Kahler v. Kansas

Supreme Court of the United States · 2020 · Criminal Law

589 U.S. 271 (2020)

Updated August 1, 2026

Criminal Lawinsanitydue processinsanity defenseM'Naghtenmoral incapacitycognitive incapacitymens rea

Facts

Kansas law allows a defendant to use evidence of mental disease or defect to show that he lacked the culpable mental state required for the charged offense, but it provides that mental disease or defect is not otherwise a defense. Thus, Kansas permits mental-illness evidence at the guilt phase only to negate mens rea, not to obtain acquittal based on inability to recognize moral wrongfulness. At sentencing, however, Kansas allows broad mental-health evidence to mitigate punishment or support commitment to a mental health facility. Kahler argued before trial that this scheme was unconstitutional because it allowed conviction of a mentally ill defendant who could not tell right from wrong.

Issue

Does the Fourteenth Amendment's Due Process Clause require a state to adopt an insanity defense that acquits a defendant who, because of mental illness, could not distinguish right from wrong when committing the crime? More specifically, does due process compel states to adopt M'Naghten's moral-incapacity test?

Rule

A state rule governing criminal liability violates due process only if it offends a principle of justice so rooted in the traditions and conscience of the people as to be ranked as fundamental. No particular insanity test, including M'Naghten's moral-incapacity test, is a fundamental baseline required by due process; states have substantial latitude to define insanity defenses and to channel mental-illness evidence between guilt and sentencing.

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In Missouri, the legislature amends its criminal code to provide that a defendant may introduce evidence of mental disease at trial only to show he lacked the culpable mental state required for the charged offense. The code also allows a convicted defendant to present any mental-health evidence at sentencing to seek mitigation or commitment to a psychiatric facility. After intentionally setting fire to a building in St. Louis, Omar Velez claims he knew the act was illegal but, because of psychosis, believed it was morally required to save his neighborhood.

If Omar argues that the Fourteenth Amendment requires the State to acquit him because he could not distinguish moral right from wrong, how should a court rule?

Explanation. Due process does not compel a State to use M'Naghten's moral-incapacity test. The majority held that a State may instead allow mental-illness evidence at the guilt phase to negate mens rea and reserve broader mental-health evidence for sentencing mitigation or commitment. Because no single insanity formulation is constitutionally required, Omar's due process claim fails.