The dissent continued in relevant part: Similarly, the APA’s Diagnostic and Statistical Manual of Mental Disorders (DSM) used to require an impairment to onset “‘before age 18 years’” to meet the definition of an intellectual disability. Atkins, 536 U. S., at 308, n. 3 (quoting DSM–IV, p. 41 (4th ed. 2000)). However, in 2013, the manual’s fifth edition (DSM–5) changed course, providing only that an impairment must onset “during the developmental period.” Hall, 572 U. S., at 721 (citing DSM–5, at 33). The revisions to the AAIDD and APA definitions have aligned those definitions more closely with that of the American Psychological Association, another authority relied upon in Hall, which also sets the cutoff at age 22. Manual of Diagnosis and Professional Practice in Mental Retardation 13, 36 (1996). These three leading clinical pronouncements provide powerful evidence of medical consensus that cannot be disregarded. Moore, 581 U. S., at ___.
The legislative policies of various States in defining intellectual disability are also central to the inquiry. Hall, 572 U. S., at 710. By my count, here, as in Hall, “in 41 States an individual in Coonce’s position . . . would not be deemed automatically eligible for the death penalty.” Id., at 716.3 Two States impose no age-of-onset requirement in the Atkins context. Neb. Rev. Stat. §28–105.01(3) (Cum. Supp. 2014); Kan. Stat. §§21– 6622(h), 76–12b01(d) (Cum. Supp. 2018); see State v. Vela, 279 Neb. 94, 151, 777 N. W. 2d 266, 307 (2010) (discussing Nebraska Legislature’s choice to omit age-of-onset requirement). Two impose an age-22 onset requirement. See Ind. Code §35–36–9–2 (2021); Utah Code §77–15a– 102(2) (2021). A fifth State, the Nation’s most populous, recently amended its law to replace its rigid age-18 onset requirement with “the developmental period, as defined by clinical standards.” Cal. Penal Code Ann. §1376(a)(1) (West Cum. Supp. 2021). A sixth dropped its rigid age18 onset requirement in 2014. Compare La. Code Crim. Proc. Ann., Art. 905.5.1(H) (West 2014) (requiring onset “before the age of eighteen years”) with La. Code Crim. Proc. Ann., Art. 905.5.1(H) (West Cum. Supp. 2021) (requiring onset “during the developmental period”).
State Supreme Courts often look to United States Supreme Court precedent. This case presents the less common scenario in which the United States Supreme Court looks to state supreme courts for guidance.