324
Opinion of the Court
herent in diagnosis of mental illness. Developments in the Law—Civil Commitment of the Mentally Ill, 87 Harv. L. Rev. 1190, 1242-1243 (1974). It is thus no surprise that many psychiatric predictions of future violent behavior by the mentally ill are inaccurate. See, e. g., Steadman, Employing Psychiatric Predictions of Dangerous Behavior: Policy vs. Fact, in Dangerous Behavior: A Problem in Law and Mental Health 123, 125-128 (C. Frederick ed. 1978); Monahan 47-49. For these reasons, it would have been plausible for Kentucky to conclude that the dangerousness determination was more accurate as to the mentally retarded than the mentally ill.
A statutory classification fails rational-basis review only when it " 'rests on grounds wholly irrelevant to the achievement of the State's objective.' " Holt Civic Club v. Tuscaloosa, 439 U. S. 60, 71 (1978), quoting McGowan v. Maryland, 366 U. S. 420, 425 (1961). See also, e. g., McDonald v. Board of Election Comm'rs of Chicago, 394 U. S. 802, 809 (1969); Kotch v. Board of River Port Pilot Comm'rs for Port of New Orleans, 330 U. S. 552, 556 (1947). Because ease of diagnosis is relevant to two of the four inquiries, it is not "wholly irrelevant" to the achievement of Kentucky's objective, and thus the statutory difference in the applicable burden of proof survives rational-basis review. In any event, it is plausible for Kentucky to have found that, for purposes of determining the acceptable risk of error, diagnosis and dangerousness are the most critical factors in the commitment decision, so the appropriate burden of proof should be tied to them.
There is a further, more far-reaching rationale justifying the different burdens of proof: The prevailing methods of treatment for the mentally retarded, as a general rule, are much less invasive than are those given the mentally ill. The mentally ill are subjected to medical and psychiatric treatment which may involve intrusive inquiries into the patient's innermost thoughts, see Meissner & Nicholi, The Psy-
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