A new study debunks a common belief revealing that most psychiatric disorders -- including depression -- do not predict future violent behavior. They also found that some delinquent youth with current psychiatric illness may also be violent.

Researchers at Northwestern University found that the substance abuse disorders are the only disorders that predict future violence. They also found that

"Our findings are relevant to the recent tragic plane crash in the French Alps. Our findings show that no one could have predicted that the pilot -- who apparently suffered from depression - - would perpetrate this violent act," said Linda Teplin, corresponding author of the study. "It is not merely a suicide, but an act of mass homicide."

For the study, researchers collected and analyzed data from the Northwestern Juvenile Project, a longitudinal study of youth who were detained at the Cook County Juvenile Temporary Detention Center in Chicago between 1995 and 1998. Violence and psychiatric disorders were assessed via self-report in 1,659 youth aged 13 to 25 years interviewed up to four times between three and five years after detention.

Although they concluded that most mental disorders can't predict future violence, they did find that males with mania were more than twice as likely to report current violence than those without. But these relationships are not necessarily causal.

Delinquent youth with psychiatric illness have multiple risk factors -- such as living in violent and impoverished neighborhoods. These environments may increase their risk for violent behavior as well as worsen their psychiatric illness.

"Providing comprehensive treatment to persons with some psychiatric disorders could reduce violence," Katherine Elkington, first author of the study and an assistant professor of clinical psychology in psychiatry at Columbia University Medical School and New York Psychiatric Institute, said in a statement. "We must improve how we address multiple problems -- including violent behavior -- as part of psychiatric treatment."

The findings are detailed in the Journal of the American Academy of Child & Adolescent Psychiatry.