According to U. S. researchers' mental illness alone does not make a person more violent but when mental illness is combined with substance abuse it increases the risk to a level of statistical significance.

The study was undertaken by Eric Elbogen of the University of North Carolina and colleagues who tracked 34,653 people from 2001 and 2003 in a U. S. government health survey. They gave detailed information about their mental health, history of violence and other issues.

Three years later these people were again questioned about any violence they had engaged in during this period such as sexual assault, attacking someone with a knife or gun, injuring someone in a physical fight or arson.

The researchers found that the people with severe mental illness but no substance abuse were in no way more likely to admit to violence than the average person. While people who reported drug or alcohol abuse along with severe mental illness were found to be three times as likely to have been violent during the same period.

"Our study shows that a link between mental illness and violence does exist, but it's not as strong as most people think," said study author Eric B. Elbogen, an assistant professor in the forensic psychiatry program at the University of North Carolina (UNC) at Chapel Hill School of Medicine.

"The reality is that you still have people with mental illness who do commit violent acts. But what this shows is that there are non-mentally ill people who commit violent acts, too," Elbogen said.

"These findings challenge the perception some people have, and which you often see reflected in media coverage, that mental illness alone makes someone more dangerous. Our study shows that this perception is just not correct," study co-author Dr. Sally Johnson of the University of North Carolina added in a statement.

The study listed the top ten predictors of future violence and severe mental illness and substance abuse ranked ninth behind factors such as youth, history of juvenile detention, violence and physical abuse, parental criminal history and unemployment. The people, who had a combination of three factors, a prior history of violence, substance abuse and severe mental illness, were found to be nearly 10 times as violent as those with mental illness only.

"We found that several other factors -- such as history of past violence or substance abuse or a recent divorce or loss of one's job -- are much more predictive of future violence than mental illness alone. Only when a person has both mental illness and substance abuse at the same time does that person's risk of future violence outweigh anyone else's," Elbogen said.

The findings appear in the February issue of the Archives of General Psychiatry.