Mental health needs of people with mental retardation have become increasingly recognized in recent years, both in the United States and other parts of the world, as demonstrated by a proliferation of professional journal articles, books, conferences and newsletters reporting both relevant practical studies and conceptual issues (MacLean, 1993). These persons are often referred to as being "dually diagnosed."
This current interest in the diagnosis and treatment of mental illness in people with mental retardation is spearheaded by the National Association for the Dually Diagnosed (N.A.D.D.), founded in 1982 as a multidisciplinary and parent association specifically concerned with these issues. Most N.A.D.D. members reside in the United States or Canada. N.A.D.D. has been instrumental in directing national and international attention to clinical, programmatic, and policy issues concerning persons who have mental illness and mental retardation. Other organizations with similar interests include the American Association on Mental Retardation, which has organized a special interest group on mental health services, the American Psychiatric Association, which has a committee on developmental disabilities, and the European Association for Mental Health in Mental Retardation.
Results of recent studies (Benson, 1985; Jacobson, 1990; Reiss, 1994) have shown that twenty to sixty percent of persons with mental retardation have symptoms which appear to meet criteria for a psychiatric diagnosis as outlined in Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revised (DSM-III-R) (American Psychiatric Press, 1987). Since each of these studies surveyed a particular group of persons with mental retardation, living in a particular setting, at least some of the variance in prevalence rates probably reflects particular demographic differences in the groups (King, et al. 1994), as well as significant differences in the research methodology employed. No general prevalence study has yet been reported for persons with mental retardation comparable to the widely cited National Institute of Mental Health Epidemiologic Catchment Area program, which assesses prevalence rates in the general population (Burke & Regier, 1988). While many experts feel that psychiatric disorders are more prevalent in people with mental retardation as compared with a nonretarded population (Menolascino & Fleisher, 1993), Reiss noted in 1994 that no research had at that time been reported directly comparing the rates of mental disorders in people with mental retardation versus the general population.
Although most experts feel that the types of psychiatric disorders experienced by persons with mental retardation generally represent the full range of diagnostic classifications (Popper, 1988; Sovner & Hurley, 1989), the assessment of psychopathology in this population is clinically challenging. Mental disorders in individuals with mental retardation are often unrecognized, undiagnosed, and untreated. Some of the underestimation probably results from a phenomenon called "diagnostic overshadowing," where the presence of mental retardation decreases the diagnostic significance of an accompanying mental health disorder (Reiss et al. 1982). Sovner listed in 1986 four non-specific factors associated with mental retardation that influence the diagnostic process: intellectual distortion, the effects of the mentally retarded person's diminished ability to think abstractly and communicate independently; psychosocial masking, the effect of disabilities on the content of psychiatric symptoms; cognitive disintegration, the tendency of mentally retarded people to become disorganized under emotional stress; and baseline exaggeration, the fact that during a period of emotional stress, deficits in maladaptive behavior may significantly increase in severity.
Although the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders series was designed primarily to assess psychopathology in persons with relatively normal intelligence and psychosocial functioning, earlier versions (Third Edition, Revised, 1987) listed an advisory subcommittee on mental retardation. …