The increased risk of persons with schizophrenia committing violent crime may be largely mediated by co-existing substance abuse problems, according to a study in the May 20 issue of JAMA.

Many studies have reported on the association between major mental disorder and violence, including some that specifically have examined the relationship with schizophrenia. "These reports typically find that schizophrenia is related to a 4- to 6-fold increased risk of violent behavior, which has led to the view that schizophrenia and other major mental disorders are preventable causes of violence and violent crime," the authors write. They add that considerable uncertainty exists as to what is the cause of this elevated risk. Some studies have indicated that substance abuse may play a role.

Seena Fazel, M.D., of the University of Oxford, Warneford Hospital, Oxford, England, and colleagues examined the relationship of schizophrenia with violent crime and the possible role of substance abuse. The study included data from nationwide Swedish registers of hospital admissions and criminal convictions from 1973-2006. Risk of violent crime in patients after diagnosis of schizophrenia (n = 8,003) was compared with that among general population controls (n = 80,025). Potential confounders (age, sex, income, and marital and immigrant status) and mediators (as substance abuse) were measured at the beginning of the study. To study familial confounding, the researchers also investigated risk of violence among unaffected siblings (n = 8,123) of patients with schizophrenia.

The researchers found that among patients with schizophrenia, 1,504 (13.2 percent) had at least 1 violent offense compared with 4,276 (5.3 percent) of general population controls (adjusted odds ratio, 2.0). The rate of violent crime in individuals diagnosed as having schizophrenia and substance abuse (27.6 percent) was significantly higher than in those without substance abuse (8.5 percent), which resulted in adjusted odds ratios of 4.4 for violent crime in schizophrenia with substance abuse and 1.2 in schizophrenia without substance abuse.

The risk increase among those with substance abuse was significantly less pronounced when unaffected siblings were used as controls (28.3 percent of those with schizophrenia had a violent offense compared with 17.9 percent of their unaffected siblings), suggesting significant familial (genetic or early environmental) confounding of the association between schizophrenia and violence.

"We demonstrate that the risk of violent crime in schizophrenia in patients without comorbid substance abuse is only slightly increased. In contrast, the risk is substantially increased among patients with comorbidity and suggests that current practice for violence risk assessment and management in schizophrenia may need review," the authors conclude.