Insanity defense and the Supreme Court

DESCRIPTION: A legal defense in which a defendant pleads not guilty on the grounds that a severe mental disorder rendered them incapable of understanding the nature or wrongfulness of their actions at the time of the offense.

SIGNIFICANCE: The insanity defense is rooted in the common law principle that criminal liability requires criminal intent. The US Supreme Court has examined the issue since the late nineteenth century and has addressed it in cases involving both due process protections and public safety concerns.

Historically, the standards governing the insanity defense were the province of the states, with intrastate consistency maintained by appellate court decisions and state statutes. Inconsistencies appeared among states on several issues—variation in legal definitions, allowable evidence, and post-verdict treatment. Most states relied on the M’Naghten Rule (created in 1843 in England by the House of Lords), which restricts insanity to a mental condition severe enough to prevent the defendant from understanding the nature or consequences of their act. Individuals found not guilty by reason of insanity were often institutionalized for an indeterminate period.

In the mid-twentieth century, two contrasting cultural trends encouraged the Supreme Court to examine the procedures and consequences of the insanity defense. These trends were an emerging concern with the civil liberties of people with mental illness and a growing fear of crime that highlighted the need for the state to protect the public from all individuals capable of committing violent crimes.

A series of Court decisions protected the due process rights of a defendant to plead not guilty by reason of insanity. The Court ruled in Ake v. Oklahoma (1985) that a psychiatric expert must be provided for an indigent defendant who wished to plead insanity. In Riggins v. Nevada (1992), the Court ruled that defendants who plead not guilty by reason of insanity may refuse, at the time of their testimony, to take medication that will reduce or eliminate symptoms. It also found, in Jones v. United States (1983) and Foucha v. Louisiana (1992), that the release of someone found not guilty by reason of insanity depends on whether the individual remains mentally ill and dangerous, not the expiration date of the sentence for the alleged offense.

At the same time, the Court recognized the police powers of the state to protect the public from dangerous people. In Leland v. Oregon (1952), the Court upheld state laws that made an insanity plea more difficult by placing the burden of proof on the defendant. In Kansas v. Hendricks (1997), the Court refused to extend due process safeguards to sexually violent predators, who may be institutionalized indefinitely beyond their prescribed sentence.

Public concern around the insanity defense intensified after John Hinckley Jr. was found not guilty by reason of insanity for his 1981 assassination attempt on President Reagan. Several states tried to reduce the use of insanity pleas by adopting a “guilty but mentally ill” (GBMI) verdict, which allows for both treatment and punishment. Other state courts continued to employ definitions of insanity broader than that outlined in the M’Naghten Rule, encompassing “irresistible impulses.” The Court has reviewed and generally upheld such definitional variations.

In a major twenty-first-century decision, the Court ruled in Kahler v. Kansas (2020) that the Constitution does not require states to maintain an insanity defense based on moral incapacity. This affirmed that states have broad discretion in determining the scope or existence of an insanity defense, including the use of approaches that focus on criminal intent rather than moral incapacity, so long as defendants can still present mental illness as mitigating evidence. Although the scope of insanity definitions varies, the underlying relevance of criminal intent remains broadly accepted and constitutionally solid, and this rule remains unchanged in Supreme Court doctrine.

Bibliography

“Davis v. United States.” Legal Information Institute, 16 Dec. 1895, www.law.cornell.edu/supremecourt/text/160/469. Accessed 10 Apr. 2026.

Heilbrun, Kirk, et al. Wrightsman’s Psychology and the Legal System. 10th ed., Cengage, 2024.

“Kansas, Petitioner v. Leroy Hendricks.” Legal Information Institute, 23 June 1997, www.law.cornell.edu/supremecourt/text/521/346. Accessed 10 Apr. 2026.

O’Connor, Katie. “Taking the Fight for the Insanity Defense to the States.” Psychiatric News, vol. 55, no. 21, 2020, doi:10.1176/appi.pn.2020.11a22. Accessed 10 Apr. 2026.

"SC: Past Psychiatric Records Not Required to Prove Legal Insanity." The Supreme Court of the Philippines, 27 Feb. 2025, sc.judiciary.gov.ph/sc-past-psychiatric-records-not-required-to-prove-legal-insanity. Accessed 10 Apr. 2026.

Slobogin, Christopher. Justifying the Insanity Defense. Oxford University Press, 2023.

Steadman, Henry J., et al. Before and After Hinckley: Evaluating Insanity Defense Reform. Guilford Press, 1993.

“Terry Foucha, Petitioner v. Louisiana.” Legal Information Institute, 18 May 1992, www.law.cornell.edu/supremecourt/text/504/71. Accessed 10 Apr. 2026.

"US Supreme Court Sides with Kansas over Insanity Defense." American Bar Association, 23 July 2020, www.americanbar.org/groups/committees/death_penalty_representation/project_press/2020/summer/us-supreme-court-sides-with-kansas. Accessed 10 Apr. 2026.