The U.S. Supreme Court on Monday turned down an opportunity to consider whether states can ban the insanity defense in criminal cases.

Most states permit a defendant to claim the defense of not guilty by reason of insanity.  It's not a medical term, it's a legal one, generally meaning that a person could not understand the difference between right and wrong and was, therefore, unable to act with criminal intent. 

Though long permitted, it has never been popular.  Between 1979 and 1995, five states decided to ban it -- Idaho, Kansas, Montana, Nevada and Utah.  A factor in the changing public sentiment was John Hinckley Jr's successful insanity plea when he was put on trial for shooting President Ronald Reagan in 1981.

The Supreme Court on Monday declined to take up a challenge from lawyers for an Idaho man, John Delling, who was convicted of killing two of his friends while, his lawyers contend, he was in the grip of severe delusions caused by acute paranoid schizophrenia. Because Idaho's law says that a defendant's mental condition "shall not be a defense to any charge of criminal conduct," he was unable to plead not guilty by reason of insanity.

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Delling's lawyers argued that the Constitution's guarantee of due process demands that the insanity defense be available because it has strong roots in the legal system. And, they said, the ban on cruel and unusual punishment "forbids criminal punishment that violates broadly and deeply held Anglo-American legal practices."  

Idaho defended its law, contending that "moral incapacity is only one of four different historical approaches to insanity in criminal cases, no one of which is constitutionally required."

Three justices, Stephen Breyer, Ruth Bader Ginsburg and Sonia Sotomayor, dissented from the court's refusal to take up the case.  "The law has long recognized that criminal punishment is not appropriate for those who, by reason of insanity, cannot tell right from wrong," they said.

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