WASHINGTON, June 29 — The Supreme Court upheld Arizona's limited approach to the insanity defense on Thursday, ruling that states are not obliged to permit a defendant to argue that mental illness prevented him from forming the requisite intent to commit a crime.
The case was brought by an Arizona man who was a teenager suffering from paranoid schizophrenia when he shot and killed a police officer. He was convicted of violating a law that makes it a crime to kill a police officer intentionally, and he argued that the delusions caused by his illness had prevented him from forming that specific intent.
Arizona law permits defendants to argue that because of mental illness, they lack the ability to appreciate the wrongfulness of their action. But the state does not permit the introduction of evidence of a lack of cognitive capacity, either as part of the definition of insanity itself or as part of a defense of the lack of criminal intent. Only a handful of states take such a limited approach to the insanity defense.
The court has never ruled that the Constitution requires states to allow an explicit insanity defense. Writing for the court, Justice David H. Souter said that approaches among the states varied so widely that it was "clear that no particular formulation has evolved into a baseline for due process" and that the constitutional guarantee of due process "imposes no single canonical formulation of legal insanity." That leaves the insanity defense "substantially open to state choice," Justice Souter said.
The Arizona procedures meet the test of due process, Justice Souter concluded in an opinion that was joined by four other justices: Chief Justice John G. Roberts Jr. and Justices Antonin Scalia, Clarence Thomas and Samuel A. Alito Jr.
Justice Stephen G. Breyer provided a sixth vote for most parts of the opinion, Clark v. Arizona, No. 05-5966. But rather than affirm the conviction of the defendant, Eric M. Clark, who was sentenced to life in prison, Justice Breyer said the Arizona courts should make sure the law had been applied "with sufficient directness and precision" in Mr. Clark's case.
Justices Anthony M. Kennedy, John Paul Stevens, and Ruth Bader Ginsburg dissented, saying in an opinion by Justice Kennedy that the state's categorical exclusion of evidence was irrational and unjustified.
One justification that the majority cited was avoiding juror confusion. Justice Souter said it was one thing for a defendant to show by expert testimony that he is mentally ill, but it was another to extrapolate from such evidence to a conclusion that the defendant could not have formed the specific intent to commit a crime, for which the legal term is the Latin mens rea.
Such evidence "can easily mislead," he said, adding: "It is very easy to slide from evidence that an individual with a professionally recognized mental disease is very different, into doubting that he has the capacity to form mens rea, whereas that doubt may not be justified."
In criminal law, a defendant who is claiming a defense of insanity has the burden of demonstrating mental illness, while the government retains the burden of proof on the issue of criminal intent. That means that once a defendant puts insanity as an issue into a trial on the question of criminal intent, the government has to show that the defendant did have the capacity, a difficult burden for the prosecution to meet when the defendant is clearly mentally ill.
In this case, Mr. Clark's trial took place before a judge without a jury. The judge found that Mr. Clark was indisputably mentally ill but that he still knew his action was wrong.