Prevalence of mental illness among Australian and New Zealand people in prison: A systematic review and meta-analysis of studies published over five decades.

Background: The higher rate of mental illness, including severe psychotic disorders, among people in prisons compared to the general community is well-established. However, there have been no reviews or attempts to pool data on the reported prevalence of mental illness across prisons in Australia and New Zealand.

Methods: A systematic search of electronic databases from 1966 to the end of 2020 was conducted to identify studies reporting rates of 'any' mental illness, and separately rates of psychotic illness, among adult men and women in Australian and New Zealand prisons. A meta-analysis was performed according to PRISMA guidelines (PROSPERO ID: CRD42021241946), with separate analyses conducted for lifetime and current prevalence rates.

Results: Seventeen studies, with a total of 10,209 people in prison, met inclusion criteria. The pooled lifetime prevalence of 'any' mental illness was 69.0% (95% confidence interval = [0.56, 0.79]), and the pooled lifetime prevalence for psychosis was 9.1% (95% confidence interval = [0.07, 0.12]). Meanwhile, the pooled prevalence for 'any' current mental illness was 54.0% (95% confidence interval = [0.39, 0.68]) and for any current psychosis was 6.4% (95% confidence interval = [0.04, 0.10]). There was substantial heterogeneity between studies, with evidence that lifetime rates of psychosis have increased over time, with true prevalence estimates in 95% of all comparable populations falling between 2.7% and 26.4%.

Conclusion: The prevalence of mental illness, including severe psychotic illness, is high in Australian and New Zealand prisons. Furthermore, there is evidence that the prevalence of psychosis may be increasing over time, indicating more effort is needed to ensure diversion of people with mental illness away from the criminal justice system and into healthcare pathways.