What did the U.S. Supreme Court decide in Atkins v. Virginia about the death penalty?

In the 2002 case Atkins v. Virginia, the U.S. Supreme Court ruled that executing individuals with intellectual disabilities is unconstitutional.1

The Court determined that such executions violate the Eighth Amendment, which prohibits cruel and unusual punishment. Relying on the concept of "evolving standards of decency," the justices concluded that the death penalty is not a suitable or proportional punishment for offenders with intellectual disabilities, as it does not measurably advance the goals of deterrence or retribution. Consequently, the Court held that the Constitution places a substantive restriction on the state's power to impose the death penalty on these individuals, effectively reversing the lower court's judgment.1