The insanity defense is a long-standing doctrine in criminal law holding that punishment requires moral and cognitive culpability (mens rea), and that a defendant suffering from a severe mental disease or defect at the time of the act may lack that culpability. Where the defense succeeds, the typical outcome is not outright release but a verdict such as "not guilty by reason of insanity" (NGRI) or "guilty but mentally ill," usually followed by involuntary commitment to a secure psychiatric facility.
Several legal standards coexist across jurisdictions:
- M'Naghten Rule — originating from the 1843 English case of Daniel M'Naghten, who shot Edward Drummond believing he was Prime Minister Robert Peel. It asks whether the defendant, due to a defect of reason, did not know the nature of the act or did not know it was wrong. It remains the dominant test in England and Wales and in many U.S. states.
- Irresistible Impulse Test — supplements M'Naghten by asking whether the defendant could control their conduct even if they knew it was wrong.
- Model Penal Code (ALI) Test — drafted by the American Law Institute in 1962, asks whether the defendant lacked "substantial capacity" to appreciate the criminality of the conduct or to conform conduct to the law.
- Durham Rule — a broader "product" test from Durham v. United States (D.C. Cir. 1954), now largely abandoned.
In the United States, the federal standard was tightened by the Insanity Defense Reform Act of 1984, passed after John Hinckley Jr. was found NGRI for the 1981 shooting of President Ronald Reagan. It shifted the burden of proof to the defense and narrowed the test to whether the defendant could appreciate the wrongfulness of the act.
The U.S. Supreme Court held in Kahler v. Kansas (2020) that the Constitution does not require states to adopt a moral-incapacity insanity test, allowing significant state variation. Some states — including Idaho, Kansas, Montana, and Utah — have abolished the traditional defense, permitting mental illness only to negate mens rea.
Example
In 1982, a U.S. federal jury found John Hinckley Jr. not guilty by reason of insanity for the attempted assassination of President Ronald Reagan, prompting Congress to pass the Insanity Defense Reform Act of 1984.
Frequently asked questions
Rarely. Defendants found not guilty by reason of insanity are typically committed to a secure psychiatric hospital, often for periods longer than a comparable prison sentence would have been.
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