In 2002, the Supreme Court held in Atkins v. Virginia
that it's unconstitutional to execute someone with an intellectual disability. Florida interpreted that meant that if your IQ is 70 or less, you could not be executed; if your IQ is 71 or higher, not only could you be executed by there was no room to present evidence that your true mental capacities were poorly reflected by the IQ test.
In a 5-4 decision today authored by Occasional Swing Justice Anthony Kennedy, the Supreme Court held that this rigid rule was insufficient in guarding against the risk that a person with an intellectual disability would be executed, for now sparing the life of convicted murderer Freddie Lee Hall, whose IQ tested between 71-80 in the years after his conviction. As Justice Kennedy explained:
Florida’s rule disregards established medical practice in two interrelated ways. It takes an IQ score as final and conclusive evidence of a defendant’s intellectual capacity, when experts in the field would consider other evidence. It also relies on a purportedly scientific measurement of the defendant’s abilities, his IQ score, while refusing to recognize that the score is, on its own terms, imprecise....
Intellectual disability is a condition, not a number. See DSM–5, at 37. Courts must recognize, as does the medical community, that the IQ test is imprecise. This is not to say that an IQ test score is unhelpful. It is of considerable significance, as the medical community recognizes. But in using these scores to assess a defendant’s eligibility for the death penalty, a State must afford these test scores the same studied skepticism that those who design and use the tests do, and understand that an IQ test score represents a range rather than a fixed number. A State that ignores the inherent imprecision of these tests risks executing a person who suffers from intellectual disability.
This Court agrees with the medical experts that when a defendant’s IQ test score falls within the test’s acknowledged and inherent margin of error, the defendant must be able to present additional evidence of intellectual disability, including testimony regarding adaptive deficits.
Justice Alito penned the dissent on behalf of the four you'd expect, decrying the majority's reliance on the American Psychiatric Association's judgment over that of state legislatures, and basically arguing that as long as a decent number of states would execute Hall, the Court had no basis upon which to hold that there was now a consensus that doing so would violate society’s standards of decency.
The Court's opinion is here.