History of the Insanity Defense

The precedent in English and later American courts for defendants to not be held responsible for their crimes due to their mental stability dates back to 1581. It was the first time an official legal document distinguished between those who could understand the difference between good and evil and those who couldn't. By the 18th century, the "wild beast" test was developed in which if the defendant did not understand the significance of their crimes "no more than an infant, a brute, or a wild beast" they would not face the consequences of their crimes.

The M'Naughten Test was developed in 1843 following the case of Daniel M'naughten and was later codified by the British Courts. M'Naughten was a Scottish woodcutter who murdered the secretary to the Prime Minister, Sir Robert Peel, in an attempt to murder the Prime Minister. He believed that the Prime Minister was behind multiple misfortunes that had happened to him. During the trial, he was acquitted after being found "not guilty by reason of insanity".  Queen Victoria was not pleased with the outcome and requested that the verdict be reviewed by the House of Lords. The verdict was reversed and the M'Naughten rule was born based on how the lords came to their own verdict. It was determined that a defendant should not be held responsible for their actions only if they did not they were wrong when the crime was committed. The rule later carried into the Americas and in 1998, 25 states and the District of Columbia still used it to test legal insanity.

By the mid-twentieth century, there was growing dissatisfaction with the M'Naughten Test. It was criticized for being out of date and too rigid by both psychiatric and legal communities. Critics called for medical evidence of mental illness to be brought into the insanity defense equation. In 1954, an appellate court discarded the M'Naughten Test for the favored Durham Rule, which declared that if a criminal's crime was "the product of mental disease or mental defect" the defendant would not be held responsible. It marked the replacement of moral considerations with more neutral scientific determinations. However, the rule proved too vague to actually be applied to law. Critics worried that defendants would use alcoholism or other disorders linked to antisocial behavior as excuses for their crimes. So far, 22 states have rejected the Durham Rule.

The American Law Institute (A.L.I.) set out to create a model insanity defense statute in 1962. It softened the M'Naughten test while including medical and psychiatric evidence like the Durham Rule. It holds that a defendant will not be held responsible if "as a result of a mental disease or defect, he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law". The A.L.I. standard does not include mental illness that only manifests itself in a criminal conduct, thus a serial killer whose only symptom is killing people would not be included.

As of 1998, states were largely split between 2 standards with 22 states using the A.L.I. standard, while 26 continued to use the M'Naughten test.

Source
https://www.pbs.org/wgbh/pages/frontline/shows/crime/trial/history.html