Yes. In a 6-3 opinion delivered by Justice John Paul Stevens, the Court held that executions of mentally retarded criminals are "cruel and unusual punishments" prohibited by the Eighth Amendment. Since it last confronted the issue, the Court reasoned that a significant number of States have concluded that death is not a suitable punishment for a mentally retarded criminal. Moreover, the Court concluded that there was serious concern whether either justification underpinning the death penalty - retribution and deterrence of capital crimes - applies to mentally retarded offenders, due to their lessened culpability.
What is ATKINS v. VIRGINIA