Mentally abnormal homicide in New Zealand as defined by legal and clinical criteria: A national study.

Background: Homicides by people with mental illness have been studied using either clinical or legal categorization of the homicide as abnormal. No previous study has employed both definitions in the same population.

Method: A retrospective study of all homicides in New Zealand between 1988 and 2000 considered mentally abnormal homicide using a legal definition (when the courts deemed a contribution of mental illness was present) and a clinical definition (defined as the presence of a discharge diagnosis from inpatient mental health treatment) of 'mentally abnormal'. Rates, characteristics and time trends were investigated.

Results: Of the 844 cases, 7.1% met legal criteria for being mentally abnormal, while 7.7% had ever received a diagnosis for a psychotic illness, and a further 14.5% had been admitted to a psychiatric hospital for any other reason. The majority (60%) of perpetrators with a psychotic diagnosis received a mental health disposition from the court. Of these, 60% were first diagnosed with their psychotic illness prior to the homicide, while 28% were first diagnosed at the time of the offence and a further 12% after imprisonment. Of all those who received a psychotic diagnosis, 89% had post-conviction admissions or a mental health disposition.

Conclusion: Legal and clinical definitions of mentally abnormal homicide detect similar rates of mentally abnormal homicide, but illustrate somewhat different dimensions of the relationship between mental illness and homicide.