Psychosis and the Risk of Stranger Homicides

Psychosis increases the risk of committing homicide, but it remains unclear whether it also affects victim selection. Individual cases of stranger homicide elicit a lot of public attention and outrage, even though evidence of their incidence is scarce.

Forensic psychiatric reports of 389 patients who had committed homicide in Finland during 1980-2014 were examined to determine the relationship between the offender and the victim. The stranger homicide incidence derived from perpetrators with psychosis was compared to a comparative incidence derived from a group of perpetrators without psychosis (other mental disorders were not excluded) over the time frame 2003-2014. Stranger homicide incidence rates were calculated using Finnish population averages of the study years, assuming a Poisson distribution and reported as per 100 000 person-years among potential victims in the Finnish general population.

Three hundred and eighty nine patients with psychosis had committed 414 homicides, with 40 complete stranger victims and 15 victims known for less than 24 h. Complete stranger homicide incidence committed by individuals with psychosis was 0.022 per 100 000 person-years and 0.13 for individuals without psychosis. When also including victims known for < 24 h, the incidence was 0.031 for individuals with psychosis and 0.28 for individuals without psychosis per 100 000 person-years.

Nine out of ten stranger homicides are committed by individuals without psychosis. However, on the basis of a 3.1% prevalence of psychotic disorders in Finland, individuals with psychosis have about a 3- to 5-fold risk of committing stranger homicides as compared to individuals without psychosis.