The recent gun violence in our country has brought increased attention to mental illness. This focus may unfairly increase the stigma mental illness carries.
Surveys suggest that many Americans over-estimate mental illness as a cause of violence. In one national survey, more than half of Americans believed that people with schizophrenia were likely to commit violent acts. About a third thought that people with major depression were likely to do so.
But research suggests that public perception does not reflect reality. Violence by people with mental illness — like aggression in the general population — is the result of many factors that combine in complex ways. These include family history, personal stressors (such as divorce or grief) and economic stressors (such as poverty and homelessness). Substance abuse is often tightly woven into this mix. It is therefore hard to figure out how one factor contributes to violence.
Studies of violence are often flawed. Some studies rely on "self-reporting," or people's own memories of whether they have acted violently. This kind of study may estimate violence rates to be lower than they actually are. People may forget what they did in the past, or may be embarrassed about or unwilling to admit to violent behavior.
Other studies have looked at people already involved with the criminal justice system. Researchers compare arrest rates, for example, of people in prison who have mental illness with people who don't. Because the studies take place among people who have already been arrested, the results may not be an accurate measure of violence rates in the average community.
Finally, some studies have not controlled for other factors that contribute to violent behavior (whether an individual is mentally ill or not). These include poverty, family history, personal adversity and stress.
One of the first studies to address these design flaws was the MacArthur Violence Risk Assessment Study. Instead of relying on just one source for rates of violence, the researchers used three. They:
The study found that 31% of people who had both a substance abuse disorder and a psychiatric disorder (a "dual diagnosis") committed at least one act of violence in a year, compared with 18% of people with a psychiatric disorder alone. This confirmed other research that substance abuse is a key contributor to violent behavior.
The investigators also looked at rates of violence in one area in Pittsburgh, PA. They wanted to control for environmental factors and substance use. They found no significant difference in the rates of violence among people with mental illness and other people living in the same neighborhood. In other words, when substance use wasn't a factor, the rates of violence seemed to reflect factors common to a particular neighborhood rather than the symptoms of a psychiatric disorder.
A study from the University of Oxford also tried to determine how much mental illness contributes to violent behavior. Researchers used data from a Swedish registry of hospital admissions and criminal convictions. They compared rates of violence among people with psychiatric disorders with those among peers in the general population. They found that people with bipolar disorder or schizophrenia were more likely — to a modest degree — to commit assaults or other violent crimes.
However, when the researchers compared patients with bipolar disorder or schizophrenia with their unaffected siblings, the rates were less different. This suggested that genetics or social factors that affect all family members, such as poverty and early exposure to violence, are partially responsible for violent behavior.
These two studies paint a complex picture of mental illness and violence. They suggest that violence by people with mental illness is due to many factors that overlap and interact.
Mental health professionals are under pressure to determine the likelihood that their patients will commit a violent act. It is impossible to make predictions about what any person will do.
Violent acts tend to occur when people are highly emotional. The same people may not show any signs of the intent to commit violence when meeting with a mental health professional. And even when someone explicitly expresses the intent to harm someone else, the relative risk for acting on that plan is still influenced by other factors:
Treatment of mental illness and substance abuse may help reduce rates of violence. Medication is not likely to be the only answer for lowering risk. The ideal treatment is long term and includes a range of psychosocial approaches. These include community outreach, family support, psychotherapy, conflict management, substance abuse treatment and hospital stays (when necessary).
This may be difficult to achieve in the real world, where budgets are tight. On the other hand, if we want to see less violence in our society, it may be worth investing in better coordinated mental health services. Many of the solutions to these challenges will not come from clinicians, but from policy makers.
Michael Craig Miller, M.D. is the former editor-in-chief of the Harvard Mental Health Letter and an assistant professor of psychiatry at Harvard Medical School. Dr. Miller has an active clinical practice and has been on staff at Beth Israel Deaconess Medical Center for more than 30 years.