322
Opinion of the Court
orders 29 (3d rev. ed. 1987) (hereinafter Manual of Mental Disorders); American Assn. on Mental Retardation, Mental Retardation: Definition, Classification, and Systems of Support 5, 16-18 (9th ed. 1992) (hereinafter Mental Retardation); S. Brakel, J. Parry, & B. Weiner, The Mentally Disabled and the Law 16-17, 37 (3d ed. 1985) (hereinafter Mentally Disabled); Ky. Rev. Stat. Ann. § 202B.010(9) (Michie 1991). By the time the person reaches 18 years of age the documentation and other evidence of the condition have been accumulated for years. Mental illness, on the other hand, may be sudden and may not occur, or at least manifest itself, until adulthood. See, e. g., Manual of Mental Disorders 190 (onset of schizophrenia may occur any time during adulthood); id., at 220, 229 (onset of depression usually is during adulthood). Furthermore, as we recognized in an earlier case, diagnosis of mental illness is difficult. See Addington v. Texas, 441 U. S. 418, 430 (1979). See also Mentally Disabled 18. Kentucky's basic premise that mental retardation is easier to diagnose than is mental illness has a sufficient basis in fact. See, e. g., id., at 16; Ellis & Luckasson, Mentally Retarded Criminal Defendants, 53 Geo. Wash. L. Rev. 414, 438-439 (1985).
This difference between the two conditions justifies Kentucky's decision to assign a lower standard of proof in commitment proceedings involving the mentally retarded. In assigning the burden of proof, Kentucky was determining the "risk of error" faced by the subject of the proceedings. Addington v. Texas, supra, at 423. If diagnosis is more difficult in cases of mental illness than in instances of mental retardation, a higher burden of proof for the former tends to equalize the risks of an erroneous determination that the subject of a commitment proceeding has the condition in question.1 See G. Keppel, Design and Analysis 65-68 (1973).
1 Justice Souter suggests that this description of the function of burdens of proof is inconsistent with Addington v. Texas, 441 U. S. 418 (1979). See post, at 339-341 (dissenting opinion). His reasoning, however, would
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