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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:
  • Those with psychotic disorders
  • Those with affective disorders
  • Those with mental retardation
  • Those charged with violent crimes
  • Those standing trial before a judge only (bench trials)
  • Older
  • Educated
  • Single
  • Female
And defendants least likely to be successfully acquitted on insanity defenses are:
  • Those with personality disorders
  • Those charged with sex offences
  • Those standing trial before a judge and jury (jury trials)
  • Males
  • Young
  • Uneducated

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

  • Andrea Yates

    Yates was charged with drowning her children in a bathtub in Texas in 2001. She was found to have suffered from and been hospitalized for postpartum psychosis, but was ultimately convicted by a jury and sent to prison. Her conviction was later overturned.
  • John Hinckley

    Hinckley was charged with the attempted murder of President Ronald Reagan and acquitted in 1981. Since that time he has spent most of his life institutionalized and completing treatment programs for his condition. Since arguing that his disorder is now in remission, Hinckley has been allowed temporary releases and unsupervised day and overnight visits.
  • Tery Foucha

    Tery Foucha was a young Louisiana man accused of aggravated burglary and illegally discharging of a firearm under the heavy influence of drugs in 1984. Foucha was found not guilty by reason of insanity and hospitalized in the state mental hospital for four years. He was granted release after four years once the psychiatrists treating him determined that his illness had gone into remission, despite still being considered "dangerous". The Supreme Court in Foucha v. Louisiana found that individuals found not guilty by reason of insanity should no longer be institutionalized for their crimes if their mental illness had gone into remission. In practice, however, these sorts of individuals are supervised after release in the community and monitored for signs that their condition has returned.
  • Daniel M'Naghten

    M'Naghten was a Scottish woodcutter charged with the murder of a man he believed to be the British Prime Minister at the time, Sir Robert Peel, in the early 1840's. The jury used the "wild beast test" to determine that M'Naghten should be considered mentally-deranged and unable to understand the seriousness of his actions. He was institutionalized for 22 years in Broadmoor Mental Institution until his death in 1865. The famous M'Naghten Rule used by American courts today is derived from the outcome of this case, and the "miscarriage of justice" it was widely considered to produce. The M'Naghten Rule states that individuals who clearly "knew" their act was wrong, despite being mentally ill or not, should be able to be convicted of their crimes. The case resulted in the so-called "right-or-wrong test" of insanity pleas, where the defendant should be convicted if he or she:
    • was aware of what they were doing during the act
    • was aware of the difference between what is morally right and what is morally wrong
  • Archie Brawner

    The "Brawner Rule" that resulted from the 1972 case of Brawner v. United States determined that individuals suffering from a mental disease or defect that prevents them from appreciating the wrongfulness of their conduct (or from acting in accordance with the law) should be deemed not guilty by reason of insanity. Archie Brawner had gotten riled up at a party one night and had fired some gunshots wildly, some of which had struck and killed Billy Ford. He was convicted of 2nd-degree murder and on rehearing the court determined the appropriate standard to use for assessing insanity. Furthermore, only certain serious mental disorders could qualify for insanity defenses. According to the Brawner Rule, psychopaths or sociopaths do not qualify for exemption due to insanity because they are typically repeat-offenders without a recognizable affliction that prevents them from understanding right from wrong.
  • Monte Durham

    Monte Durham was young 26-year old with a long history of mental disorder and petty theft, charged of burglary and then acquitted on grounds that the crime was the product of his mental condition. The "Durham Rule" that resulted from the 1954 case of Durham v. United States determines that an individual is not guilty by reason of insanity if his or her mental disorder plays a central role in the commission of the illegal act. Since the outcome of Durham was considered too vague, open-ended, overly-simplistic, and in conflict with the M'Naghten Rule, many US states in the 1980's reformed their insanity defence laws, and some abolished the insanity defence altogether, and made it more difficult to argue insanity pleas in a court of law.


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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