This Cornell Law School piece is the best short summary I have found of the various insanity defenses.

The McNaughton rule -- not knowing right from wrong

The first famous legal test for insanity came in 1843, in the McNaughton case. Englishman Daniel McNaughton shot and killed the secretary of the British Prime Minister, believing that the Prime Minister was conspiring against him. The court acquitted McNaughton "by reason of insanity," and he was placed in a mental institution for the rest of his life. However, the case caused a public uproar, and Queen Victoria ordered the court to develop a stricter test for insanity.

The "McNaughton rule" was a standard to be applied by the jury, after hearing medical testimony from prosecution and defense experts. The rule created a presumption of sanity, unless the defense proved "at the time of committing the act, the accused was laboring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing or, if he did know it, that he did not know what he was doing was wrong."

The McNaughton rule became the standard for insanity in the United States and the United Kingdom, and is still the standard for insanity in almost half of the states.

The Durham rule -- "irresistible impulse"

Monte Durham was a 23-year-old who had been in and out of prison and mental institutions since he was 17. He was convicted for housebreaking in 1953, and his attorney appealed. Although the district court judge had ruled that Durham's attorneys had failed to prove he didn't know the difference between right and wrong, the federal appellate judge chose to use the case to reform the McNaughton rule.

Citing leading psychiatrists and jurists of the day, the appellate judge stated that the McNaughton rule was based on "an entirely obsolete and misleading conception of the nature of insanity." He overturned Durham's conviction and established a new rule. The Durham rule states "that an accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect."

The Durham rule was eventually rejected by the federal courts, because it cast too broad a net. Alcoholics, compulsive gamblers, and drug addicts had successfully used the defense to defeat a wide variety of crimes. The Model Penal Code: turning responsibility to the jury The American Law Institute - Model Penal Code.

In 1972, the American Law Institute, a panel of legal experts, developed a new rule for insanity as part of the Model Penal Code. This rule says that a defendant is not responsible for criminal conduct where (s)he, as a result of mental disease or defect, did not possess "substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law." This new rule was based on the District of Columbia Circuit's decision in the federal appellate case, United States v. Brawner, 471 F.2d 969 (1972).

The Federal rule: Reagan gets into the act

In 1984, Congress passed, and President Ronald Reagan signed, the Comprehensive Crime Control Act. The federal insanity defense now requires the defendant to prove, by "clear and convincing evidence," that "at the time of the commission of the acts constituting the offense, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts" (18 U.S.C. § 17). This is generally viewed as a return to the "knowing right from wrong" standard. The Act also contained the Insanity Defense Reform Act of 1984, 18 U.S.C. § 4241, which sets out sentencing and other provisions for dealing with offenders who are or have been suffering from a mental disease or defect.