Exposure to violence significantly increases the chance that a person will commit a violent crime in the subsequent week – whether or not that person has an existing mental illness.

Absolute risk was highest among people with schizophrenia, with a violent crime rate of 177 per 10,000 individuals, compared with 22 per 10,000 before the trigger event. But although that was significantly higher than the rate seen among patients with bipolar disorder and normal controls, those groups also experienced significant increases in violent behavior after a violent experience (83 vs. 13 per 10,000 and 70 vs. 9 per 10,000, respectively), Amir Sariaslan, PhD , wrote July 13 online in JAMA Psychiatry ( doi: 10.1001/jamapsychiatry.2016.1349 ).

The Swedish national study, which comprised more than 3 million people, found other triggers for transient violent criminality among both patients and controls. These included traumatic brain injuries, unintentional injuries, self-harm, substance intoxication, and parental bereavement, wrote Dr. Sariaslan of the University of Oxford (England), and his colleagues.

“We also observed that the triggers had less effect as follow-up time increased,” the team noted. “These findings support the hypothesis that recent exposure to a stressful life event, an intentional or unintentional injury, or having been diagnosed with substance intoxication increases the short-term risk of interpersonal violence.”

The study comprised 64,595 patients diagnosed with psychotic disorders and in 2.76 million controls. Since it spanned 13 years, each person also was able to serve as his own individual control as well. The model examined both absolute and relative risks, and controlled for a large number of sociodemographic, clinical, and psychosocial variables.

Exposure to violence was the strongest precipitating factor for violent crime, exerting an increased relative risk of almost 13-fold among patients with schizophrenia, and 8-fold among both patients with bipolar disorder and controls.

The increased risks associated with traumatic brain injury were 6.7 for those with schizophrenia, 4.3 for those with bipolar disorder and almost 8 for controls. For self-harm, the risk hovered around fourfold for all groups.

The risks for unintentional injuries ranged from 3.5-4.8; and for substance intoxication, from 3.0-4.0.

The increased risks associated with parental bereavement showed a slightly different pattern, being sharply increased among those with schizophrenia, compared with controls (5.0 vs. 1.7), the investigators wrote.

“An explanation for this finding is that elevated levels of social support from family members and close friends in the controls may be protective against violence,” they suggested.

The study also determined that the effects of these triggers weakened over time. Again, this observation was most obvious with exposure to violence, and in the group with schizophrenia. The increased risk of committing a violent crime dropped from a high of 12.7 in the first week to baseline by the second week.

The finding of an increased risk of violent behavior after an incident of self-harm is a novel one, the investigators added.

“Our findings suggest that self-harming patients, particularly those with psychoses, are an important group to be assessed for interpersonal violence in addition to the routinely examined risk of suicide,” they wrote.

The study was supported by the Wellcome Trust, and grants from Swedish governmental agencies. None of the authors had any financial disclosures.