Homicide and mental illness in New Zealand, 1970-2000
Simpson AI, McKenna B, Moskowitz A, Skipworth J and Barry-Walsh J;
Commented by , 24 Nov 2004
Aim of the study
Homicides by people with a serious mental illness attract a lot of public attention worldwide and this may contribute to the stigma for people with illnesses such as schizophrenia. The issue of homicide associated with mental illness got even more complex following the enormous efforts of deinstitutionalisation in many countries.
Previous studies in Denmark, the UK and Germany found little or no effect of deinstitutionalisation on rates of homicide. The current study tried to analyse the annual rate of homicide committed by people with serious mental illness in New Zealand between 1970 and 2000 in order to detect an association with deinstitutionalisation or with total homicide rates.
Deinstitutionalisation in New Zealand was started in the 1960s where hospital beds per 100,000 population have been steadily falling until 1998. In addition, the authors attempted to characterise homicide perpetrators and victims, and tried to determine the contact of the perpetrators with mental health services before the offence.
Methods
The authors used a number of New Zealand government databases to retrospectively assess homicide in New Zealand from 1970 to 2000. A legal definition was used to identify mentally abnormal homicide perpetrators, i.e. unfit to stand trial, not guilty by reason of insanity, convicted and sentenced to psychiatric committal, or convicted of infanticide.
Time trends were analysed by displaying the percentage of homicide committed by mentally abnormal people in annual groupings. In addition change over time was analysed using a Poisson regression model.
Results
Overall, 8.7% of the 1498 homicides were committed by mentally abnormal people. The annual rate of such homicides was static over the period analysed and was 1.3 per million population. During the years 1970 to 1990 total homicides increased by over 6% per year, while during 1990 to 2000 there was a decline.
However, the percentage of all homicides committed by mentally ill people declined from 19.5% in 1970 to 5.0% in 2000. It is noteworthy that 10% of the perpetrators had been treated in a hospital during the month before the crime, while 29% had never had a contact with a psychiatric hospital. In 74% of the cases, the victims of the offences were known to the perpetrator.
Discussion
One of the main results of the study was that in the last years people with mental illness contributed proportionately less to total homicide now than they did in 1970. A limitation of the study is the use of legal definitions to identify groups of interest. This is problematic because such legal definitions may in part reflect changes in legal policy.
They will also miss people who were convicted of homicide and in whom the illness might have contributed to the offence but failed to reach the criteria of the insanity test. In addition original data were collected by non-researchers and not specifically for clinical research. Nevertheless it is reassuring to see that deinstitutionalisation did not seem to be associated with an increased risk of homicide by people who are mentally ill.