Little is known whether differences exist in motivation and reactive/instrumental offense behaviors between murder offenders with and without psychotic disorder. To contribute to better prevention strategies, the aim of this study was to investigate offense characteristics in murder offenders with a psychotic, nonpsychotic or no psychiatric diagnosis, and whether these factors differ according to sex. This data study examined the population of murder offenders between 1997 and 2005 (N = 435) in Victoria, Australia. Apparent motive for murder was allocated to one of six classifications. Assignations of reactive versus instrumental offense categories, co-offending, victim-offender relationship were determined by review of case material. Mental health service usage and prevalence of mental illnesses were identified through data with the statewide register of contacts with the public mental health system. Of the 435 offenders, 43 (9.9%) had been diagnosed with a psychotic illness. Gender differences between and within offenders with and without a psychotic disorder were analyzed. Murder offenders with a psychotic disorder were 3.19 (95% confidence interval [CI] = [1.27, 8.03]) times more likely to be motivated by revenge than nonpsychotic offenders and those with no diagnosis (odds ratio [OR] = 2.46, 95% CI = [1.10, 5.53]). The diagnostic groups did not differ on premeditation of their offenses or prior convictions. Perpetrators with psychotic illness were significantly more likely to kill family members and others known to them. It appears that for the psychotic disorder group, the perception of having been wronged in some way is a potential risk marker for planning and committing a serious offense. Care should be taken to improve adherence and access to care to avoid possible future serious violence especially in female patients with a high burden of mental illness like schizophrenia or severe mood disorders.
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