Last Monday, the Supreme Court issued its decision in Kahler v. Kansas and upheld Kansas’s significant narrowing of the insanity defense. Justice Kagan—joined by the five conservative justices—wrote for the majority, rejecting Kahler’s claim that Kansas had impermissibly narrowed the insanity defense such that it violated due process.

The insanity defense has a rich history in the Anglo-American tradition but its form has changed over time and current state law is a patchwork. The most influential formulation of the insanity defense is M’Naghten’s Rule, set forth in England in 1843. Under M’Naghten, insanity is a complete defense (1) when a defendant’s illness leaves him unable to distinguish right from wrong (moral capacity) or (2) when a defendant’s illness leaves him unable to understand the nature or quality of his act (cognitive capacity). In the case of murder, a defendant who lacks only moral capacity understands that he is killing another person but—because of mental illness—believes that the killing is morally justified, perhaps because he has a delusion that God has decreed it.  A defendant who has the moral capacity but lacks cognitive capacity knows that it would be morally wrong to murder but does not understand that he is killing a person, for example because he has a delusion that the victim is a dragon.

Kahler, who shot his estranged wife, her grandmother, and his two daughters, claimed that mental illness prevented him from knowing right from wrong when he pulled the trigger. But Kansas, by statute, allows mental illness as a defense only when it prevented the defendant from having the “culpable mental state required as an element of the crime charged” and did not allow for a moral incapacity defense. Thus, only the dragon-slayer – and neither the killer acting according to a deific decree nor Kahler—would have a defense of insanity in Kansas.

Kahler argued that eliminating M’Naghtens moral capacity prong of the insanity defense violated due process. But, the bar for whether a state criminal liability law violates due process—established in Leland v. Oregon— is high, violating 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.”

Though the majority and Justice Breyer’s dissent both undertake a historical analysis to answer this question, they reach different conclusions. Kagan agrees that the insanity defense – in some form—has been recognized for centuries but rejects the argument that there is a “single canonical formulation of legal insanity.” She points to the volitional capacity test—commonly added to M’Naghten beginning in the mid-19thcentury— that allowed defendants to escape liability if their crime was the result of irresistible impulses and the later innovation to simply ask whether the criminal act was the product of the defendant’s mental illness, among other variations. Thus, she finds only a general principle, which Kansas’s narrowed insanity defense does not violate. She argues that this is strengthened by the fact that Kansas allows mental illness generally to be accounted for at sentencing.

The dissent finds more than a general principle: it locates moral capacity in the irreducible core of the insanity defense. The preeminent common-law jurists, Breyer finds, linked the affirmative defense of insanity to the absence of reason, free will, and moral understanding, with the metaphors of the wild beast and the infant underscoring the logic that those who were incapable of “perceiving the wrongful character of [their] act[s]” could not be liable for them. Breyer argues that it is contrary to this historical legacy to provide a defense to the mentally ill defendant who mistakenly believes he is shooting a dog when he shoots a man but deny that defense to the defendant who believes a dog has commanded him to shoot a man.

A further twist in Breyer’s argument is that what Kansas leaves defendants with is nothing more than rebutting the mens rea component of the government’s case, which is always available to defendants. Thus, the insanity defense that has survived through the centuries presumably must do more.

Breyer also notes that giving judges the discretion to consider mental illness at sentencing after a guilty verdict is very different from the complete defense of insanity, which gives people a right not to be punished. That a mentally ill defendant might be placed in a hospital setting after his conviction does not change the fact that this right was denied nor does it eliminate the stigma of a guilty verdict.

The court’s decision in Kahler leaves open several questions. Does Kansas’s narrowing of the insanity defense violate the Eighth Amendment’s prohibition on cruel and unusual punishment? (That question was not properly before the court in Kahler.) Could complete abolition of the insanity defense survive a due process challenge? Kagan does not think so and seems intent on telegraphing this to the states, grounding much of her opinion in her agreement with the dissent that insanity has been recognized “for hundreds of years” as “relieving responsibility for a crime.” Whether states heed her message – and what courts do in response—remains to be seen.