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The Supreme Court on Monday ruled against a Kansas man who argued his constitutional rights were violated when the state refused to allow him to bring an insanity defense.

In a 6-3 opinion, the Court said an insanity defense that considers whether a defendant can distinguish between right and wrong is not constitutionally required.

Under the law in Kansas, a defendant can argue mental illness only to prove that he did not intend to commit the crime. Otherwise, mental illness cannot be used as a defense. Four other states have also abolished an insanity defense.

The ruling could inspire other states to do so and will make it more difficult for the mentally ill to defend themselves.

“Today’s decision leaves much, if not most, of the scope of insanity defenses to individual states,” said Steve Vladeck, CNN Supreme Court analyst and professor at the University of Texas School of Law.

“Kansas is one of only five states that has all-but abolished the defense – a step that the majority upheld today. But the most important implication of the decision may be in opening the door to additional states that want to follow suit,” he said.

The man, James Kahler, was sentenced to death for killing his wife and other family members in 2009. His lawyers sought to raise a defense that he was mentally ill and did not know right from wrong in part because he was experiencing overwhelming obsessive compulsion episodes.

The so-called “insanity defense” bars such an individual from being held criminally responsible.

“Contrary to Kahler’s view, Kansas takes account of mental health at both trial and sentencing. It has just not adopted the particular insanity defense Kahler would like,” Justice Elena Kagan wrote in the majority opinion.

“That choice is for Kansas to make – and, if it wishes, to remake and remake again as the future unfolds. No insanity rule in this country’s heritage or history was ever so settled as to tie a State’s hands centuries later.”

Kagan stressed that Kansas law allows a defendant to “present psychiatric and other evidence of mental illness” through testimony to prove that “he had no intent to kill” to defend himself against a criminal charge.

“The defendant can use that evidence to show that his illness left him without cognitive capacity to form the requisite intent,” she said. And, she said, Kansas permits a defendant to offer whatever mental health evidence he “deems relevant at sentencing.”

“If a mentally ill defendant had enough cognitive function to form the intent to kill, Kansas law directs a conviction even if he believed the murder morally justified,” she added.

Writing for Kagan’s three liberal colleagues, Justice Stephen Breyer argued that “a defendant who, due to mental illness, lacks sufficient mental capacity to be held morally responsible for his actions cannot be found guilty of a crime.”

“Here” Breyer said, “Kansas has not simply redefined the insanity defense.”

“Rather,” he said, “it has eliminated the core of a defense that has existed for centuries: that the defendant, due to mental illness, lacked the mental capacity necessary for his conduct to be considered morally blameworthy.”

This story has been updated with additional details of the ruling.