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The purpose of this paper is to determine: the extent to which an intellectual disability diagnosis meets current diagnostic and statistical manual of mental disorders (DSM) diagnostic criteria; the prevalence of reported autism spectrum disorders (ASD); and the extent to which assessment of developmental issues is central to the diagnosis of psychotic disorder, in patients discharged with a diagnosis of psychotic disorder and intellectual disabilities.

Of all patients discharged with psychotic disorder during a four‐year period (n=3339), chart reviews were completed on those also diagnosed with intellectual disability or borderline IQ.

The findings if this paper are threefold: only 39 percent of the 41 individuals discharged with a diagnosis of psychotic disorder and intellectual disability met documented DSM criteria for intellectual disability; the prevalence of reported ASD was much lower than expected; and the average number of different discharge diagnoses per individual over time was 4.8. Schizophrenia diagnoses were made early in the diagnostic process and tended to persist even when ASD concerns were documented.

The results support the need to systematically assess the developmental issues of patients with intellectual disability as part of the psychiatric diagnostic formulation. Differential diagnoses of psychotic‐like behaviours seen in people with intellectual disability, and alternative frameworks for understanding these behaviours, which in turn should guide more effective interventions and treatment, are discussed.