Colorado v. Connelly

The M'Naghten rule is a test for criminal insanity. Under the M'Naghten rule, a criminal defendant is not guilty by reason of insanity if, at the time of the alleged criminal act, the defendant was so deranged that she did not know the nature or quality of her actions or, if she knew the nature an quality of her actions, she was so deranged that she did not know that what she was doing was wrong.

The M'Naghten rule on criminal insanity is named for Daniel M'Naghten, who, in 1843, tried to kill England's prime minister Sir Robert Peel. M'Naghten thought Peel wanted to kill him, so he tried to shoot Peel but instead shot and killed Peel's secretary, Edward Drummond. Medical experts testified that M'Naghten was psychotic, and M'Naghten was found not guilty by reason of insanity.

The M'Naghten rule was adopted in most jurisdictions in the United States, but legislatures and courts eventually modified and expanded the definition. The definition of criminal insanity now varies from jurisdiction to jurisdiction, but most of them have been influenced by the M'Naghten rule.