Cite as: 536 U. S. 304 (2002)
Opinion of the Court
records, and the administration of a standard intelligence test which indicated that Atkins had a full scale IQ of 59.5
The jury sentenced Atkins to death, but the Virginia Supreme Court ordered a second sentencing hearing because the trial court had used a misleading verdict form. 257 Va. 160, 510 S. E. 2d 445 (1999). At the resentencing, Dr. Nelson again testified. The State presented an expert rebuttal witness, Dr. Stanton Samenow, who expressed the opinion that Atkins was not mentally retarded, but rather was of "average intelligence, at least," and diagnosable as having antisocial personality disorder.6 App. 476. The jury again sentenced Atkins to death.
5 Dr. Nelson administered the Wechsler Adult Intelligence Scales test (WAIS-III), the standard instrument in the United States for assessing intellectual functioning. AAMR, Mental Retardation, supra. The WAIS-III is scored by adding together the number of points earned on different subtests, and using a mathematical formula to convert this raw score into a scaled score. The test measures an intelligence range from 45 to 155. The mean score of the test is 100, which means that a person receiving a score of 100 is considered to have an average level of cognitive functioning. A. Kaufman & E. Lichtenberger, Essentials of WAIS-III Assessment 60 (1999). It is estimated that between 1 and 3 percent of the population has an IQ between 70 and 75 or lower, which is typically considered the cutoff IQ score for the intellectual function prong of the mental retardation definition. 2 Kaplan & Sadock's Comprehensive Textbook of Psychiatry 2952 (B. Sadock & V. Sadock eds. 7th ed. 2000).
At the sentencing phase, Dr. Nelson testified: "[Atkins'] full scale IQ is 59. Compared to the population at large, that means less than one percentile. . . . Mental retardation is a relatively rare thing. It's about one percent of the population." App. 274. According to Dr. Nelson, Atkins' IQ score "would automatically qualify for Social Security disability income." Id., at 280. Dr. Nelson also indicated that of the over 40 capital defendants that he had evaluated, Atkins was only the second individual who met the criteria for mental retardation. Id., at 310. He testified that, in his opinion, Atkins' limited intellect had been a consistent feature throughout his life, and that his IQ score of 59 is not an "aberration, malingered result, or invalid test score." Id., at 308.
6 Dr. Samenow's testimony was based upon two interviews with Atkins, a review of his school records, and interviews with correctional staff. He did not administer an intelligence test, but did ask Atkins questions taken from the 1972 version of the Wechsler Memory Scale. Id., at 524-525,
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