WASHINGTON -- A divided U.S. Supreme Court strengthened the rules against the execution of mentally disabled people, striking down a Florida law that imposed a strict cutoff based on the results of an intelligence test.

Voting 5-4, the justices reinforced a 2002 decision that said states can't constitutionally execute people who are mentally disabled. Tuesday's ruling said Florida and as many as eight other states were being too restrictive in defining the condition.

"The death penalty is the gravest sentence our society may impose," Justice Anthony Kennedy wrote for the majority. "Persons facing that most severe sanction must have a fair opportunity to show that the Constitution prohibits their execution. Florida's law contravenes our nation's commitment to dignity and its duty to teach human decency as the mark of a civilized world."

Under Florida's law, only inmates who scored 70 or lower on an intelligence test were eligible to be considered mentally disabled and thereby exempt from capital punishment. The majority said inmates with scores just above that level are entitled to make the case that their intellectual functioning is actually lower.

Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan joined the majority opinion.

The ruling means a new hearing for Freddie Lee Hall, a convicted murderer whose IQ had been measured at various times as 71, 73 and 80.

Hall was sentenced to death for the 1978 murder of Karol Hurst, who was 21 and seven months pregnant when Hall and an accomplice forced her into her car in a supermarket parking lot. She was found in a wooded area where she had been beaten, sexually assaulted and shot. Hall also was convicted of murdering a deputy sheriff the same day.

Mental-health experts generally consider an IQ score of 70 to mark the dividing line for mental disability. The question was how to handle people who score 71 to 74, close enough to prevent statistical confidence that they are really above the threshold.

Kennedy said Florida's approach ignored the test's margin of error. The state "disregards established medical practice," he said.

"The professionals who design, administer, and interpret IQ tests have agreed, for years now, that IQ test scores should be read not as a single fixed number but as a range," he wrote.

Kennedy said eight other states -- Kentucky, Virginia, Alabama, Arizona, Delaware, Kansas, North Carolina and Washington -- used similar cutoffs or had laws that could be interpreted that way.

Chief Justice John Roberts and Justices Samuel Alito, Clarence Thomas and Antonin Scalia dissented. Alito said the majority gave too much weight to the views of professional organizations, including the American Psychiatric Association.

"What counts are our society's standards -- which is to say, the standards of the American people -- not the standards of professional associations, which at best represent the views of a small professional elite," Alito wrote. He said society as a whole hadn't formed a consensus around the issue.

The Supreme Court assesses whether given practices are barred by the Eighth Amendment's prohibition of cruel and unusual punishment by considering, in the words of a 1958 decision, the "evolving standards of decency that mark the progress of a maturing society." In doing so, Alito said, it had always "meant the standards of American society as a whole."

"Now, however," he wrote, "the court strikes down a state law based on the evolving standards of professional societies, most notably the American Psychiatric Association."

Twenty states had allowed the execution of mentally disabled people before the court outlawed the practice in the 2002 Atkins v. Virginia decision.

The Atkins decision gave states only general guidance. It said a finding of mental retardation required proof of three things: "subaverage intellectual functioning," meaning low IQ scores; a lack of fundamental social and practical skills; and the presence of both conditions before age 18. The court said IQ scores under "approximately 70" typically indicate retardation.

Lawyers for Hall, now 68, said he has been treated as mentally disabled since he was a child. They said he has limited intellectual functioning and short-term memory as well as a speech impediment that hinders his ability to communicate.

"Today the court has ensured that Florida will honor the court's judgment in Atkins that persons with intellectual disability are constitutionally protected from execution," Eric Pickard, Hall's lawyer, said in an emailed statement.

The court returned the case, Hall v. Florida, to the lower courts for a fresh assessment of Hall's condition.

"Freddie Lee Hall may or may not be intellectually disabled," Kennedy wrote, "but the law requires that he have the opportunity to present evidence of his intellectual disability, including deficits in adaptive functioning over his lifetime."

In earlier decisions limiting the use of the death penalty and other harsh punishments under the Eighth Amendment, Kennedy has often joined the court's liberal wing. He wrote several of those decisions.

"The Eighth Amendment's protection of dignity," he wrote in Tuesday's opinion, "reflects the nation we have been, the nation we are, and the nation we aspire to be. This is to affirm that the nation's constant, unyielding purpose must be to transmit the Constitution so that its precepts and guarantees retain their meaning and force."

Kennedy was also in the majority in cases striking down the death penalty for the mentally disabled, for juvenile offenders and for nonhomicide crimes and in ones limiting the use of life without parole sentences for juvenile offenders.

Information for this article was contributed by Greg Stohr of Bloomberg News and by Adam Liptak of The New York Times.

A Section on 05/28/2014