The Supreme Court on Monday ruled that states can effectively eliminate the insanity defense for criminal defendants who suffer from mental illness.

The 6-3 ruling holds that a Kansas law preventing the exoneration of defendants who claim a diminished mental state is not unconstitutional.

In an unusual alignment for the bench, Justice Elena Kagan Elena KaganREAD: Supreme Court justices mourn death of Ginsburg, 'an American hero' Democrats, advocates seethe over Florida voting rights ruling Supreme Court denies push to add Green Party candidates to Montana ballot MORE, considered among the more liberal justices, wrote the majority opinion and was joined by her five conservative colleagues. Three liberal justices — Stephen BreyerStephen BreyerREAD: Supreme Court justices mourn death of Ginsburg, 'an American hero' Ginsburg death sets up battle over future of court Trump's Supreme Court list reveals influence of Clarence Thomas MORE, Sonia Sotomayor Sonia SotomayorNames to watch as Trump picks Ginsburg replacement on Supreme Court READ: Supreme Court justices mourn death of Ginsburg, 'an American hero' READ: Supreme Court justices offer tributes to Ruth Bader Ginsburg MORE and Ruth Bader Ginsburg Ruth Bader GinsburgRegina King accepts Emmy wearing Breonna Taylor shirt, urges viewers to vote Ocasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat Mural of Ruth Bader Ginsburg pops up blocks away from White House MORE — dissented.

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"Defining the precise relationship between criminal culpability and mental illness involves examining the workings of the brain, the purposes of the criminal law, the ideas of free will and responsibility," Kagan wrote. "It is a project demanding hard choices among values, in a context replete with uncertainty, even at a single moment in time. And it is a project, if any is, that should be open to revision over time, as new medical knowledge emerges and as legal and moral norms evolve."

"Which is all to say that it is a project for state governance, not constitutional law," Kagan continued.

The Kansas law prevents defendants from arguing that their diminished mental state impaired their ability to understand right from wrong and that they should not be held criminally responsible for their offenses. Under the law, defendants can still argue that they lacked the necessary intent to commit the crime, and can use insanity to ask for reduced sentences.

The law was challenged by James Kahler, who in 2011 was sentenced to death for killing his two daughters, his estranged wife and his wife's grandmother in a 2009 rampage.

His lawyers had unsuccessfully argued before trial that the state law preventing Kahler from using the insanity defense was unconstitutional. Kahler also presented evidence of insanity at sentencing after his conviction, but the jury imposed the death penalty.

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Four other states — Alaska, Idaho, Montana and Utah — have adopted the same rules about the insanity defense. Kahler's lawyers argued to the Supreme Court that the Kansas rule undermines a long-held legal principle.

"This rule defies a fundamental, centuries-old precept of our legal system: People cannot be punished for crimes for which they are not morally culpable," they wrote in a 2018 brief.

Breyer wrote in a dissent that the rule effectively administers an arbitrary disparity over who can argue insanity. He presented a hypothetical contrasting two different mentally-ill defendants: the first was shown at trial to have killed someone because they were convinced that the victim was a dog, and the second killed someone out of the belief that a dog ordered them to.

"Under Kansas’ changed law, the defendant in Prosecution One could defend against the charge by arguing that his mental illness prevented him from forming the mental state required for murder (intentional killing of a human being)—just as any defendant may attempt to rebut the State’s prima facie case for guilt," Breyer wrote.

"The defendant in Prosecution Two has no defense," he continued. "Because he acted with the requisite level of intent, he must be convicted regardless of any role his mental illness played in his conduct."

Updated at 11:47 a.m.