Background: Mental capacity is central to legal and ethical debates on the use of compulsion in psychiatry.
Aims: To describe the clinical epidemiology of mental incapacity in patients with psychiatric disorders, including interrater reliability of assessments, frequency in the psychiatric population and associations of mental incapacity.
Method: Cross-sectional studies of capacity to consent to treatment for psychiatric patients were systematically reviewed from Medline, EMBASE and PsycInfo databases. Information on the reliability of assessments, frequency and associations of mental incapacity was extracted.
Results: Out of 37 papers reviewed, 29 different capacity assessment tools were identified. Studies were highly heterogeneous in their measurement and definitions of capacity. Interrater reliabilities between tools were high. Studies indicate incapacity is common (median 29%) but the majority of psychiatric in-patients are capable of making treatment decisions. Psychosis, severity of symptoms, involuntary admission and treatment refusal were the strongest risk factors for incapacity.
Conclusions: Mental capacity can be reliably assessed. The majority of psychiatric in-patients have capacity, and socio-demographic variables do not have a major impact but clinical ones do.