As with any field of scientific inquiry, our understanding of the condition of intellectual disability is improved and enhanced over time by continuing, rigorous study and analysis. The scientific study and the diagnosis of intellectual disability involve issues important to scholars and clinicians. But amici believe that there is no need for this Court to become enmeshed in the details and intricacies of those scholarly efforts in order to resolve the instant case and to provide guidance to lower courts in their task of fairly adjudicating cases under Atkins v. Virginia, 536 U.S. 304 (2002). This Court need only affirm that States must conform to the basic framework of the clinical definition of intellectual disability. The clinical definition of intellectual disability consists of three prongs.2 The first prong requires impairment in intellectual functioning. This Court made clear in Hall v. Florida, 134 S. Ct. 1986 (2014), that States cannot artificially ration the number of Atkins-eligible capital defendants by arbitrarily rejecting universally accepted scientific principles in evaluating test scores. The second prong requires deficits in adaptive behavior. As the Court observed in Hall, “Intellectual disability is a condition, not a number.” 134 S. Ct. at 2001. Although the precise clinical terminology has evolved over time, the core principles about the interpretation of adaptive deficits have been well settled among clinicians for decades. Central to this clinical consensus is agreement that the inquiry must focus on deficits in adaptive skills, and not some form of “balancing” those deficits with supposed strengths that an individual might appear to possess. Equally well accepted is the fact that many individuals with intellectual disability may also have other conditions or disabilities (known as “co-morbidity” or “dual diagnosis”) and those other conditions do not affect the diagnosis of intellectual disability. Texas has distorted the clinical definition’s carefully crafted and scientifically tested requirements for the second prong. It has devised a formula of exclusionary “factors,” a formula that rests heavily on stereotypes about people with intellectual disability. This approach is wholly inconsistent with accepted scientific standards. Deviating from the basic clinical framework of the definition inevitably leads to inaccurate and unreliable results, and protects only a sub-set of defendants with intellectual disability. This deliberate decision to reject clinical standards in the adjudication of death penalty cases is inconsistent with this Court’s holdings, and incompatible with the Eighth Amendment.
Brief of Amici Curiae, The American Association on Intellectual and Developmental Disabilities (AAIDD), and The ARC of the United States, In Support of Petitioner,
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