Are people who suffer from mental illnesses more likely to commit violent crimes?
That question has been on the nation's mind since a 22-year-old community college dropout with a history of odd behavior was charged with shooting 19 people outside a Tucson supermarket this month, killing six and wounding Rep. Gabrielle Giffords (D-Ariz.), who police say was his target.
The answer may seem obvious to the general public, given the popularity of movies, TV shows and books in which mentally unbalanced individuals are portrayed as homicidal maniacs. Three-quarters of Americans view mentally ill people as dangerous, according to a 1999 study in the American Journal of Public Health. Another 1999 study from the same journal found that 60% of Americans believed patients with schizophrenia — a condition characterized by disordered thought processes, paranoid delusions and auditory hallucinations — were likely to commit violent acts.
But while the data show that people with certain psychiatric problems do commit violent crimes at a higher rate than those who are seemingly healthy, the vast majority of homicides, arsons and assaults are perpetrated by people who are not considered severely mentally ill.
What's more, other factors, such as unemployment, divorce in the last year and a history of physical abuse, are better predictors of violent behavior than a diagnosis of schizophrenia, according to a 2009 study in the Archives of General Psychiatry.
"If a person has severe mental illness without substance abuse and history of violence, he or she has the same chances of being violent during the next three years as any other person in the general population," the study found.
Data from the National Institute of Mental Health's Epidemiologic Catchment Area study indicate that 2.3% of Americans in good mental health had committed a violent act in the course of a year. Among those with schizophrenia or a major mood disorder, the rate was 7%, the study reported.
But it's important to keep those figures in perspective, said Duke University medical sociologist Jeffrey Swanson, who compiled those figures and published them in the journal Hospital and Community Psychiatry. Yes, the rate is three times higher among mentally ill people, but it still means that 93% of them were not violent, he said.
"It's unproductive to besmirch a whole group of people recovering from illnesses as if they were all dangerous — when in fact they're not," he said.
Swanson's research has also found that 19.7% of substance abusers have committed a violent act in the last year, making that demographic nearly three times more dangerous than the mentally ill.
It's also a much smaller group: About 21.8 million Americans over the age of 12 are thought to have recently used an illegal substance and about 17.6 million adults may have issues with alcohol abuse. Though those two groups may overlap, the total number of substance abusers easily dwarfs the 2.4 million adults who suffer from schizophrenia, according to the National Institutes of Health.
Though the link between violence and schizophrenia may be small, it certainly exists, said forensic psychiatrist Seena Fazel of the University of Oxford.
In 2009, Fazel and colleagues conducted a meta-analysis of 20 studies that looked for a link between mental illness and violence. Though the relationship may have appeared weak when looking at one or two of the studies, the overall trend was clear.
"If you look at the issue in different ways, different countries, different periods of time, there seems to be a consistent problem," said Fazel, who published the results in 2009 in the journal PLoS Medicine. "That's not a nice thing to say, but there it is."
For those with severe mental illness, the risk of violence seemed to be about twice that of the general population, but still small. The link was stronger for certain types of crimes, such as homicide and arson, he added.
And just as in the general population, severely mentally ill people who also had a substance abuse problem have a greater disposition toward violence. In Fazel's study, schizophrenic individuals who also had drug or alcohol dependencies were about four times more likely to commit a violent act than those who were schizophrenic but clean.
Drug and alcohol abuse is comparatively common among psychiatric patients. The 2009 study in the Archives of General Psychiatry found that 46% of those with a severe mental illness had a history of drug or alcohol use.
The reason for the link between mental illness and substance abuse isn't clear, Fazel said. It could be that the same genetic traits or environmental factors predisposing a person to mental illness also cause a person to use drugs or alcohol, he said. Perhaps some patients drink or use drugs to self-medicate, or to counteract unpleasant side-effects of their medications, he added.
In any event, the only mentally ill people who present a real risk are the ones who don't get treatment for their disease and remain unmedicated, said E. Fuller Torrey, founder of the Treatment Advocacy Center, a nonprofit organization in Arlington, Va., that supports involuntary treatment for the severely mentally ill who refuse to get it on their own.
When John Hinckley Jr. tried to assassinate President Reagan in 1981, lay people and experts alike were quick to explain his actions based on Freudian ideas about psychosocial causes rather than a chemical imbalance in the brain, Torrey said.
"Now that kind of thinking has been replaced, and properly so, with the idea that this is a physical disease," he said.