Insanity is a legal term for any mental disease or defect severe enough to prevent someone from being legally responsible for his or her actions. Under many systems of criminal law, someone found to be insane cannot be guilty of a crime. Thus, insanity is used as a defense in some criminal cases. However, a defendant found not guilty by reason of insanity may still be considered dangerous. Such a defendant could not be imprisoned but could be hospitalized until he or she was no longer mentally ill.
The insanity defense focuses on the defendant’s state of mind at the time of the crime. This is different from incompetency to stand trial. Incompetency focuses on the defendant’s state of mind at the time of trial. A defense of diminished capacity permits a defendant to introduce evidence showing that he or she lacked the mental state of intention necessary to be convicted of a crime.
In early English history, people who were thought to be insane were not prosecuted for criminal behavior. Instead, they were committed to asylums. Eventually, a standard was required to determine when someone was or was not insane to determine whether to prosecute. The landmark case in the modern development of the insanity defense is M’Naghten’s Case. This case was decided in the United Kingdom in 1843. Daniel M’Naghten had been charged with murder. The court judged him not guilty by reason of insanity. Doctors who examined M’Naghten found that he was prevented from being responsible for his actions by a mental illness. The case led to a new standard for the insanity defense called the M’Naghten Rule. This rule recognized a defendant as insane if, at the time the defendant committed the act, he or she met certain conditions. The conditions were that the defendant either did not know the nature and quality of the act or did not know what he or she was doing was wrong. British law and a large number of U.S. states still use some form of the M’Naghten Rule. More legal tests have been added for the insanity defense since M’Naghten.
If a defendant is acquitted because of insanity, the court gives a verdict of not guilty by reason of insanity. All states have procedures for committing a defendant to a psychiatric institution following such a verdict. Some states also allow a defendant to be found guilty but mentally ill. This verdict allows for conviction, but it also requires treatment as part of the sentence.
