[1] The case fleshed out standards first announced by the Court in Atkins v. Virginia, which left the determination of what constitutes intellectual disability to the states.

They found that there was "substantial evidence in the record" to support a finding that Hall had been "mentally retarded his entire life" but questioned the evidence presented by the defense experts:[3][4] nothing of which the experts testified could explain how a psychotic, mentally-retarded, brain-damaged, learning-disabled, speech-impaired person could formulate a plan whereby a car was stolen and a convenience store was robbed.The Florida Supreme Court affirmed the sentence.

There was also significant evidence in school reports and court records of Hall's intellectual disability, a trial judge noted that he had been "mentally retarded his entire life".

[6] The Court prohibited states in borderline cases from relying only on intelligence test scores to determine whether a death row inmate is eligible to be executed.

[7] The Court further held that the states may not use a "rigid rule" that denies leniency to defendants with severe mental disabilities simply because they score above 70 on an IQ test.

[2] Justice Anthony M. Kennedy wrote for the majority that this "rigid rule, the court now holds, creates an unacceptable risk that persons with an intellectual disability will be executed, and thus is unconstitutional.

[11] In his dissent, Alito criticized the majority's reliance on the views of medical experts, saying that the Justices had overruled Atkins "based largely on the positions adopted by private professional organizations".

[4] He also argued that by overruling Atkins, the court had replaced the framework, established in previous Eighth Amendment cases, with a "uniform national rule that is conceptually unsound, and likely to result in confusion".