Experts long have sought to understand the link between mental illness and violence.

Eric Elbogen of the University of North Carolina and colleagues tracked 34,653 people who gave detailed information from 2001 and 2003 in a US government health survey about their mental health, history of violence and other issues.

These people were then questioned three years later about whether during that period they had engaged in violence such as sexual assault, attacking someone with a knife or gun, injuring someone in a physical fight or arson.

Those with severe mental illness but no substance abuse were no more likely than the average survey participant to admit to violence during the three years, the researchers wrote in the Archives of General Psychiatry.

But those who reported drug or alcohol abuse and also had severe mental illness were three times as likely as those with mental illness alone to have been violent, they added.

"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 in a telephone interview.

Having severe mental illness and substance abuse combined ranked only ninth on the study's list of the top 10 predictors of future violence, behind factors such as youth, history of juvenile detention, violence and physical abuse, parental criminal history and unemployment.

"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," the University of North Carolina's Dr Sally Johnson added in a statement.