Is an insanity defense a constitutional right?

The question in the title of this post is prompted by this effective Washington Post article, which is headlined, "Supreme Court is asked to find that insanity defense is a constitutional right." Here is how it gets started:

There’s no doubt John Joseph Delling knew what he was doing. His carefully planned 2007 crime spree lasted weeks, covered 6,500 miles and culminated in two people dead and one seriously wounded.

He had his reasons, too. Delling, then 21, had become “a type of Jesus,” he later explained, and the men he attacked, two of them former classmates he had not seen in years, were stealing his “energy.” An MRI of his brain would have revealed the damage the men had already caused, he told authorities. “I had to defend myself,” he said.

As the nation confronts another act of unfathomable madness, Delling’s story is one chapter in a distressing and violent genre: the loner who tries to impress a movie star by shooting the president; the mother who drowns her children to save them from damnation; the black-clad shooter who seems to step from the movie screen to kill.

But Delling’s case presents an intriguing legal question as well. He committed his crimes in Idaho, which is one of only four states — Kansas, Montana and Utah are the others — in which a defendant may not use insanity as a defense to criminal charges.

Delling’s lawyers are now at the Supreme Court, asking the justices to rule that the Constitution mandates that such a defense be available for those who, because of mental illness, are not responsible for the mayhem they create. “For centuries, the moral integrity of the criminal law has depended, in part, on the insanity defense,” Stanford law professor Jeffrey L. Fisher wrote in a petition on Delling’s behalf.

Punishment is traditionally justified on the basis of an individual consciously choosing evil over good, Fisher wrote. “Laws such as Idaho’s abandon that basic tenet,” he said. Fisher contends that Idaho’s law violates the Constitution’s guarantee of due process of law, as well as the Eighth Amendment’s prohibition of cruel and unusual punishment.

All states and the federal government once allowed the insanity defense. But that changed with the public outrage over John W. Hinckley Jr.’s acquittal for reasons of insanity in his assassination attempt on President Ronald Reagan in 1981. Many states and the federal government reacted by shifting the burden of proving insanity to the defense. But five states, including Nevada, abolished the insanity plea.

The highest courts in four of those states have upheld the laws. The Nevada Supreme Court, however, struck down that state’s statute, saying that the insanity defense recognizes a “fundamental principle” that people cannot be convicted of crimes when mental illness prevents them from knowing that their conduct is wrong. The other state supreme courts have found otherwise, and so far, the U.S. Supreme Court has not found reason to accept appeals in any of those rulings.

Despite its prominence in television crime dramas, the insanity defense is rarely invoked and is successful only about a quarter of the time, according to the most widely quoted study of its use. In its last examination of the issue in 2006, the court ruled that Arizona could narrow the insanity defense to exclude some defendants. The justices said they did not need to address the more fundamental question of whether an insanity defense is constitutionally mandated.

Especially in the wake of the Supreme Court's recent juvenile Eighth Amendment jurisprudence, I could readily imagine SCOTUS saying severe mental illness must be considered at trial OR sentencing in some way.  And, whatever the Justices might think on the merits, this issue surelt seems worthy of its consideration.