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Adults with intellectual disabilities experience higher rates of mental ill health than the general population. Despite this, the epidemiological knowledge base remains limited. The purpose of this article is to review mental health epidemiological studies relevant to adults with intellectual disabilities, published since January 2008.Several studies have aimed to build the epidemiological evidence base, particularly with regards to problem behaviours, which appear to be remitting–relapsing conditions rather than necessarily being chronic. Most of such work confirms prevalence and incidence rates, and conducts exploratory analyses to determine factors independently related to mental ill health. Down syndrome protects against problem behaviours and mental ill health (except dementia that occurs at a higher rate), whereas epilepsy does not appear to affect risk for mental ill health. Dementia is four times more common in older persons with intellectual disabilities without Down syndrome than in the general population. Persons with borderline intellectual disabilities also experience higher rates of mental ill health than the general population, but receive fewer treatments.This work builds usefully upon previous studies. Further hypothesis-based analyses are needed.