Mentally ill people are often seen as dangerous in popular culture - a perception that is fed by a large body of research and outsize media attention to serious violence perpetrated by people with psychiatric disorders.
A new study turns the tables to find that that people suffering from mental illness are almost five times more likely to be the victims of murder than the general population.
The public perception of mentally ill people as dangerous makes them more likely targets of violence, suggest the researchers.
"Continual stigmatizing portrayals may make the violent victimization of an already marginalized section of society more likely," said Dr. Roger Webb and colleagues from the University of Manchester in an opinion piece in the journal BMJ.
In a population-wide study of homicides in Sweden from 2001 to 2008 that was published on March 5 in BMJ, Swedish and American researchers found that of 615 murders during that period out of a total population of 7 million people, 22 percent of the murder victims were mentally ill.
Mental illness is a well-known risk factor for suicide and accidental death, but this study is the first established association of it with an increased risk for homicide.
Previous small American studies of emergency room patients and psychiatric outpatients showed a possible correlation between mental illness and risk for homicide, but the sample sizes were too small to examine risks associated with specific mental disorders and did not account for confounding socioeconomic factors.
Murder risk was highest for mentally ill people who also used drugs heavily, though it was also high for those with personality disorders, depression, anxiety disorders, and schizophrenia. Demographic factors like sex and age did not dampen the correlation between mental illness and vulnerability to homicide.
A demographic risk factor for murder among the mentally ill in the study was low socioeconomic status, which made victims more likely to live in deprived neighborhoods with poor health services and higher baseline homicide rates.
The murder victims may also have been in closer contact with other mentally ill people, and lacked awareness of personal safety risks because of cognitive symptoms of their mental illnesses.
The study concluded that violence against the mentally ill is a much larger public health problem than violence perpetrated by the mentally ill.
The homicide rate in Sweden (1.1 per 100,000 people) and other western European countries is much lower than that of the United States (7 per 100,000 people), which means Americans are likely to have a much higher number of mentally ill murder victims.
The researchers believe that "although some reactions towards mentally ill people are pro-social, feelings of uneasiness, fear, and a desire for social distance are common and may increase the risk of victimization."
They suggest that improved housing, employment, and substance abuse treatment can ease mentally ill people's vulnerability to murder, and that mental health clinics should work more closely with the criminal justice system to develop personal safety and conflict management skills among the mentally ill.
Doctors should also assess their mentally ill patients' risk of abuse, bullying, drug use, and violent crime - whether they are at risk of perpetrating it or being victims of it.
"These new research findings therefore deserve to be disseminated widely, so that professional groups and agencies working in mental health, as well as the media and general public, are aware that mentally ill people are at increased risk of becoming victims of someone else's violence."