ABSTRACT
In people with intellectual disabilities (ID) mental illness is three to four times more common (Deb, Thomas and Bright, 2001) than in those without ID. Estimates of point prevalence vary from 10 to 39 per cent (Borthwick-Duffy, 1994; Borthwick-Duffy and Eyman, 1990; Bouras and Drummond, 1992; Cooper, 1997; Corbett, 1979; Deb et al., 2001; Eaton and Menolascino, 1982; Göstason, 1985; Hagnell, Ojesjo, Otterbeck and Rorsman, 1994; Iverson and Fox, 1989; Jacobson, 1982; Lund, 1985; Reid, 1994; Reiss, 1990). The reasons for the differing estimates include the use of different samples (such as community versus institution-based samples) and diagnostic criteria. If conditions like personality disorders, autism, behavioural disorders, dementia and attention deficit hyperactivity disorders are excluded, then there is little difference between the ID and non-ID populations’ prevalence rates (Deb et al., 2001). There are also difficulties in diagnosing some mental disorders – the criteria that are often employed have been based on those who do not have an ID and may be even less valid for those with more severe ID. This may be one reason why not all studies agree that there may be a higher prevalence of schizophrenia in people with ID (Borthwick-Duffy and Eyman, 1990; Corbett, 1979; Göstason, 1985; Iverson and Fox, 1989; Jacobson, 1982; Lund, 1985; Reid, 1994). An increased vulnerability to mental health problems is likely to be due to an increased incidence of brain abnormalities including associated epilepsy, physical and sensory problems, and social and psychological adversities.
