States cannot deem convicts mentally fit for execution based on an IQ test score alone, the Supreme Court ruled Tuesday, making it less likely the mentally disabled will face the death penalty.

At issue was a Florida law that prevented death row inmates from introducing additional evidence of intellectual disability if they scored above 70 on an IQ test, a rule the high court said violated the Eighth Amendment’s prohibition on cruel and unusual punishment. Eight other states have similar laws, including Arizona, Alabama, Utah, Idaho, Kansas, Oklahoma, South Carolina and Arkansas.

The decision deals another blow to the death penalty, following weeks of controversy owing to the botched execution of Clayton Lockett in Oklahoma.

Close video Supreme Court strikes down law setting death penalty IQ limit The U.S. Supreme Court struck down a law used by Florida and a few other states that set a strict cut-off, based on IQ test scores, of eligibility for the death penalty. NBC’s Pete Williams reports. The U.S. Supreme Court struck down a law used by Florida and a few other states that set a strict cut-off, based on IQ test scores, of eligibility for the death penalty. NBC’s Pete Williams reports. share tweet email Embed

creates an unacceptable risk that persons with intellectual disability will be executed, and thus is unconstitutional.” The other four conservative Justices joined a dissent authored by Justice Samuel Alito.

Justice Anthony Kennedy, joining the four Democratic-appointed Justices, wrote that Florida’s rule “

“An IQ score is an approximation, not a final and infallible assessment of intellectual functioning,” wrote Kennedy. “Intellectual disability is a condition, not a number.”

The death row inmate challenging Florida’s law is Freddie Lee Hall, who was convicted of kidnapping, raping and murdering a pregnant newlywed and then killing a sheriff’s deputy while robbing a convenience store in 1978. Kennedy writes in his opinion that family and teachers “identified [Hall] on numerous occasions as ‘mentally retarded.’” Nevertheless, Hall scored slightly above 70 on a number of IQ tests, rendering him fit for execution under Florida law. A 2002 Supreme Court opinion, Atkins v. Virginia, held that execution of the mentally disabled violates the Eighth Amendment’s prohibition on cruel and unusual punishment.

Alito’s dissent accuses Kennedy of holding the Constitution hostage to the decisions of medical professionals he cites in his opinion. “Under our modern Eighth Amendment cases, 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.

“Intellectual disability is a condition, not a number.” Justice Anthony Kennedy on death penalty ruling

Florida is fourth nationwide in the number of inmates executed since 1976, when the Supreme Court ruled that the death penalty was constitutional. The state leads the country in the number of death row inmates who were later exonerated. Last year, Florida Republican Governor Rick Scott signed a law intended to make sure death row inmates are executed sooner.

“Florida’s law contravenes our Nation’s commitment to dignity and its duty to teach human decency as the mark of a civilized world,” Kennedy wrote. “The States are laboratories for experimentation, but those experiments may not deny the basic dignity the Constitution protects.”

Kennedy is the author of several key decisions narrowing the death penalty. In the 2005 case Roper v. Simmons, Kennedy wrote the opinion barring capital punishment for people who commit crimes as minors. In 2007, Kennedy wrote the opinion in Kennedy v. Louisiana, holding that applying the death penalty in rape cases violated the Eighth Amendment.

“This case continues a long line of Kennedy opinions establishing boundaries on the death penalty,” said Adam Winkler, a law professor at UCLA School of Law.“While he often prefers states rights to federal power, he is foremost a believer in the Constitution’s limits on all government power to deny people basic dignity. Dignity is a principle he’s referred to repeatedly, in both the gay rights and death penalty contexts.”