Cite as: 509 U. S. 312 (1993)
Opinion of the Court
and guardians to participate as parties in proceedings to commit the mentally retarded but not the mentally ill. As we have noted, see supra, at 321-322, by definition, mental retardation has its onset during a person's developmental period. Mental retardation, furthermore, results in "deficits or impairments in adaptive functioning," that is to say, "the person's effectiveness in areas such as social skills, communication, and daily living skills, and how well the person meets the standards of personal independence and social responsibility expected of his or her age by his or her cultural group." Manual of Mental Disorders 28-29. See also Mental Retardation 5-6, 15-16, 38-41. Based on these facts, Kentucky may have concluded that close relatives and guardians, both of whom likely have intimate knowledge of a mentally retarded person's abilities and experiences, have valuable insights that should be considered during the involuntary commitment process.
Mental illness, by contrast, may arise or manifest itself with suddenness only after minority, see supra, at 322, when the afflicted person's immediate family members have no knowledge of the medical condition and have long ceased to provide care and support. Further, determining the proper course of treatment may be far less dependent upon observations made in a household setting. Indeed, we have noted the severe difficulties inherent in psychiatric diagnosis conducted by experts in the field. Addington v. Texas, 441 U. S., at 430. See also Mentally Disabled 18. In addition, adults previously of sound mental health who are diagnosed as mentally ill may have a need for privacy that justifies the State in confining a commitment proceeding to the smallest group compatible with due process. Based on these facts, Kentucky may have concluded that participation as parties by relatives and guardians of the mentally ill would not in most cases have been of sufficient help to the trier of fact to justify the additional burden and complications of granting party status. To be sure, Kentucky could have provided rel-
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