What is Insanity Defense?

Legal Definition
The insanity defense, also known as the mental disorder defense, is a defense by excuse in a criminal case, arguing that the defendant is not responsible for their actions due to an episodic or persistent psychiatric disease at the time of the criminal act. This is contrasted with an excuse of provocation, in which defendant is responsible, but the responsibility is lessened due to a temporary mental state. It is also contrasted with a finding that a defendant cannot stand trial in a criminal case because a mental disease prevents her from effectively assisting council, from a civil finding in trusts and estates where a will is nullified because it was made when a mental disorder prevented a testator from recognizing the natural objects of her bounty, and from involuntary civil commitment to a mental institution, when a person is found to be gravely disabled or to be a danger to herself or to others.

Exemption from full criminal punishment on such grounds dates back to at least the Code of Hammurabi. Legal definitions of insanity or mental disorder are varied, and include the M'Naghten Rule, the Durham rule, the 1953 British Royal Commission on Capital Punishment report, the ALI rule (American Legal Institute Model Penal Code rule), and other provisions, often relating to a lack of mens rea ("guilty mind"). In the criminal laws of Australia and Canada, statutory legislation enshrines the M'Naghten Rules, with the terms defense of mental disorder, defense of mental illness or not criminally responsible by reason of mental disorder employed. Being incapable of distinguishing right from wrong is one basis for being found to be legally insane as a criminal defense. It originated in the M'Naughten Rule, and has been reinterpreted and modernized through more recent cases, such as People v. Serravo.

In the United Kingdom, Ireland, and the United States, use of the defense is rare; however, since the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991, insanity pleas have steadily increased in the UK. Mitigating factors, including things not eligible for the insanity defense such as intoxication (or, more frequently, diminished capacity), may lead to reduced charges or reduced sentences.

The defense is based on evaluations by forensic mental health professionals with the appropriate test according to the jurisdiction. Their testimony guides the jury, but they are not allowed to testify to the accused's criminal responsibility, as this is a matter for the jury to decide. Similarly, mental health practitioners are restrained from making a judgment on the issue of whether the defendant is or is not insane or what is known as the "ultimate issue".

Some jurisdictions require the evaluation to address the defendant's ability to control their behavior at the time of the offense (the volitional limb). A defendant claiming the defense is pleading "not guilty by reason of insanity" (NGRI) or "guilty but insane or mentally ill" in some jurisdictions which, if successful, may result in the defendant being committed to a psychiatric facility for an indeterminate period.
-- Wikipedia
Legal Definition
See Insanity and diminished capacity

A person accused of a crime can acknowledge that they committed the crime, but argue that they are not responsible for it because of their mental illness, by pleading "not guilty by reason of insanity."
An important distinction: "Not guilty by reason of insanity" and "diminished capacity"
Although a defense known as "diminished capacity" bears some resemblance to the "reason of insanity" defense (in that both examine the mental competence of the defendant), there are important differences. The most fundamental of these is that, while "reason of insanity" is a full defense to a crime -- that is, pleading "reason of insanity" is the equivalent of pleading "not guilty" -- "diminished capacity" is merely pleading to a lesser crime.

One of the most famous recent uses of the insanity defense came in United States v. Hinckley, concerning the assassination attempt against then-President Ronald Reagan.
The history of "not guilty by reason of insanity"
The insanity defense reflects a compromise on the part of society and the law. On the one hand, society believes that criminals should be punished for their crimes; on the other hand, society believs that people who are ill should receive treatment for their illness. The insanity defense is the compromise: basically, it reflects society's belief that the law should not punish defendants who are mentally incapable of controlling their conduct.

In the 18th century, the legal standards for the insanity defense were varied. Some courts looked to whether the defendant could distinguish between good and evil, while others asked whether the defendant "did not know what he did." By the 19th century, it was generally accepted that insanity was a question of fact, which was left to the jury to decide.
The M'Naghten rule -- not knowing right from wrong
The first famous legal test for insanity came in 1843, in the M'Naghten case. Englishman Daniel M'Naghten shot and killed the secretary of the British Prime Minister, believing that the Prime Minister was conspiring against him. The court acquitted M'Naghten "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 "M'Naghten 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 M'Naghten 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
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).

Obviously, this standard is very vague. It leaves a number of factors up to the jury to determine, given the facts of a case and the testimony of experts. About half the states have adopted the Model Penal Code rule for insanity.
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 offence, 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.