Charles Guiteau Trial

The insanity defense has been mounted by criminal defendants for several centuries. In fourteenth century England, the insanity plea was available to a person who was "deprived of and memory so as not to know what he [was] doing, no more than an infant, a brute, or a wild beast." England later fashioned the M'Naghten rule, which allowed the insanity defense if the defendant did not know what she was doing or did not know that what she was doing was wrong.

The M'Naghten rule migrated to the United States, but it has since been modified. Some states made the insanity defense available to defendants who acted under an "irresistible impulse," allowing for a form of physical insanity to supplement M'Naghten's recognition of mental insanity. Many states now allow the insanity defense if the defendant lacked "substantial capacity" to appreciate his or her conduct. On the federal level, the "irresistible impulse" test has been abolished. On the federal level and in some states, defendants on the federal level must prove their insanity. A few states require the prosecutor to prove the defendant's sanity. A successful claim of insanity does not set a defendant free: insane defendants are sent to secure mental health facilities.