The US Supreme Court on Monday declined to take up a case from Idaho testing whether the federal Constitution requires states to provide criminal defendants with a right to claim they are innocent by reason of insanity.

All but four states – Idaho, Montana, Utah, and Kansas – permit criminal defendants to assert the insanity defense. The four states dropped the provision in the early 1980s after John Hinckley was found not guilty by reason of insanity in his attempted assassination of President Ronald Reagan.

The high court has never ruled on whether the 14th Amendment’s due process clause and/or the Eighth Amendment’s ban on cruel and unusual punishment require those four states to provide an insanity defense.

The court did not comment on its refusal to take up the case. However, three justices dissented from the action.

“The law has long recognized that criminal punishment is not appropriate for those who, by reason of insanity, cannot tell right from wrong,” wrote Justice Stephen Breyer in a three-page dissent joined by Justices Ruth Bader Ginsburg and Sonia Sotomayor. “If a defendant establishes an insanity defense, he is not criminally liable, though the government may confine him civilly for as long as he continues to pose a danger to himself or to others by reason of his mental illness,” Justice Breyer wrote.

In contrast, Idaho’s law mandates that a defendant’s “mental condition shall not be a defense to any charge of criminal conduct.”

Rather than a blanket insanity defense, Idaho’s approach permits defense lawyers to present evidence at trial that their client’s mental illness undercut his or her ability to form the necessary criminal intent.

Also, judges in Idaho are required at sentencing to consider the defendant’s capacity to appreciate the wrongfulness of the criminal conduct.

The issue arose in an appeal on behalf of John Joseph Delling, who pleaded guilty to two counts of second-degree murder and was sentenced to life in prison.

Mr. Delling, who has a history of mental illness, was under the delusion that former high school classmates and other associates were attempting to sap his “energy” and deplete his “power” in a way that would eventually kill him. Instead of allowing that to happen, Delling drew up a list of seven people he believed he needed to kill to save his own life.

Delling shot his first victim several times, but the man lived. He shot his second and third targets in the head. Both died. Prosecutors said that, at the time of Delling’s arrest, four other names remained on his kill list.

Delling’s lawyers said their client suffered from extensive delusions and believed he was acting in self-defense. They said Delling lacked the ability to form a rational understanding of the wrongfulness of his conduct.

Under those circumstances, they said, he should be permitted to claim the insanity defense.

Prosecutors in Idaho disagreed. They said Delling had been methodical in planning his crimes and, despite his mental illness, had been able to form clear criminal intent to kill two human beings.

“He killed in the delusional belief that the people he marked for death were ‘trying to steal his powers’ and that their actions would result in his death,” Idaho Deputy Attorney General Kenneth Jorgensen wrote in his brief to the court.

“The evidence at sentencing showed an escalation of violence in Delling’s behavior over time,” Mr. Jorgensen said. “Delling had, however, successfully hidden the extent of his delusional thinking from mental health specialists prior to the murders.”

Jorgensen also argued that Idaho law entitles convicted criminals who are insane to the same treatment as those who are civilly committed on grounds of insanity.

Delling’s lawyers said their client is not being afforded the same treatment as someone civilly committed. A patient who is civilly committed is entitled to be released after successful treatment, they said.

In contrast, Delling is being housed at Idaho’s Maximum Security Institution in solitary confinement, and must serve his full sentence even if his treatment is fully successful, Delling’s lawyers said.

The Idaho Supreme Court upheld Delling’s conviction, rejecting arguments that he should have been allowed to claim the insanity defense because he is incapable of appreciating the wrongfulness of his conduct.

“This disagreement over whether the Constitution mandates an insanity defense strikes at the heart of the integrity of the criminal justice system. And this case presents an ideal vehicle for resolving this issue,” Stanford law Prof. Jeffrey Fisher wrote in his brief urging the justices to take the Delling case.

“Almost every state, the District of Columbia, and the federal government continue to agree that punishing the insane offends the fundamental principle of justice that only those who are morally responsible for their actions may be convicted of serious crimes,” Professor Fisher wrote. “Idaho’s model …, flouts this principle and is at odds with both the weight of history and the modern consensus among the states.”

In his dissent, Breyer said the court should hear Delling’s appeal and examine whether Idaho’s modification of the insanity defense violates the 14th Amendment’s due process clause.

“The difference between the traditional insanity defense and Idaho’s standard is that the latter permits the conviction of an individual who knew what he was doing, but had no capacity to understand that it was wrong,” Breyer said.

The case was Delling v. Idaho (11-1515).