someone unlawfully enters your residence, you don’t have to try to escape before defending yourself.3Legal Information Institute. Castle Doctrine One limitation applies in nearly every state: deadly force is not justified to protect property alone. Shooting someone who is stealing your car or vandalizing your fence will not qualify as self-defense, because no human life is at risk.
Defense of Others
The same principles extend to protecting someone else. You can use reasonable force to defend a third party facing an imminent threat, as long as that third party would have been justified in using force themselves. You step into the shoes of the person you are protecting, and the force still must match the danger.
Alibi
An alibi defense attacks the prosecution’s case at its foundation. The defendant says they were somewhere else entirely when the crime happened. If you can prove you were in another city at the time of a robbery, the prosecution’s theory collapses because you physically could not have done it. This is a factual defense, not a justification or an excuse. It simply denies involvement.
Because alibi evidence can be fabricated, courts take procedural safeguards seriously. In federal cases, the prosecution can request written notice of an alibi defense, and the defendant must respond within 14 days. That notice must identify every location the defendant claims to have been and provide the name, address, and phone number of each alibi witness.4Legal Information Institute. Federal Rules of Criminal Procedure 12.1 – Notice of an Alibi Defense Most states have similar disclosure rules. Missing the deadline can mean losing the right to call alibi witnesses at trial.
The strength of an alibi depends heavily on corroboration. Surveillance footage, time-stamped receipts, cell phone location data, and testimony from disinterested witnesses all carry far more weight than a family member saying you were home. An alibi that falls apart at trial can actively hurt the defense by making the defendant look dishonest.
Duress
Duress applies when someone commits a crime because another person threatened to kill or seriously injure them, or someone close to them, if they refused. The classic scenario is being forced at gunpoint to drive a getaway car. The defense acknowledges that the defendant did the act but argues they had no real choice.
Courts impose strict requirements. The threat must be immediate and credible. A vague warning about future harm will not qualify. The defendant must have had no safe way to escape or avoid complying.5Legal Information Institute. Duress If a reasonable person in the same situation could have walked away, called police, or found any other way out, the defense fails.6United States Court of Appeals for the Armed Forces. Core Criminal Law Subjects – Defenses – Duress The threatened harm must also be worse than the crime committed. Being threatened with a slap does not justify armed robbery.
Duress is almost universally unavailable as a defense to murder. Courts have consistently held that being threatened with death does not justify taking someone else’s life.7Ninth Circuit District and Bankruptcy Courts. Manual of Model Criminal Jury Instructions – 6.5 Duress, Coercion or Compulsion A small number of states allow duress to reduce a murder charge to manslaughter, but outright acquittal on a murder charge through duress is effectively off the table.
Necessity
Necessity looks similar to duress, but the threat comes from circumstances rather than another person. A driver who speeds through a red light to rush a passenger having a heart attack to the hospital might raise necessity. So might someone who breaks into a cabin during a blizzard to avoid freezing to death.
The defense requires several elements working together:
- The threat was imminent, happening now or about to happen rather than something that might occur later.
- The defendant had no legal alternative. If calling 911 would have worked, breaking the law was not necessary.
- The harm avoided was greater than the harm caused by the crime.
- The defendant did not create the emergency they are now using to justify their actions.
Necessity is a genuinely difficult defense to win.8Legal Information Institute. Necessity Defense Courts interpret it narrowly, because a broad necessity defense could justify almost anything. Most states also exclude murder and other high-level felonies, just as they do with duress.
Insanity
The insanity defense is an excuse defense that generates enormous public attention relative to how rarely it succeeds. The argument is that, because of a severe mental disease or defect, the defendant should not be held criminally responsible for what they did. This is a legal determination, not a medical one. A clinical diagnosis of mental illness does not automatically qualify someone for the defense.
Legal Tests for Insanity
Different jurisdictions apply different standards. The oldest and most common is the M’Naghten test, used in roughly half the states, which asks whether the defendant understood what they were doing or knew it was wrong at the time of the crime.9Legal Information Institute. M’Naghten Rule Other states use variations. The irresistible impulse test asks whether the defendant could control their behavior even if they knew it was wrong. The substantial capacity test, from the Model Penal Code, sets a somewhat lower bar by asking whether the defendant lacked substantial ability to appreciate the wrongfulness of their conduct or conform their behavior to the law.
In the federal system, the standard is defined by statute. The defendant must prove they had a severe mental disease or defect that made them unable to appreciate either the nature of their actions or that those actions were wrong. The burden is on the defendant, and the defendant must meet it by clear and convincing evidence.10Office of the Law Revision Counsel. 18 USC 17 – Insanity Defense A handful of states, including Kansas, Idaho, Montana, and Utah, have effectively abolished the traditional insanity defense. About 13 states offer a guilty but mentally ill verdict as an alternative, which acknowledges mental illness without eliminating criminal responsibility.
