While mental illness alone doesn't incite an individual to violence, a new study has found that this coupled with substance abuse may indicate that a person is likely to engage in violence in future.
Published in the Archives of General Psychiatry, the study was basically conducted to examine the association between mental illness and violent behaviour.
Dr. Eric B. Elbogen and Dr. Sally C. Johnson, both from the University of North Carolina at Chapel Hill School of Medicine, however, found that a mental illness alone did not predict whether an individual would show violent behaviour in future.
They instead found that individuals with both mental illness and substance abuse were at higher risk for future violence.
The researchers revealed that they assessed data collected as part of the National Epidemiological Survey on Alcohol and Related Conditions, a survey conducted by the National Institute on Alcohol Abuse and Alcoholism.
A total of 34,653 individuals were interviewed about their mental health, demographics, history of violence and other risk factors between 2001 and 2003.
The participants answered questions about any violent behaviour-including fighting, sexual assault and starting fires-perpetrated between then and a second interview that was conducted between 2004 and 2005.
At the first interview, 10.87 percent of participants were diagnosed with schizophrenia, bipolar disorder or major depression alone, 21.41 percent with substance abuse or dependence alone and 9.4 percent with a severe mental disorder plus substance abuse or dependence.
The researchers said that having a mental illness alone at the first interview did not predict whether an individual would have violent behaviour before the second interview, but individuals with both mental illness and substance abuse or dependence were at higher risk for future violence.
"The highest risk was shown for dual-disordered subjects with a history of violence, who showed nearly 10 times higher risk of violence compared with subjects with severe mental illness only," the authors write.
The researchers revealed that other factors that predicted future violence included a history of juvenile detention, physical abuse or having witnessed parental fighting; a recent divorce, unemployment or victimization; or being younger, male or lower-income.
According to them, most of those factors were present more often in individuals with mental illness.
"Because severe mental illness did not independently predict future violent behaviour, these findings challenge perceptions that mental illness is a leading cause of violence in the general population," the authors write.
"Still, people with mental illness did report violence more often, largely because they showed other factors associated with violence. Consequently, understanding the link between violent acts and mental disorder requires consideration of its association with other variables such as substance abuse, environmental stressors and history of violence," they conclude.