Not guilty by reason of insanity is a plea by a criminal defendant who admits the criminal act, but claims he/she was so mentally disturbed at the time of the crime that he/she lacked the mental capacity necessary to commit a crime.
In this context, "not guilty" does not mean the person did not commit the criminal act for which he or she is charged. It means that when the person committed the crime, he or she could not tell right from wrong or could not control his or her behavior because of severe mental defect or illness. Such a person, the law holds, should not be held criminally responsible for his or her behavior. The legal test for insanity varies from state to state..
What happens to a defendant after a judge or jury returns a finding of insanity depends on the crime committed, and on the state in which the trial takes place. Usually, those found "not guilty by reason of insanity are institutionalized in a special hospital for severely mentally ill persons who have committed crimes. After a period of time, the person may request a hearing to determine if he or she is no longer a danger to self or others or no longer mentally ill, and is therefore eligible to be released.
One of the most famous cases of a person acquitted using the insanity defense was the acquittal of John Hinckley, Jr., who attempted to assassinate President Reagan.