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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.
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.
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.
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