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The Insanity Defense


Defendants found Not Guilty by Reason of Insanity (NGRI) are those whom a judge and/or jury has found to be so mentally-disordered during the commission of a crime that the individual should not be held criminally responsible for that crime. This does not necessarily mean that all mentally-disordered persons are not criminally responsible, nor does it mean that criminal responsibility requires the individual to be perfectly healthy. Mental disorder does not equate with being not guilty of one's crimes.


How often the insanity defence is actually used remains difficult to determine, but researchers have estimated that insanity pleas are probably used in only 1% of all US felony cases. What is more certain is that insanity defence claims are very rarely successful. One multiple-statewide study found a 20% - 25% success rate for insanity pleas (Callahan et al 1991), while another (conducted in Colorado) found a 44% success rate, and another (conducted in Wyoming) found a 2% success rate. The largest study, which examined insanity please in 35 US states (Cirincione and Jacobs 1999), found that on average only slightly over 30 defendants are acquitted of crimes by reason of insanity each year in the United States.

Defendants most likely to be successfully acquitted on insanity defenses: And defendants least likely to be successfully acquitted on insanity defenses are:

The Insanity Defense Reform Act

In response to the acquittal of John Hinckley, charged with attempting to assassinate President Reagan in the early 1980's but acquitted on grounds of insanity, US Congress passed the Insanity Defence Act of 1984, making it harder for defendants to escape criminal responsibility by reasons of insanity. The Insanity Defense Reform Act largely upheld the decision in M'Naghten and reversed the subsequent decision in Brawner, concluding that, for a person to be acquitted on grounds of insanity, his or her mental disorder must have removed the individual's ability to distinguish between right and wrong during the commission of the crime. Individuals cannot be acquitted based on "irresistible impulses" and must show that the mental disorder is "severe," which precludes the legitimacy of many "less serious" disorders such as personality disorders and fugue states. Below is Title 18 of Section 17 of the United States Code, which lays out the terms of the insanity defense:

18 USCS § 17

§ 17. Insanity defense

(a) Affirmative defense. It is an affirmative defense to a prosecution under any Federal statute that, at the time of the commission of the acts constituting the offense, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts. Mental disease or defect does not otherwise constitute a defense.

(b) Burden of proof. The defendant has the burden of proving the defense of insanity by clear and convincing evidence.

Famous Cases


Bartol and Bartol, 2008. Criminal Behavior: A Psychosocial Approach. Pearson-Prentice-Hall: Upper Saddle River, NJ.
United States v. Brawner, 471 F.2d 969, 991 (D.C. Cir. 1972)
Beyond accommodation: Reconstructing the insanity defense to provide an adequate remedy for postpartum psychotic women Journal of Criminal Law and Criminology, 1 January 2003.



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