Background: Mental illness is more prevalent in people with intellectual disabilities (ID) than in the normal population. The association between mental illness and severity of ID is also of importance in the understanding and treatment of maladaptive and challenging behaviours. The aim of this study was to investigate the association between severity of ID and prevalence of mental illness.
Methods: Using the Mini PAS-ADD, an instrument designed to identify psychiatric symptoms in people with ID, informants were interviewed about the presence of symptoms in 96 participants with moderate, severe and profound ID, and asked about the use of psychotropic medication.
Results: Mental illness, particularly anxiety, depression and psychosis, was far more prevalent in participants with moderate ID than in people with severe and profound ID. The use of psychotropic medication was not significantly different between the groups.
Conclusions: The prevalence of psychiatric illness decreases with severity of ID. The usefulness of psychiatric illness models, in explaining maladaptive and challenging behaviours, also decreases with severity of ID. Drug treatment may become more complicated, and behavioural and environmental interventions may become relatively more important, as severity of ID increases.