Background: Previous studies have shown an increased risk of criminal offending by persons with severe mental illness (SMI). Some suggest other risk factors such as substance abuse, victimisation and parental history of offending may be associated with the increased risk.
Aim: To assess the impact of substance abuse and other risk factors on offending rates in people with SMI compared to those with no mental illness (NMI).
Methods: This is part of a longitudinal record-linked whole-population study of 467,945 children born in Western Australia (WA) between 1980 and 2001. It includes linkages between the WA psychiatric case register, WA Corrective Services data and other statewide registers. This analysis created a cohort of 184,147 people born 1983 to 1991 to explore the impact of exposure to a variety of risk factors on rates of offending.
Results: People with SMI had a higher rate of offending than those with no mental illness with an unadjusted incidence rate ratio (IRR) of 3.91 (95% CI 3.61-4.24). Adjusting for substance abuse reduced the rate ratio by 60%: IRR 1.57 (95% CI 1.44-1.72). Minimal change was seen when adjusting for other factors (e.g. sociodemographics, victimisation, parental offending history): adjusted IRR 1.55 (95% CI 1.42-1.70).
Conclusion: Our analysis shows people with SMI have a higher rate of offending than those with no mental illness. Substance abuse has a major impact on this rate. Results suggest the need for a greater focus addressing the issue of substance abuse to reduce the rate of offending in this population.