Lawgivers have wrestled since biblical times with the issues of whether and when brainsick people are morally responsible for bad acts.

In English law, a standard for insanity verdicts dubbed the M’Naghten rule was adopted in 1843, and it eventually took root in the former colonies.

Named for a deranged, would-be political assassin, the rule said an accused person should be acquitted if, during the offense, he was “labouring under such a defect of reason, from a disease of the mind, as to not know the nature and quality of the act he was doing; or, if he did know it, that he did not know he was doing what was wrong.”

The M’Naghten test was used in most U.S. courts well into the 20th century, before the American Law Institute, a think tank, proposed a more liberal rule.


The ALI model, drafted in 1962, softened the rigid “did not know” language of M’Naghten. It said a defendant should be found not guilty if, during the offense, he lacked “substantial capacity to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of the law.” Over time, most states and the District of Columbia replaced M’Naghten with the ALI standard.


In federal courts, the ALI rule was in effect in 1981 when John W. Hinckley Jr. shot and wounded President Ronald Reagan and three others outside a Washington hotel.

Hinckley’s insanity acquittal ignited a nationwide backlash. Under the Insanity Defense Reform Act of 1984, signed by Reagan, federal courts reverted to the stricter M’Naghten rule, with even tougher language than the original. Most states followed suit, and some abolished the defense altogether.


In D.C. Superior Court, though, the looser ALI rule still applies.

As for the handful of states that did away with the insanity defense, the U.S. Supreme Court is set to hear arguments Oct. 7 in Kahler v. Kansas, in which a death-row inmate asserts that abolishing the defense was unconstitutional.