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Insanity Defense

A criminal defendant who is found to have been legally insane when he or she committed a crime may be found not guilty by reason of insanity. In some cases, the defendant may be found guilty but sentenced to a less severe punishment due to a mental impairment. In states that allow the insanity defense, defendants must prove to the court that they did not understand what they were doing; failed to know right from wrong; acted on an uncontrollable impulse or some variety of these factors.

Below you will find basic information and legal issues related to the insanity defense, plus links to FindLaw's various articles about legal insanity. See Defending Yourself Against a Criminal Charge for related information.

Legal Insanity: Background

The first known recognition of insanity as a defense to criminal charges was recorded in a 1581 English legal treatise stating that, "If a madman or a natural fool, or a lunatic in the time of his lunacy" kills someone, they cannot be held accountable. British courts came up with the "wild beast" test in the 18th Century, in which defendants were not to be convicted if they understood the crime no better than "an infant, a brute, or a wild beast."

Besides the fact that courts no longer use the terms "lunatic" or "wild beast," current laws allowing for the insanity defense follow a similar logic. The legal basis for insanity was codified into British law in the mid 19th Century with the M'Naughten Rule, which is used in a majority of U.S. states and other jurisdictions around the world today.

See Current Application of the Insanity Defense and Status of the Insanity Defense to learn more.

How Courts Test for Legal Insanity

Depending on the jurisdiction, courts use one or a combination of the following tests for legal insanity:

  • The "M'Naghten Rule" - Defendant either did not understand what he or she did, or failed to distinguish right from wrong, because of a "disease of mind."
  • The "Irresistible Impulse" Test - As a result of a mental disease, defendant was unable to control his impulses, which led to a criminal act.
  • The "Durham Rule" - Regardless of clinical diagnosis, defendant's "mental defect" resulted in a criminal act.
  • The "Model Penal Code" Test for Legal Insanity - Because of a diagnosed mental defect, defendant either failed to understand the criminality of his acts, or was unable to act within the confines of the law.

The Insanity Defense: State Laws

A few states do not allow the insanity defense against criminal charges, including Idaho, Kansas, Montana, and Utah. All four of these states, with the exception of Kansas, allow "guilty but insane" verdicts, which often provide for institutionalization in lieu of prison. Most states that recognize legal insanity use either the M'Naghten Rule (sometimes in combination with the Irrestistible Impulse Test) or the Model Penal Code. Only New Hampshire uses the Durham standard.

See The Insanity Defense Among the States for a complete list.

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