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Fact Sheet: The Insanity Defense

About the fact-sheet series: Fact sheets summarize current literature into a short (2 page) document intended for distribution. Fact-sheets are extremely useful for academics, professionals or laypeople who are in contact with offenders, victims, corrections or the legal system in any way. They provide a means to disseminate empirically based information in a way that is both quick and useful. Fact sheets undergo the EAPL-S peer review process and editing before publication.

About the author: This article was written as a guest post by Paola Giannetakis, a PhD student at the University of Huddersfield (United Kingdom) and member of the International Research Centre for Investigative Psychology.

View this document in it's full glory by downloading the pdf here.



Remember to also read our fact sheet on mentally ill offenders!

What is the Insanity Defense?

The legal defense by reason of insanity is primarily used in criminal prosecutions and is based on the assumption that at the time of the crime, the defendant was not sound of mind, and therefore, was incapable of appreciating the nature of the crime and differentiating right from wrong behavior. Allowing such typology of defense is based on the principle that civilized societies do not punish people who do not know what they are doing or are incapable of controlling their conduct.

Fitness to Stand Trial

The legal system distinguishes between two essential components used to evaluate an individual’s fitness to stand trial: competency and insanity. Competency refers to the capacity of a defendant to assist the case attorney and comprehend the contents of the allegations, while insanity refers exclusively to the mental state of the defendant at the time of the crime. The legal standards for defining insanity vary from state to state and from country to country. For example, in the US, a criminal act is not only regarded illegal, but it must also be accompanied by mens rea (a guilty mind). The defense of insanity derives from the M’Naghten rules in 1843, asserting, “It must be clearly proved that, at the time of committing the act, the party accused was labouring under such a defect of reason, from disease of the 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 he was doing what was wrong.” (Post, 1963). Although the M’Naghten rule is still used, there are limitations.

Conceptualizations of Insanity

There is a twofold conception of the insanity defense. One type of insanity may be defined as “cognitive insanity”, where a defendant was suffering from a mental disease or defect at the time of the crime, which impaired his/her psychological ability to appreciate the wrongfulness of the act. A second type of insanity is based on the concept of “irresistible impulse”, where a defendant is psychologically able to appreciate and distinguish between right and wrong behaviors, but who has suffered from a mental disease leading to an inability to control his/her actions. Is also possible to define insanity as being:

  1. Legally determined: the defendant is not considered criminally responsible if, as a result of a mental disorder or defect, he/she lacked the capacity to appreciate his/her misconduct as a violation of the law.
  2. Morally determined: the defendant lacks criminal responsibility if, as a result of a mental disorder or defect, he/she lacked the capacity to appreciate his/her misconduct as a violation of what a society deems unethical.
  3. Subjectively determined: the defendant is not considered criminally responsible if, as a result of mental disease or defect, he/she believed to be morally justified in committing his/her behavior despite an awareness of his/her actions as being illegal and/or a violation of social-moral standards.

Insanity in Court

During the trial a defendant claiming insanity could receive a verdict of “not guilty by reason of insanity” (NGRI) or “guilty but insane or mentally ill” (GBMI). It should be clearly stated that both NGRI and GBMI are types of verdicts and not types of defenses. GBMI means that although a defendant is held criminally responsible, the defendant is regarded mentally ill and requires psychological treatment, while NGRI establishes that a defendant is found not guilty on the basis of mental status at the time of the crime. It is often a misconception that a not guilty verdict means a defendant is released to the public following the trial; however, defendants regarded not guilty by reason of insanity are sent into the custody of a mental institution for a length of time that, in some cases, can be longer than a prison sentence time with a guilty verdict.

Determining Insanity

One major issue in determining insanity is that insanity is a legal concept, not a psychiatric one. This means that just suffering from a mental disorder is not sufficient to prove insanity. There are a number of mental disorders that could potentially influence behavior, but the issue is to determine if the disorder caused an individual to lack mens rea at the time of the crime committed. In the United States, trials in which a defendant pleads not guilty by reason of insanity represent 1% of all the criminal cases, and the defense is legally proven in only 25% of these cases. This type of trial requires the evaluation and testimony of forensic psychiatrists or forensic psychologists. These professionals must determine if a defendant was unable to appreciate and distinguish between right and wrong behaviors, at the time of the crime. There are two clinical tests used to evaluate a defendant’s mental competency and appropriate labeling of legally-mentally insane, they are: the Slobogin Mental Screening Evaluation (Slobogin et al, 1984), and the Rogers Criminal Responsibility Assessment Scales (Rogers, 1984). Ultimately, the assessment of insanity is a complex and time-consuming process. Forensic psychiatrists and psychologists must evaluate information from a number of sources and conduct clinical diagnostic interviews and tests.


Legal insanity remains a largely misunderstood and misrepresented issue. It is important for the public to learn about the value and complexity of this defense, and that offenders who meet the criteria are not simply “getting off easy”.

Quick summary

  • The insanity defense is based on the idea that some defendants can have a lack of mens rea (criminal intent), making them not legally accountable for crime.
  • It is hard to determine legal insanity, and even harder to successfully defend it in court.
  • Insanity defendants receive a verdict of NGRI (Not Guilty by Reason of Insanity) or GBMI (Guilty but Mentally Ill).

Where can I get more information?

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  1. American Law Institute. (1962). Model penal code
  2. Boehnert, C. E. (1989). Characteristics of successful and unsuccessful insanity pleas. Law and Human Behavior, 13(1), 31-39.
  3. Commonwealth v. Rodgers, 7 Metcalf 500 (Mass. 1844)
  4. Finkel, N. J. (1991.) The insanity defense. Law and Human Behavior, 15 (5), 533-555.
  5. Goldstein, A. S. (1980). The insanity defense. Westport, Conn: Greenwood Press.
  6. Greene, E., & Heilbrun, K. (2011). Wrightsman's psychology and the legal system. Belmont, CA: Wadsworth Cengage Learning.
  7. Huckabee, H. M. (2000). Mental disability issues in the criminal justice system: What they are, who evaluates them, how and when. Springfield, Ill: C.C. Thomas.
  8. Perlin, M., The Jurisprudence of the Insanity Defense (Carolina Academic Press, 1994).
  9. Post, C. G. (1963). An Introduction to the Law. Englewood Cliffs, NJ: Prentice-Hall
  10. Rogers, R. (1984). Rogers criminal responsibility assessment scales (R-CRAS) and test manual. Odessa, FL: Psychological Assessment Resources.
  11. Slobogin, C., Melton, G. B., & Showalter, C. R. (1984). The feasibility of a brief evaluation of mental state at the time of offense. Law and Human Behavior, 8, 305-321.
  12. Slovenko, R. (1999). The Mental Disability Requirement in the Insanity. Defense.Behavioral Sciences & the Law Vol. 17, Issue 2, p165-180.
  13. United States v. Brawner, 471 F. 2d 969 (D.C. Cir., 1972).

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