Whether the Eighth Amendment permits the execution of offenders with mental retardation.
Holding
No. The Eighth Amendment categorically prohibits the execution of offenders with mental retardation because the punishment is excessive and therefore cruel and unusual.
Reasoning
The Court applied the Eighth Amendment principle that punishment must be proportionate to the offender and the offense. Under the Court's established approach, the Amendment draws meaning from the “evolving standards of decency that mark the progress of a maturing society,” rather than being fixed solely by understandings from the founding era.
The clearest objective evidence of contemporary standards is state legislation. Since Penry, a substantial and consistent legislative movement had emerged: eighteen death-penalty States had enacted statutes barring execution of offenders with mental retardation, Congress had adopted the same rule for federal capital punishment, and no State had enacted legislation restoring the practice. The direction and breadth of that change supported the conclusion that a national consensus had developed.
Actual execution practices reinforced the legislative evidence. Even in States that retained authority to execute offenders with mental retardation, such executions were uncommon. The Court also treated the views of professional organizations, religious groups, international actors, and public polling as additional, though not dispositive, support for the conclusion that the practice had become unusual and unacceptable.
The Court independently agreed with the legislative judgment because mental retardation reduces personal culpability. Although affected defendants may know right from wrong and may be competent to stand trial, their impairments in reasoning, judgment, communication, impulse control, and adaptive functioning make them generally less morally blameworthy than the class of offenders for whom capital punishment is reserved.
Neither principal penological justification for death sufficiently supports execution of this class of offenders. Retribution depends on an offender's personal moral culpability, which is diminished by mental retardation. Deterrence is also weak because the cognitive and behavioral impairments that reduce culpability make it less likely that these defendants will process the threat of execution and control their conduct in response to it; exempting them does not lessen deterrence for offenders without mental retardation.
Mental retardation also creates a heightened risk of an unreliable capital sentence. Defendants may be more vulnerable to false confessions, less able to assist counsel, less persuasive witnesses, and more likely to appear remorseless to jurors. Further, presenting mental retardation as mitigation can operate as a double-edged sword by encouraging a finding of future dangerousness. These risks justified a categorical constitutional rule rather than reliance on individualized sentencing alone.