Kerker et al. (2004) reviewed 200 peer-reviewed articles, selected 52 based on stringent criteria, and concluded that between 5% and 12% of children and between 17% and 36% of adults with intellectual disabilities (ID) also had mental health disorders. Janicki, Davidson and colleagues (Janicki, et al., 2002; Davidson, et al., 2003) reported that prevalence rates of psychiatric diagnoses among adults with ID did not change with increasing age. A larger number may not actually have a formal psychiatric diagnosis but may be treated with psychotherapy, psychoactive medication or both by a mental health professional (Davidson et al., 2003; Holland, 2003; Jacobson, 2003; Reiss, 1990). Reiss (1994) suggested that mental health disorders may be under-reported owing to the phenomenon of diagnostic overshadowing, i.e. the tendency to incorrectly attribute symptoms of frank mental illness to behavioural abnormalities associated with ID.
Behavioural or psychiatric disorders that may have been accepted by institutional staff are often not tolerated in community placements. Hence, the presence of additional mental health problems is a principal threat to social integration (Borthwick, 1988; Bruininks et al., 1988; Bruininks et al., 1987; Crawford et al., 1979; Hill & Bruininks, 1984; Pagel & Whitling, 1978). As a consequence, the presence of mental health problems impairs the quality of life of persons with ID (Shalock & Keith, 1993), or cause regression of adaptive or developmental functioning (Russell & Tanguay, 1981). It may also create unnecessary escalation of family stressors and impair family functioning (Reiss, 1990).