The M’Naghten Rule is the oldest insanity defense test in American criminal law, and roughly half the states still use some version of it. The question it asks is narrow: at the moment the crime occurred, did a mental illness prevent the defendant from understanding what they were doing, or from knowing it was wrong? If the answer to either is yes, the defendant can be found not guilty by reason of insanity. If the answer is no, the defense fails, even when serious mental illness is present.1Legal Information Institute. M’Naghten Rule
That narrowness is the single most important thing to understand about the rule. It tests cognition only. A defendant who knew exactly what they were doing and knew it was wrong, but could not stop themselves because of mental illness, is not insane under M’Naghten.
The Two-Prong Test
Every defendant starts out presumed sane. To overcome that presumption, the defense has to show that at the time of the offense, the defendant’s mental state met one of two conditions.1Legal Information Institute. M’Naghten Rule
The first prong covers defendants who did not know what they were doing. A mental illness has to have so distorted the defendant’s perception that they did not understand the physical nature of their actions. The textbook illustration is someone so delusional that they believed they were squeezing a lemon when they were in fact strangling a person.
The second prong covers defendants who knew what they were doing but, because of mental illness, could not grasp that it was wrong. Someone who kills while delusionally convinced that God has commanded the act might fit here. They understand the physical action; they cannot comprehend its wrongfulness.
Both prongs are about knowledge. Neither asks whether the defendant could control themselves. That is the line that separates M’Naghten from broader insanity tests, and the line that makes it so difficult to meet.
What Counts as a Disease of the Mind
The cognitive impairment has to come from what the rule calls a “disease of the mind.” Courts treat that as a legal category rather than a medical one. Judges decide whether a given condition qualifies, looking for an internal condition that affects the capacity to reason. Organic brain damage, schizophrenia, and severe psychotic episodes have historically cleared the bar.
Personality disorders and psychopathy almost never do. Someone with antisocial personality disorder generally understands what they are doing and knows it is wrong. They may lack empathy or guilt, but the rule does not test for empathy or guilt. It tests for knowledge.
Voluntary intoxication is also excluded. Being drunk or high, however severely, is not a disease of the mind. Some jurisdictions carve out a narrow exception for “settled insanity,” where prolonged substance abuse has produced a permanent mental condition that exists independently of being intoxicated. The threshold is high.
How the Defense Works in Court
In most U.S. jurisdictions, the defendant carries the burden of proving insanity. This reverses the usual criminal law pattern, where the prosecution has to prove every element of the offense. For insanity, the defense has to affirmatively establish that the defendant met the M’Naghten criteria.
At the federal level, the defendant must prove insanity by clear and convincing evidence, a demanding standard that sits between the civil preponderance of the evidence and the criminal beyond a reasonable doubt.2Office of the Law Revision Counsel. 18 US Code 17 – Insanity Defense State standards vary. Some require clear and convincing evidence; others use the lower preponderance standard.
Expert Testimony
Both sides usually bring psychiatrists or psychologists to testify about the defendant’s mental condition at the time of the offense. This testimony matters because the insanity question is retrospective: what was the defendant’s mental state days, weeks, or months earlier, when the crime actually occurred?
Federal expert witnesses face a significant limit. Under Rule 704(b) of the Federal Rules of Evidence, no expert may state an opinion on whether the defendant actually had the mental state required for a crime or a defense to it. A psychiatrist can describe symptoms, diagnosis, and likely mental condition. The psychiatrist cannot tell the jury that the defendant was legally insane. That conclusion is reserved for the jury.3Legal Information Institute. Federal Rules of Evidence Rule 704 – Opinion on an Ultimate Issue
What Happens After an Insanity Acquittal
An insanity acquittal is not a release. A defendant found not guilty by reason of insanity is typically committed to a secure mental health facility. At the federal level, commitment continues until the person’s mental condition improves enough that releasing them would not create a substantial risk of bodily injury to another person or serious property damage.4Office of the Law Revision Counsel. 18 US Code 4243 – Hospitalization of a Person Found Not Guilty Only by Reason of Insanity
Commitment can run far longer than any prison sentence would have. In Jones v. United States, the Supreme Court held that a person acquitted by reason of insanity is not entitled to release just because they have been hospitalized longer than the maximum prison term for the original charge. The defendant in that case had been charged with a misdemeanor carrying a one-year maximum sentence and was still confined years later. The Court said there is “no necessary correlation” between the length of a hypothetical criminal sentence and the time needed for recovery.5Justia U.S. Supreme Court Center. Jones v. United States
Release requires either a court finding that the person no longer poses a substantial risk, or a conditional discharge under a supervised treatment plan. For offenses involving bodily injury or serious property damage, the person must prove suitability for release by clear and convincing evidence. For other offenses, the standard drops to a preponderance of the evidence.4Office of the Law Revision Counsel. 18 US Code 4243 – Hospitalization of a Person Found Not Guilty Only by Reason of Insanity Conditional release can be revoked if the person fails to follow their treatment plan.
