BACKGROUND: People with intellectual disabilities experience higher rates of mental disorders, contributing to restrictive practices and premature mortality. Prevalence data are essential to understanding the patterns of disease, and for the development of tailored interventions.

AIMS: To systematically examine the burden and pattern of mental disorders in people with intellectual disabilities across the lifespan.

METHOD: We searched six databases (inception to 17 October 2024), and conducted a manual search up to 15 December 2024, for systematic reviews on the prevalence of mental disorders in people with intellectual disabilities with or without neurodevelopmental conditions. We conducted a narrative synthesis of prevalence rates, including those stratified by intellectual disability level, sex, age and autism, where available, and compared these with published prevalence rates in people without intellectual disabilities (International Prospective Register of Systematic Reviews, no. CRD42024610611).

RESULTS: We included 26 systematic reviews: 7 meta-analyses and 19 narrative reviews. Compared with the general population, the prevalence of schizophrenia (3.55-4.80%), anxiety (5.4-5.5%) and obsessive-compulsive disorders (2.4%) appeared higher, whereas that of mood (6-7%), personality and post-traumatic stress disorders appeared lower. Study quality was moderate to critically low. For syndromic intellectual disabilities we noted high anxiety rates in fragile-X, Williams and 22q11.2 deletion syndromes, and in those with co-occurring autism. We found gaps on dementia, bipolar, substance use and eating disorders, and limited data stratified by intellectual disability level, sex and age.

CONCLUSIONS: This umbrella review confirms the high prevalence of mental disorders among people with intellectual disabilities, and highlights limited evidence for several conditions and population subgroups, including for minoritised ethnic groups. Standardised, high-quality epidemiological research is needed to shape clinical care and public mental health policy.