The insanity defense is, unlike TV stories, a very narrow path to win an acquittal: It was successful in only one felony-level case between 1995 and ’97 in Minnesota, according to a study by Center for Applied Research and Policy Analysis at Metropolitan State University in St. Paul.

The study also shows that of 586,000 indictments in four states &045; California, Georgia, Montana and New York) &045; insanity plea was employed in only 5,300 cases, or 0,9 percent of the total. A total of 1,385 of them, 26 percent of insanity pleas and 0.23 percent of the indictments, resulted in an acquittal.

Mental illness alone does not automatically waive all criminal responsibility, according to the study.

To the insanity defense, about half the states, including Minnesota, apply a standard called the McNaughton rule named after a British man

McNaughton, who killed the prime minister’s secretary, believed the premier was conspiring against him. It states

defendants be acquitted, “only if they labored under such a defect of reason, from disease of mind, as not to know the nature and quality of the act he was doing or, if he did know it, that he did not know what he was doing was wrong.”

Unlike other not-guilty pleas, the majority of states put the burden of proof on the defense in an insanity-plea case. In Minnesota, the law requires the defense prove insanity by a preponderance of the evidence.

Winning an acquittal does not mean the accused walks free. On the contrary, he may be confined in a treatment facility until mental health officials determine he no longer poses a danger to anyone.