So a team of researchers from Sweden and the USA assessed mental disorders and homicides across the entire population of Swedish adults between 2001 and 2008.
Mental disorders were grouped into the following categories: substance use disorder; schizophrenia; mood disorders including bipolar disorder and depression; anxiety disorders and personality disorders. Results were adjusted for several factors such as sex, age, marital status, educational level, employment status and income.
Of 7,253,516 adults in the study, 141 (22%) out of 615 homicidal deaths were among people with mental disorders.
After adjusting for several factors, the results show that people with any mental disorder were at a five-fold increased risk of homicidal death, relative to people without mental disorders.
The risk was highest among those with substance use disorders (approximately nine-fold), but was also increased among those with personality disorders ((3.2-fold), depression (2.6-fold), anxiety disorders (2.2-fold), or schizophrenia (1.8-fold) and did not seem to be explained by substance use.
One explanation for the findings may be that those with mental disorders are more likely to live in high deprivation neighbourhoods, which have higher homicide rates, say the authors. They may also be in closer contact with other mentally ill people and be less aware of their safety risks owing to symptoms of the underlying illness.
They suggest that interventions to reduce these risks "should include collaborations between mental health clinics and the criminal justice system to develop personal safety and conflict management skills among people with mental illness."
Improved housing, financial stability, and substance abuse treatment may also reduce vulnerability to violent crime, they add.
A key implication of these new findings is that clinicians should assess risk for the full array of adverse outcomes that may befall people with mental health problems, say Roger Webb and colleagues at the University of Manchester, in an accompanying editorial. This would include being a victim of violence as well as committing it, abuse and bullying, suicidal behaviour, accidental drug overdoses, and other major adverse events linked with intoxication or impulsivity.
These risks go together, and people with mental illness, as well as their families, should receive advice on avoiding various types of harm, they suggest.
They acknowledge that some important questions remain unanswered, but suggest that national mental health strategies "should reflect the broad nature of safety concerns in mental healthcare, while anti-stigma campaigns among the public should aim to counter fear of mentally ill people with sympathy for the risks they face."