Homicides by mentally ill persons have led to political concerns about deinstitutionalisation.

To provide accurate information about the contribution of mental illness to homicide rates.

Retrospective study of homicide in New Zealand from 1970 to 2000, using data from government sources. ‘Mentally abnormal homicide’ perpetrators were defined as those found unfit to stand trial, not guilty by reason of insanity, convicted and sentenced to psychiatric committal, or convicted of infanticide. Group and time trends were analysed.

Mentally abnormal homicides constituted 8.7% of the 1498 homicides. The annual rate of such homicides was 1.3 per million population, static over the period. Total homicides increased by over 6% per year from 1970 to 1990, then declined from 1990 to 2000. The percentage of all homicides committed by the mentally abnormal group fell from 19.5%in 1970 to 5.0% in 2000. Ten percent of perpetrators had been admitted to hospital during the month before the offence; 28.6% had had no prior contact with mental health services. Victims were most commonly known to the perpetrator (74%).

Deinstitutionalisation appears not to be associated with an increased risk of homicide by people who are mentally ill.

The extent to which forensic psychiatric rehabilitation alters an individual's level of risk is unclear.

To highlight some essential features of a forensic psychiatric rehabilitation system, and to discuss risk assessment in this context to create a conceptual framework for risk research and practice.

The applicability of risk assessment instruments to forensic psychiatric rehabilitation was examined. Core processes and elements considered essential in this type of rehabilitative work were reviewed.

Current risk research has limited application to rehabilitation. Future research aimed at analysing forensic psychiatric rehabilitation will be hampered by the complexity of the treatment systems and the number of methodological issues relevant to this type of research.

Novel research approaches are suggested to analyse further the risk factors and processes important in forensic psychiatric rehabilitation.