What Happens After an Insanity Acquittal
A successful insanity defense does not mean walking free. In the federal system, a person found not guilty by reason of insanity faces a hearing within 40 days of the verdict to determine whether they should be committed to a psychiatric facility. Most states have similar procedures. In practice, people acquitted by reason of insanity are almost always committed, and they frequently spend as long in a mental institution as they would have spent in prison. Sometimes longer, because release depends on demonstrating they are no longer dangerous rather than on serving a fixed sentence.
Entrapment
Entrapment applies when the government induced someone to commit a crime they would not have committed on their own. This is not about undercover officers simply providing an opportunity to break the law, which is legal. The defense targets situations where law enforcement manufactured criminal intent that did not previously exist. An undercover officer buying drugs from someone already dealing is not entrapment. An officer who befriends a vulnerable person over months, repeatedly pressures them, and eventually persuades them to obtain drugs for the first time is in different territory.11Ninth Circuit District and Bankruptcy Courts. Manual of Model Criminal Jury Instructions – 5.2 Entrapment
Subjective and Objective Tests
Federal courts and most states use the subjective test, which focuses on the defendant’s predisposition. If the government raised the idea, the prosecution must prove beyond a reasonable doubt that the defendant was already inclined to commit the offense before any government contact.12Legal Information Institute. Jacobson v. United States, 503 U.S. 540 (1992) Evidence of predisposition includes prior criminal history, reputation, how quickly the person agreed, and any statements showing existing criminal intent.
A minority of states use the objective test, which ignores the defendant’s character and asks whether the government’s conduct was so outrageous that it would have induced a law-abiding person to commit the crime. The practical difference is significant. Under the subjective test, a defendant with a prior record for the same type of crime will have a hard time claiming entrapment, even if the government’s tactics were aggressive. Under the objective test, the defendant’s background is irrelevant.
Mistake of Fact
A mistake of fact defense argues that the defendant misunderstood a key fact in a way that eliminates the required mental state for the crime. If someone walks out of a restaurant wearing a coat identical to their own, genuinely believing it is theirs, they lacked the intent to steal. Without that intent, the crime of theft has not been committed.13Legal Information Institute. Mistake of Fact
The mistake must be honest and reasonable. The defendant must actually have believed the wrong fact, and a reasonable person could have made the same error. A far-fetched or reckless misunderstanding will not qualify. The defense is also limited by the type of crime. For offenses requiring specific intent, such as theft or fraud, a genuine factual mistake can negate the mental element entirely. For strict liability offenses, like certain regulatory violations, mistake of fact is irrelevant because intent is not part of the equation.
Mistake of fact should not be confused with mistake of law, which claims the defendant did not know their conduct was illegal. Mistake of law almost never works. Courts expect people to know the law, and “I didn’t know that was a crime” fails as a defense in nearly every situation.
Involuntary Intoxication
Involuntary intoxication applies when someone commits a crime while under the influence of a substance they did not knowingly consume. The most common examples are drinks spiked without the person’s knowledge or unexpected side effects from a prescribed medication. The defense works because the intoxication was not voluntary, and if the person was so impaired they could not form the mental state required for the crime, they lack the necessary intent.14Legal Information Institute. Involuntary Intoxication
Voluntary intoxication is a different story. Getting drunk or high by choice is generally not a defense, though a few states allow it to negate specific intent for certain crimes. Someone who unknowingly ingests a drugged beverage and then causes a car accident has a credible defense. Someone who drank ten beers and did the same thing does not.
Who Has to Prove What
Most of these defenses are affirmative defenses, meaning the defendant does not just poke holes in the prosecution’s case. The defendant raises a new claim and carries the burden of proving it. The prosecution still must prove every element of the crime beyond a reasonable doubt, but once the defendant raises an affirmative defense like insanity or duress, the defendant must support that claim with evidence.15Legal Information Institute. Affirmative Defense
The standard varies. For the federal insanity defense, the defendant must prove it by clear and convincing evidence.10Office of the Law Revision Counsel. 18 USC 17 – Insanity Defense For entrapment in federal court, the burden shifts to the prosecution to prove predisposition beyond a reasonable doubt once the defendant raises the defense.12Legal Information Institute. Jacobson v. United States, 503 U.S. 540 (1992) Alibi works differently from the others. It is not an affirmative defense at all, because the defendant is simply denying involvement. The prosecution still carries the full burden of proving the defendant committed the crime.
These distinctions matter because choosing the wrong defense strategy can backfire. Raising an affirmative defense often means conceding that you committed the act, which closes off other avenues. A defendant who claims self-defense is admitting they used force. A defendant who claims insanity is admitting they did what the prosecution alleges. That concession is irreversible if the defense fails, which is why deciding which defense to raise is one of the most consequential calls a defense attorney makes.