Atkins v. Virginia

Majority reasoning

The majority held that objective evidence of a national consensus should guide Eighth Amendment analysis, and pointed to the fact that 18 states plus the federal government had banned executing the mentally retarded since Penry v. Lynaugh in 1989, with no state moving in the opposite direction. The Court reasoned that people with mental retardation have diminished capacities in reasoning, judgment, and impulse control that reduce their moral culpability below the level needed to justify the death penalty's retributive purpose, since even average murderers' culpability has been held insufficient without aggravating depravity. The majority also concluded that executing such offenders would not meaningfully advance deterrence, because the same cognitive impairments that reduce culpability also make it less likely they process the threat of execution when deciding how to act, and because non-retarded offenders remain fully subject to the death penalty regardless of the exemption. Finally, the Court reasoned that people with mental retardation face a heightened risk of wrongful execution because they are often poor witnesses, less able to assist counsel, and prone to appearing remorseless, and their mental limitations can even be misused by juries as evidence of future dangerousness. The Court left it to individual states to develop procedures for determining which offenders qualify as mentally retarded, following its approach in Ford v. Wainwright regarding insanity.

Dissent reasoning

Chief Justice Rehnquist, joined by Scalia and Thomas, argued that only legislative enactments and the practices of sentencing juries should count as objective evidence of a national consensus under the Eighth Amendment, and that the majority improperly relied on foreign laws, opinion polls, and the views of professional and religious organizations that have no democratic accountability and were rejected as irrelevant in Stanford v. Kentucky and Penry. He argued the opinion polls cited were not shown to meet basic standards of scientific reliability, lacking disclosed sampling methods or targeted populations, and that no comprehensive jury data existed showing juries view death as disproportionate for mentally retarded offenders. Justice Scalia, joined by Rehnquist and Thomas, separately argued that only 18 of 38 death penalty states (47%) had banned the practice, a far weaker showing of consensus than in prior cases like Coker, and that this figure was itself inflated since many statutes were prospective-only or contained exceptions. Scalia further argued that the Eighth Amendment permits punishments not considered cruel in 1791, that sentencing juries are fully capable of weighing mental retardation as a mitigating factor case-by-case, and that the majority's retribution and deterrence arguments were unsupported speculation contradicted by juries that continue to sentence mentally retarded defendants to death for extreme crimes. He warned the categorical rule invites defendants to feign retardation with no corresponding risk, unlike feigned insanity.