Insanity Defense Essay

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The insanity defense is an affirmative defense to a criminal charge. Affirmative defenses are those in which the defendant tries to limit or completely eliminate criminal liability by offering an excuse or justification for the act. (Self-defense and duress are also affirmative defenses.) In the insanity defense’s traditional form, the defendant enters a plea of “not guilty by reason of insanity” (NGRI). Defendants who plead NGRI do not deny having committed the offense, but rather argue that at the time the crime was committed, they were unable to form the necessary intent and therefore should not be held legally responsible for their acts. The insanity defense has evolved over time, and jurisdictions have adopted different standards or tests. Four states—Montana, Idaho, Utah, and Kansas—do not permit insanity as a defense.

Insanity is a legal concept, not a psychological or psychiatric one. The American legal tradition recognizes two elements to any criminal offense: the actus reus (criminal act) and the mens rea (guilty mind). The inclusion of the second element, mens rea, requires that the prosecution establish culpability (or blameworthiness) by demonstrating that the accused not only committed the offense, but also intended to commit the offense. Although proving the defendant committed the act often supersedes the need to literally prove the intent (it is generally accepted that most acts are intended), the mens rea requirement opens up the opportunity for the defense to launch a diminished capacity or insanity defense in response to a criminal charge.

The traditional insanity defense, the M’Naughten Rule, which was adopted from the English Common Law, states that a defendant’s criminal conduct can be excused if “at the time of the committing of the act, the party accused was laboring under such a defect of reason, arising from a 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 what he was doing was wrong.” The M’Naughten Rule established a cognitive test that was supplemented in some jurisdictions by an “irresistible impulse” test (a test of whether the defendant could control his or her conduct). Appellate court decisions established the Durham Rule, which required that the act be a “product” of the mental defect or illness, and the Brawner Rule, which endorsed the American Legal Institute’s (ALI’s) Model Penal Code definition of insanity. The ALI expanded the cognitively based M’Naughten Rule to allow for an insanity defense based on either the defendant’s capacity to appreciate the wrongfulness of his or her conduct or his or her capacity to conform his or her conduct to the law.

The most substantial insanity defense reform followed John Hinckley, Jr.’s 1982 NGRI acquittal for his 1981 attempt to assassinate then-president Ronald Reagan. At the time of Hinckley’s acquittal, the insanity defense was based on the ALI’s standard and included both a cognitive and a volitional prong. A defendant’s NGRI plea would succeed if the defendant could demonstrate an inability to appreciate the wrongfulness of his or her conduct or lacked the ability to control the conduct. The public outrage that followed Hinckley’s acquittal led to swift legislative action culminating in the 1984 passage of the Insanity Defense Reform Act (IDRA). The IDRA was a sweeping reform of the insanity defense that eliminated the volitional component, shifted the burden of proof from the prosecution to the defense, and increased the evidentiary standard from “preponderance of the evidence” to “clear and convincing evidence.” Following IDRA reforms at the federal level, states began to revisit their insanity defense statutes, with many states reverting to a purely cognitive test. States have also revised their penal codes to allow for a diminished capacity defense (a defense that, if successful, might result in conviction on a lesser charge rather than a finding of not guilty) or a guilty but mentally ill verdict (a verdict that recognizes the mental illness without negating the defendant’s responsibility for the offense).

High-profile insanity defense pleas and acquittals have led to public outcries against the use of the insanity defense; however, contrary to popular misconceptions, the most comprehensive study of the insanity defense suggested that the defense is infrequently used and, when used, is rarely successful.


  1. Model Penal Code (1985). § 4.01(1). Murdock, D. (Executive Producer). (2002, October 17). Frontline: A crime of insanity [Television broadcast]. New York and Washington, DC: Public Broadcasting Service.
  2. Steadman, H. J., McGreevey, M. A., Morrissey, J. P., Callahan, L. A., Robbins, P. C., & Cirincione, C. (1993). Before and after Hinckley: Evaluating insanity defense reform. New York: Guilford Press.

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