Prevalence of Mental, Behavioural, and Neurobehavioural Disorders

Individuals with intellectual and developmental disabilities (IDD) are known to be at high risk of developing a comorbid mental or behavioural disorder. However, there has only recently been an increased focus on the epidemiology of such disorders in the older population of people with IDD. Attempting to precisely quantify the prevalence and incidence of disorders in this cohort remain difficult, due to complications associated with diverse sampling methodologies, differing definitions of ‘older’, and variations in diagnostic criteria. Nonetheless, the expanding research base since the 1980s indicates that mental and behavioural disorders occur at higher rates among older people with IDD when compared to their mainstream peers. The prevalence of mental disorders for the older cohort tends to decrease in relation to the severity of the intellectual disability, but the reverse is true for behavioural disorders. Estimated prevalence rates for mental disorders vary considerably, but many studies report rates in the 20–30% range, which is higher than for the general community. Behavioural disorders are also very common in older people with IDD but are again hard to quantify precisely. Observed differences in prevalence between people with IDD and the general community may diminish over the age of 70 as dementias become more commonplace in both populations. Finally, some research indicates a greater likelihood of mental disorder amongst the older female population of people with IDD