In adults with intellectual disability and autism, psychosis is rarely detected due to misinterpretation of psychotic symptoms being autism or idiosyncratic communication. The patients can poorly communicate the delusions and hallucinations they experience. Observable indicators are therefore necessary.

To establish reliable and valid behavioural indicators of psychosis in adults with intellectual disability and autism.

The aim was to investigate behavioural indicators of psychosis in adults with intellectual disability and autism.

Forty-three patients with intellectual disability and autism from a hospital-based study were recruited. Eight of these patients were assessed as having schizophrenia-like psychosis. They were further examined through a case-report study. Both statistical analysis and data from the case reports were included. Case report data were compared with scores from a validation study of the Psychopathology in Autism Checklist, PAC.

Behavioural disorganisation, severe decreased global functioning, suspected hallucinations and decreased sociability were observed in the eight psychotic patients. The level of disorganisation was significantly higher in the psychotic patients than in the non-psychotic patients - this included patients with “plain” autism. Adequate treatment significantly decreased the level of disorganised behaviour.

Disorganised behaviour, occurring concurrent with disorganised speech, severe decreased global functioning, suspected hallucinations and negative symptoms, were found to be reliable and valid indicators of psychosis.