When a person with a serious mental illness kills someone, the criminal case splits into two separate questions about their mind: whether they are mentally capable of participating in a trial now, and whether their mental state at the moment of the killing should excuse them from criminal responsibility. The answers lead to very different places. A defendant can end up indefinitely confined in a locked psychiatric hospital, serving a standard prison sentence with treatment layered on, or in rare cases released under strict supervision. What happens when a mentally ill person kills someone depends almost entirely on which of those tracks the case moves onto, and the public perception of easy acquittals bears little resemblance to how these cases actually resolve.
How Rarely the Insanity Defense Succeeds
Defendants raise an insanity claim in roughly one percent of all felony cases, and only about one in four of those succeed. The vast majority who try it are still found guilty. When the defense does work, the person is almost never set free. They are committed to a locked psychiatric facility, often for years, and sometimes longer than a prison sentence would have lasted.
Competency to Stand Trial
Before a murder trial can start, the court has to decide whether the defendant is mentally able to take part in it. This question has nothing to do with their mental state when the killing happened. Competency is about the present: can this person understand what’s going on in the courtroom and work with their lawyer?
Trying or sentencing a defendant who is incompetent violates due process under the Fourteenth Amendment.1Cornell Law School. Competency for Trial The test, set in Dusky v. United States in 1960, asks whether the defendant has a “sufficient present ability to consult with his lawyer with a reasonable degree of rational understanding” and “a rational as well as factual understanding of the proceedings.”2Justia U.S. Supreme Court Center. Dusky v. United States, 362 U.S. 402 (1960) When competency is in doubt, the judge orders a psychological evaluation and holds a hearing.
If the defendant is found incompetent, the trial stops. They are sent to a psychiatric facility for treatment — medication, therapy, or both — aimed at restoring competency so the case can resume.
When Competency Cannot Be Restored
Some defendants never recover enough to stand trial. In Jackson v. Indiana, the Supreme Court held that a state cannot hold someone indefinitely just because they are too impaired to be tried. Commitment can last only long enough to determine whether there is a substantial probability the person will regain competency in the foreseeable future.3Cornell Law School. Theon Jackson, Petitioner, v. State of Indiana If not, the state has to either begin standard civil commitment proceedings or release the person. In homicide cases, prosecutors typically push for civil commitment to keep the defendant in a supervised treatment setting.
The Insanity Defense
Once a defendant is found competent, the trial moves forward, and the insanity question shifts backward in time to the defendant’s mental state at the exact moment of the killing. The principle behind it is that criminal law punishes people who intended to do something wrong. If a severe mental illness kept the defendant from forming that intent, criminal responsibility doesn’t serve the purpose of the law.
Raising insanity is not a denial of the act. The defendant is admitting the killing and arguing that their mental illness was so severe they shouldn’t be criminally liable for it. It is an affirmative defense, which means the defendant has to prove it rather than the prosecution having to disprove it.
Who Has to Prove What
How much evidence the defendant needs depends on the jurisdiction. In federal court, the Insanity Defense Reform Act of 1984 requires the defendant to prove insanity by “clear and convincing evidence,” a standard that sits between the civil “preponderance of the evidence” and the criminal “beyond a reasonable doubt.”4Office of the Law Revision Counsel. 18 U.S. Code 17 – Insanity Defense Most states also place the burden on the defendant, though the exact standard varies. A handful of states still require the prosecution to disprove insanity once the defendant raises it.
The Legal Tests for Insanity
Legal insanity is not a medical diagnosis. A person can be severely mentally ill and still not qualify. The definition varies, but most states use one of a few established tests.
The M’Naghten Rule
The oldest and most restrictive standard, from an 1843 English case, focuses on what the defendant knew. A defendant is legally insane only if a mental disease or defect left them unable to understand what they were doing, or unable to tell that it was wrong. A person who knew they were stabbing someone and knew stabbing was wrong would not qualify, even while hallucinating. This is the most widely used standard in the United States.
The Model Penal Code Test
Developed by the American Law Institute, this broader test asks whether the defendant lacked “substantial capacity” either to appreciate that their conduct was criminal or to control their behavior in accordance with the law. The key addition is that second piece — the ability to control one’s actions. A defendant who knew what they were doing was wrong but genuinely could not stop themselves might qualify here but fail under M’Naghten. Several states adopted this standard; some abandoned it after the Hinckley verdict.
The Irresistible Impulse Test
Some jurisdictions use a variation often paired with M’Naghten. A defendant can be found not guilty by reason of insanity if a mental illness made it impossible to resist the impulse to commit the crime. Prosecutors counter by showing planning in advance or steps to avoid detection, both of which undercut the claim that the impulse was uncontrollable.
