On June 20, 2002 the Supreme Court decided Atkins v. Virginia where it ruled sentencing intellectually disabled individuals to death is unconstitutional. The Court held such executions are a form of "cruel and unusual punishment," which is categorically prohibited by the Eighth Amendment. The Atkins decision was silent regarding criteria by which prosecutors could show a defendant's mental capacity.
This set the stage for death row inmate Freddie Lee Hall's challenge to a relevant Florida law. The law allowed for the death penalty in cases where the defendant scored higher than 70 on an I.Q. test; Hall had scored a 71. Accordingly, Hall, who had been convicted of murder, did not qualify for an exception to capital punishment. Florida handed Hall the death sentence. Hall's lawyers contended that because his score of 71 was within the I.Q. test's margin of error, other factors should be permitted to legally establish his intellectual disability. Courts stood by the use of a cutoff I.Q. score and the sentence withstood a series of appeals.
On May 27, 2014 the Supreme Court ruled in Hall v. Florida that the Florida law was unconstitutional. The court reasoned that using the 70 I.Q. cutoff as a hard-and-fast metric of intellectual disability was arbitrary if other considerations were not considered in cases where the defendant scored close to 70. The decision comported with the American Psychological Association's position as stated in its amicus brief [PDF], "a valid diagnosis requires comprehensive assessment of adaptive behavior in conceptual, social, and practical contexts as well as general intellectual functioning."
The court noted that because Hall's score of 71 was within the test's margin of error, his lawyers should have been allowed to present additional evidence to refute the test's finding. Justice Kennedy authored the court's opinion which was joined by Justices Ginsburg, Breyer, Sotomayor and Kagan. Justice Alito filed a dissent which was joined by the Chief Justice, as well as Justices Scalia and Thomas.