Rates of homicide and homicide associated with severe mental illness in NSW between 1993 and 2016.

OBJECTIVE

To describe the characteristics of offenders found not guilty on the grounds of mental illness (NGMI) in New South Wales and rates of NGMI and other homicide verdicts.

METHOD: Demographic, legal and clinical data after referral to the NSW Mental Health Review Tribunal following an NGMI verdict for homicide matched with results from the National Homicide Monitoring Program.

RESULTS

Between 1993 and 2016, a total of 2159 homicide offenders were dealt with by the NSW courts, including 169 (7.8%) who were found NGMI. Over this period, the rate of non-NGMI homicide convictions fell from 1.83 per 100,000 per annum to 0.65 per 100,000 per annum (Kendall's tau = -0.79, p ⩽ 0.001) while the rate of NGMI homicide fluctuated, with an average annual rate of about 0.1 per 100,000 per annum (Kendall's tau = 0.17, p = 0.23). There was no association between the annual rates of NGMI and non-NGMI homicides (Pearson r = -0.3, p = 0.16) but falling rate of non-NGMI homicide meant that the proportion of NGMI offences doubled from 5.5% in the first 12 years to 11% in the second 12 years. Nearly all (88.7%) of those found NGMI had a schizophrenia-related psychosis. However, there were high rates of psychiatric comorbidity including substance use disorder (60.7%) and a history of a prior head injury (41.1%). Most (83.4%) of the NGMI offenders had previous contact with mental health services, but only half of these had received treatment with antipsychotic medication.

CONCLUSION

The fall in conviction for homicide offences in the last 24 years has not been matched by a reduction in NGMI homicide verdicts. More assertive treatment of emerging psychosis and comorbid substance use disorders, and improved continuity of care of chronic psychosis might prevent some homicides.