The Due Process Clause does not require states to allow the acquittal of defendants who, because of mental illness, cannot tell right from wrong when they committed their offenses, the U.S. Supreme Court held March 23 in Kahler v. Kansas
States are free to adopt their own standards for an insanity defense “as new medical knowledge emerges and as legal and moral norms evolve,” the Court ruled.  “Which is all to say that it is a project for state governance, not constitutional law.”
James Kahler shot and killed four family members. 
Kansas law permits an insanity defense on grounds that a defendant “lacked the culpable mental state as required as an element of the offense charged.”   Kansas law also allows a defendant to present any mental illness as a mitigating factor for sentencing, which can also lead to commitment to a mental hospital, rather than prison.
But Kansas law provides that “mental disease or defect is not otherwise a defense.”
The result is that, unlike many states, Kansas does not allow acquittal of a defendant on insanity grounds where the defendant cannot tell “right from wrong” when committing his crime. 
Kahler argued the Fourteenth Amendment’s Due Process Clause mandated that he be allowed to present such an insanity defense, and that Kansas had “unconstitutionally abolished the insanity defense” by not allowing his defense.
The Kansas Supreme Court rejected this argument.
The U.S. Supreme Court, in a 6-3 opinion by Justice Kagan, held that states are not required to use a particular insanity standard as a matter of constitutional law.
A state rule about criminal liability “violates due process only if it offends some principle of justice so rooted in the traditions and conscience of our people as to be ranked as fundamental,” the Court said. 
“The question is whether a rule of criminal responsibility is so old and venerable – so entrenched in the central values of our legal system – as to prevent a State from ever choosing another,” the Court said.  “An affirmative answer, though not unheard of, is rare.”
The Court agreed that “Kahler is right that for hundreds of years jurists and judges have recognized insanity (however defined) as relieving responsibility for a crime.”
“But neither do we think Kansas departs from that broad principle,” the Court said.  “Kansas has an insanity defense negating criminal liability – even though not the type Kahler demands.” 
Kansas has not “failed to offer any insanity defense at all,” the Court said.
Furthermore, Kansas allows any mental illness evidence to be presented as a mitigating circumstance at sentencing, which can also lead to commitment to a mental hospital, not prison.  “In sum, Kansas does not bar, but only channels to sentencing, the mental health evidence that falls outside its intent-based insanity defense,” the Court said.
The long historical record of the insanity defense is “complex – even messy,” the Court said.  Various standards have been used, dating back to the 16th century.
“In a sphere of flux and disagreement with fodder for reasonable debate about what the cognate legal and medical tests should be, due process imposes no one view of legal insanity,” the Court concluded.
“Contrary to Kahler’s view, Kansas takes account of mental health at both trial and sentencing.  It just has not adopted the particular insanity defense Kahler would like,” the Court said.
“That choice is for Kansas to make,” the Court said.  “No insanity rule in this country’s heritage or history was ever so settled as to tie a State’s hands centuries later.”
Justice Breyer, joined by Justices Ginsburg and Sotomayor, dissented.