In Atkins v. Virginia, 536 U.S. 304 (2002), the United States Supreme Court held that executing a mentally retarded offender violates the Eighth Amendment's ban on cruel and unusual punishment. 536 U.S. at 321.
The Court announced this categorical rule based on a "national consensus," evinced by prohibitions enacted by state legislatures, that mentally retarded persons are "categorically less culpable than the average criminal" and more vulnerable to wrongful execution. Id. at 315-21.
According to the Court, "to the extent there is serious disagreement about the execution of mentally retarded offenders, it is in determining which offenders are in fact retarded." Id. at 317.
The Court cited clinical definitions of mental retardation found in the Diagnostic and Statistical Manual of Mental Disorders (4th ed. 2000) (DSM-IV), published by the American Psychiatric Association (APA), and Mental Retardation: Definition, Classification, and Systems of Supports (9th ed. 1992), published by the American Association on Mental Retardation (AAMR), stating that both definitions require evidence of "subaverage intellectual functioning . . . and significant limitations in adaptive skills such as communication, self-care, and self-direction that became manifest before age eighteen." Id. at 318 & n.3.
Adopting the approach it chose when it prohibited execution of offenders who are insane, the Court left "'to the States the task of developing appropriate ways to enforce the constitutional restriction upon their execution of sentences,'" noting that "statutory definitions of mental retardation" in states that legislatively had prohibited execution "are not identical, but generally conform to the clinical definitions" promulgated by the APA and the AAMR. Id. at 317 & n.22.
The Court held that "death is not a suitable punishment for a mentally retarded criminal."
In Atkins, the Supreme Court noted a trend among the states of consideration of a person's mental capacity in determining punishment.
The Court noted that as a national consensus, "our society views mentally retarded offenders as categorically less culpable than the average criminal."
Although the Court did not determine that the mentally retarded are exempt from criminal punishment altogether, it held that it is cruel and unusual to inflict the death penalty on those with diminished mental capacity and therefore diminished culpability. (536 U.S. at 321.)
Atkins recognized also that no national consensus exists among the states as to a determination of which offenders are mentally retarded.
Accordingly, Atkins did not define the perimeters of mental retardation, but left "to the States the task of developing appropriate ways to enforce the constitutional restriction upon its execution of sentences." (536 U.S. at 317.)