The Federal Standard
The federal insanity defense, rewritten in 1984, is deliberately narrow. The defendant has to show that “as a result of a severe mental disease or defect,” they were “unable to appreciate the nature and quality or the wrongfulness” of their actions.4Office of the Law Revision Counsel. 18 U.S. Code 17 – Insanity Defense There is no mention of controlling behavior. Congress dropped that volitional piece after the Hinckley acquittal, leaving the federal standard closer to M’Naghten than to the Model Penal Code.
States That Do Not Allow the Insanity Defense
Four states — Kansas, Montana, Idaho, and Utah — have abolished the insanity defense entirely. A defendant in those states cannot argue that mental illness should excuse them from criminal responsibility. They may still introduce evidence of mental illness to challenge whether they had the specific intent required for the charged crime, such as the premeditation needed for first-degree murder, but the full insanity defense is not available.
In 2020, the Supreme Court upheld this approach in Kahler v. Kansas, ruling 6–3 that due process does not require any particular insanity test.5Justia U.S. Supreme Court Center. Kahler v. Kansas, 589 U.S. ___ (2020) A state satisfies the Constitution as long as defendants can present mental-illness evidence bearing on intent.
Guilty but Mentally Ill
About a dozen states give juries a third option: Guilty but Mentally Ill. It sounds like a middle ground, but in practice it is much closer to a straight guilty verdict than most people realize.
A defendant found guilty but mentally ill is convicted and sentenced like any other guilty defendant. The verdict acknowledges mental illness at the time of the crime and provides, in theory, for psychiatric treatment during incarceration.6Office of Justice Programs. Guilty but Mentally Ill Verdict and Due Process No state requires a reduced sentence because of this verdict, and it does not prevent the death penalty. Treatment quality inside correctional facilities is inconsistent, and research has found that defendants with this verdict are often sentenced as harshly as those found simply guilty.
What Happens After a Not Guilty by Reason of Insanity Verdict
A verdict of Not Guilty by Reason of Insanity does not mean freedom. The acquittal means the person is not held criminally responsible, but it triggers a separate legal track focused on treatment and public safety.
Automatic Psychiatric Commitment
In almost every case, the court commits the person to a secure forensic psychiatric hospital immediately after the verdict. In Jones v. United States, the Supreme Court held this automatic commitment constitutional, because the insanity verdict itself establishes both that the person committed the act and that they were mentally ill when they did.7Justia U.S. Supreme Court Center. Jones v. United States, 463 U.S. 354 (1983)
The Court also held that a person committed after an insanity acquittal is not entitled to release just because they have been hospitalized longer than they would have served in prison for the same offense. The purpose of the commitment is treatment and public protection, not punishment, so the length of a hypothetical prison sentence is irrelevant. The person stays until they have regained sanity and are no longer dangerous.7Justia U.S. Supreme Court Center. Jones v. United States, 463 U.S. 354 (1983) Confinement in a forensic facility often lasts longer than a prison term would have.
Periodic Review and Conditional Release
The committed person undergoes regular psychiatric evaluations, and the court periodically reviews whether continued confinement is justified. These hearings examine whether the illness is in remission, whether the person poses a danger, and whether they can safely live in the community. The burden at these hearings typically falls on the prosecution to show the person still meets the criteria for commitment.
When a court decides someone is ready to leave a locked facility, the move is almost never directly to full freedom. The person is placed on conditional release, a supervised arrangement that works somewhat like parole with a heavy psychiatric component. Conditions commonly include mandatory medication compliance, meetings with a forensic case monitor, substance abuse restrictions, and ongoing therapy. Monitors can intervene quickly if the person relapses, stops taking medication, or shows signs of decompensation. Any violation can send the person back to the facility.
Civil Lawsuits by the Victim’s Family
A not guilty by reason of insanity verdict applies only to criminal responsibility. It does not shield the person from a civil wrongful death lawsuit filed by the victim’s family. Criminal and civil cases operate under different standards, so a criminal acquittal has no binding effect on civil liability.
The longstanding common law rule is that mental illness does not excuse a person from liability for harm they cause. Courts have consistently held that individuals with mental illness can be found liable in civil court for injuries resulting from their actions. The civil standard of proof is also lower: the family needs to prove the case by a preponderance of the evidence, not beyond a reasonable doubt. Collecting on a judgment against someone confined to a psychiatric hospital with no assets is a separate problem.