In 2002, the court said that executing mentally disabled inmates violates the Eighth Amendment prohibition on cruel and unusual punishment. But until Tuesday, the justices left to the states the determination of who is mentally disabled.
Kennedy said the finality of capital punishment requires giving inmates the chance to present evidence of mental disability in borderline cases.
"The states are laboratories for experimentation, but those experiments may not deny the basic dignity the Constitution protects," Kennedy said in an opinion that was joined by the court's four more liberal justices.
At most, eight other states employ a similar cutoff of 70, Kennedy said. He listed those states as: Alabama, Arizona, Delaware, Kansas, Kentucky, North Carolina, Virginia and Washington. Kansas has not executed anyone in nearly 50 years and Washington recently suspended its death penalty, he said.
In dissent for the four justices to the right of Kennedy, Justice Samuel Alito said the court has no evidence that relying on test scores just above 70 is unreasonable and so should not be held unconstitutional.
Tuesday's decision came in the case of 68-year-old Freddie Lee Hall. Lawyers for Hall said there is ample evidence to show that he is mentally disabled, even though most of his multiple I.Q. tests have yielded scores topping 70. Hall has been on death row for more than 35 years since being convicted of murdering a pregnant 21-year-old woman in 1978.
In nine tests administered between 1968 and 2008, Hall scored as low as 60 and as high as 80, with his most recent scores between 69 and 74, according to the state.
A judge in an earlier phase of the case concluded Hall "had been mentally retarded his entire life." Psychiatrists and other medical professionals who examined him said he is mentally disabled.
As far back as the 1950s, Hall was considered "mentally retarded" then the commonly accepted term for mental disability according to school records submitted to the Supreme Court.
"Freddie Lee Hall may or may not be intellectually disabled, but the law requires that he have the opportunity to present evidence of his intellectual disability, including deficits in adaptive functioning over his lifetime," Kennedy said.
Kennedy relied on legal briefs filed by psychiatrists and psychologists who supported Hall to undergird his opinion. Beyond the test score, the groups said there's a consensus among the mental health professions that an accurate diagnosis also must include evaluating an individual's ability to function in society, along with finding that the mental disability began in childhood.
Alito called Kennedy's reliance misplaced. In earlier death penalty cases, Alito noted that the court took account of changing standards in American society to bolster decisions limiting executions. "Now, however, the court strikes down a state law based on the evolving standards of professional societies," Alito said.
The president of the Florida Senate, Republican lawmaker Don Gaetz, said in a statement that the decision finds no fault in the Florida law prohibiting the execution of the intellectually disabled and that it solely addresses the Florida Supreme Court's single I.Q. standard.
"Florida law, as it presently exists, provides for the utilization of more than one standard," Gaetz said. "As such, we do not anticipate the need for legislative action as a result of this ruling."
The case is Hall v. Florida, 12-10882.
In other decisions:
In a pair of unanimous rulings in favor of law enforcement authorities, the justices threw out claims from anti-Bush protesters against Secret Service agents and from the daughter of a man who was shot repeatedly by Arkansas police to end a high-speed chase.
The court divided, also 5-4, in holding that Michigan cannot sue an American Indian tribe to block the opening of a casino that is located away from the tribe's reservation.