Editorial: The insanity defense isn’t available in every state. It should be

A longstanding principle of Anglo-American law is that a defendant shouldn’t be held criminally responsible for his behavior if mental illness made it impossible for him to tell right from wrong. Yet the U.S. Supreme Court has declined to hold that the Constitution requires that defendants be able to assert an insanity defense. That may be about to change.

On Monday, the justices agreed to hear the appeal of James Kahler, who was convicted and sentenced to death after killing four members of his own family — his wife, two of his daughters and his wife’s grandmother — in Kansas, one of a handful of states that prohibit a traditional insanity defense. In Kansas, a defendant may cite a “mental disease or defect” only as a partial defense, and can be found guilty as long as he intended to commit a violent crime.

Kahler’s crimes were horrific, but he should have been able to argue that he was entitled to a verdict of not guilty by reason of insanity, as he could have done in other parts of the country. His lawyers are now asking the Supreme Court to hold that Kansas’ refusal to afford him that opportunity “defies a fundamental, centuries-old precept of our legal system: People cannot be punished for crimes for which they are not morally culpable.”

They argue that Kansas’ rule violates the 8th Amendment’s prohibition of cruel and unusual punishments and the 14th Amendment’s guarantee of due process. If the court accepts their argument, the insanity defense will acquire constitutional status and apply nationwide. That would be the just outcome.

We understand that many people are offended by the idea that someone who was insane during the commission of a crime shouldn’t be held responsible. But that idea reflects an important principle and has a long pedigree. In 1843 the British House of Lords announced what is known as the M’Naghten Rule: A defendant is insane if at the time of the crime, and as a result of a mental disease or defect, he either didn’t know the “nature and quality of the act he was doing” or he didn’t know it was wrong.

Half the states, including California, employ a form of the M’Naghten Rule. Most of the others employ a more liberal definition such as the one contained in the Modern Penal Code, which says that a mentally ill individual isn’t responsible if, at the time of the crime, he was unable to appreciate the criminality of his conduct or conform his conduct to the requirements of the law.

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Regardless of which definition is used, insanity pleas are rarely offered and usually unsuccessful, because of the high bar they set. Yet many citizens — and politicians — react indignantly to the idea of an acquittal by reason of insanity. A national firestorm followed the insanity acquittal of John W. Hinckley Jr., the deranged young man who attempted to assassinate President Reagan in 1981, and some states moved to make it harder to successfully assert the defense.

Some of the opposition to the insanity defense reflects a belief that mental illness ought not reduce culpability, and that even insane perpetrators of crimes should be tried and punished. But there is also a widespread misconception that defendants found not guilty by reason of insanity will instantly be let loose. In reality, they are generally committed to mental hospitals where they can be confined for long periods of time. (After his 1982 acquittal, Hinckley was committed to St. Elizabeths Hospital in Washington, D.C. , where he remained until 2016, though in the later years he was allowed to make visits to his parents’ home.)

One of the ironies of the debate over the insanity defense is that the definition of legal insanity is so narrow that it is of no use to many criminal defendants who, while responsible for their actions, also suffer from significant mental problems that likely contributed to their violations of the law. Their problems need to be addressed in other ways, and ideally before they enter the criminal justice system. But for the small number of defendants whose mental diseases do limit their responsibility for their actions, the insanity defense must be available — in every state of the Union.

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