How M’Naghten Compares to Other Insanity Tests
Not every jurisdiction uses M’Naghten alone, and the federal standard is its own statute. Three alternatives are worth knowing.
Irresistible Impulse
Several states pair M’Naghten with the irresistible impulse test, which adds the volitional component M’Naghten omits. A defendant can be found insane if they knew what they were doing and knew it was wrong but could not stop themselves because of mental illness. Virginia and Texas, among others, apply both tests, giving defendants two possible paths to acquittal.
The Model Penal Code Test
The American Law Institute’s Model Penal Code offers a broader standard. Under the MPC test, a defendant is not responsible if, due to mental disease or defect, they lacked “substantial capacity” either to appreciate the wrongfulness of their conduct or to conform their conduct to the law.6Legal Information Institute. Model Penal Code Insanity Defense Two differences from M’Naghten stand out. The MPC uses “appreciate” rather than “know,” reaching a deeper emotional understanding beyond intellectual awareness. And it includes the ability to conform conduct to law, so it covers defendants who understood their actions were wrong but could not control themselves. The word “substantial” also matters. M’Naghten requires a complete inability to understand; the MPC requires only that the defendant lacked substantial capacity.
The Federal Standard
Federal law has its own test under 18 U.S.C. § 17. A defendant can invoke insanity only if, at the time of the offense, a “severe mental disease or defect” made them “unable to appreciate the nature and quality or the wrongfulness” of their actions. The word “severe” was added deliberately: an ordinary mental health condition does not qualify. There is no volitional prong; a defendant who understood their actions were wrong but could not resist acting is not insane under federal law. And the defense carries the burden of proof by clear and convincing evidence.2Office of the Law Revision Counsel. 18 US Code 17 – Insanity Defense
States That Have Abolished the Defense
Four states have eliminated the insanity defense entirely: Kansas, Montana, Idaho, and Utah. A defendant in those states cannot argue for acquittal on the ground that mental illness prevented them from understanding their actions. Idaho, Montana, and Utah allow some form of a “guilty but insane” or “guilty but mentally ill” verdict, which recognizes mental illness at sentencing without providing an acquittal. In 2020, the U.S. Supreme Court upheld this approach in Kahler v. Kansas, ruling 6–3 that the Constitution does not require states to adopt an insanity test tied to a defendant’s ability to recognize that a crime was morally wrong. Defendants in those states can still introduce mental health evidence to challenge whether they had the intent the crime requires.
Guilty but Mentally Ill
About a dozen states offer a guilty-but-mentally-ill verdict as a middle-ground option. In practice it works out worse than either a conviction or an insanity acquittal. The verdict carries the same sentence as a regular conviction. No state requires a reduced sentence because of the mental illness finding. The defendant goes to prison, sometimes to a psychiatric ward that is more restrictive than the general population, and often cannot earn good-behavior credits available to other inmates. Parole typically comes with mandatory treatment conditions.
How Often the Defense Actually Succeeds
Public perception of the insanity defense bears almost no resemblance to the numbers. Studies have found the defense is raised in roughly one percent of felony cases, and of those, only about a quarter result in acquittals. Most defendants who plead insanity are convicted. Successful insanity acquittals tend to involve defendants with severe psychotic disorders and well-documented treatment histories, not defendants who construct a mental illness for trial. The image of criminals faking illness to escape punishment is a staple of popular drama and an extraordinarily rare event in actual courtrooms.