Background: About 9 million people are imprisoned worldwide, but the number with serious mental disorders (psychosis, major depression, and antisocial personality disorder) is unknown. We did a systematic review of surveys on such disorders in general prison populations in western countries.

Methods: We searched for psychiatric surveys that were based on interviews of unselected prison populations and included diagnoses of psychotic illnesses or major depression within the previous 6 months, or a history of any personality disorder. We did computer-assisted searches, scanned reference lists, searched journals, and corresponded with authors. We determined prevalence rates of serious mental disorders, sex, type of prisoner (detainee or sentenced inmate), and other characteristics.

Findings: 62 surveys from 12 countries included 22790 prisoners (mean age 29 years, 18530 [81%] men, 2568 [26%] of 9776 were violent offenders). 3.7% of men (95% CI 3.3--4.1) had psychotic illnesses, 10% (9--11) major depression, and 65% (61--68) a personality disorder, including 47% (46--48) with antisocial personality disorder. 4.0% of women (3.2--5.1) had psychotic illnesses, 12% (11--14) major depression, and 42% (38--45) a personality disorder, including 21% (19--23) with antisocial personality disorder. Although there was substantial heterogeneity among studies (especially for antisocial personality disorder), only a small proportion was explained by differences in prevalence rates between detainees and sentenced inmates. Prisoners were several times more likely to have psychosis and major depression, and about ten times more likely to have antisocial personality disorder, than the general population.

Interpretation: Worldwide, several million prisoners probably have serious mental disorders, but how well prison services are addressing these problems is not known.