Kahler v. Kansas, 589 U.S. 271

Kansas statute (§21‑5209) permits mental‑illness evidence only to negate the culpable mental state (mens rea) required for an offense; it expressly bars other insanity defenses at guilt but allows mental‑health evidence at sentencing and commitment alternatives.
James Kahler killed four family members, was charged with capital murder, argued Kansas’s rule violated due process by allowing conviction of those who cannot tell right from wrong, was convicted, and sentenced to death.
Kansas trial court excluded a moral‑incapacity insanity defense at the guilt phase; the Kansas Supreme Court affirmed, and the U.S. Supreme Court granted certiorari.
Petitioner’s core claim: due process requires States to adopt M’Naghten’s moral‑incapacity (right‑from‑wrong) test and thus forbid conviction of defendants who, because of mental illness, could not tell moral right from wrong.
Respondent’s position: Kansas retains an insanity defense (cognitive/mens rea prong), preserves broad use of mental‑illness evidence at sentencing, and states have historical and policy leeway to define insanity defenses.
Supreme Court holding: The Due Process Clause does not require States to adopt a moral‑incapacity right‑from‑wrong insanity test; States have discretion in defining insanity defenses and Kansas’s scheme is constitutional.

Issue	Plaintiff's Argument (Kahler)	Defendant's Argument (Kansas)	Held
Whether Due Process requires adoption of a moral‑incapacity (right‑from‑wrong) insanity test	Due process mandates that a defendant who, from mental illness, cannot tell moral right from wrong must be acquitted	No; history shows varied tests and States may define elements/defenses; Kansas provides cognitive‑mens‑rea defense and sentencing relief	No. Due process does not require a moral‑incapacity baseline; States have latitude to choose insanity tests
Whether Kansas has abolished the insanity defense entirely	Kansas’s rule abolishes the core insanity defense by permitting conviction of those lacking moral capacity	Kansas retains an insanity defense to negate mens rea and allows full mental‑illness evidence at sentencing and civil commitment	No. Kansas has not abolished the insanity defense; it channels certain evidence to sentencing and commitment
Whether allowing mental‑illness evidence only at sentencing (not for guilt) violates due process	Post‑conviction mitigation is insufficient because it leaves obviously insane defendants susceptible to conviction and stigma	Sentencing‑phase consideration and commitment options adequately address blame and treatment without imposing a single constitutional rule	No. Permitting mitigation at sentencing and mens‑rea defenses at guilt satisfies due process limits; choice of approach is for States

Clark v. Arizona, 548 U.S. 735 (2006) (States have latitude to select among various insanity tests; no single formulation is constitutionally required)
Leland v. Oregon, 343 U.S. 790 (1952) (due process does not compel a particular insanity test; States may choose tests based on policy and evolving science)
Powell v. Texas, 392 U.S. 514 (1968) (criminal‑responsibility doctrines reflect complex, evolving policy choices best left to States)
Montana v. Egelhoff, 518 U.S. 37 (1996) (historical practice is primary guide for substantive‑due‑process limits on state rules defining criminal liability)
Ake v. Oklahoma, 470 U.S. 68 (1985) (recognition of unsettled psychiatric science when assessing legal protections for defendants)
People v. Schmidt, 216 N.Y. 324 (1915) (Cardozo) (illustrates difference between moral‑wrong and legal‑wrong formulations of the M’Naghten test)