Prevalence of intellectual disability and comorbid mental illness in an Australian community sample

Objective: The aim of this study was to bring to light the high prevalence of Australians affected by intellectual disability and comorbid serious mental illnesses. Results from a broad scale study are used to explore the reasons for this regularly overlooked phenomenon. Methods: This study was based on secondary analysis of data collected in the national 'Disability, Ageing and Carers Survey, 1998'. The analysed data consisted of an Australian wide sample of 42 664 individuals living at home or in cared accommodation. Classification of intellectual disability and comorbid psychosis, anxiety and depressive disorder was based on the International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10). Results: The prevalence of intellectual disability in the sampled population was 1.25%. Of these people 1.3% had a psychotic disorder, 8% had a depressive disorder and 14% had an anxiety disorder that had been present for at least 6 months and was of such severity that it too was disabling. Conclusions: Findings indicate that people with intellectual disability are at high risk of developing comorbid serious mental illness. Dual diagnosis is however, often overlooked due to difficulties associated with establishing a diagnosis of a mental disorder in people with an intellectual disability, a problem which is heightened when the individual's capacity to participate in a clinical assessment is limited.