Background: Although expert opinion has asserted that there is an increased risk of violence in individuals with schizophrenia and other psychoses, there is substantial heterogeneity between studies reporting risk of violence, and uncertainty over the causes of this heterogeneity. We undertook a systematic review of studies that report on associations between violence and schizophrenia and other psychoses. In addition, we conducted a systematic review of investigations that reported on risk of homicide in individuals with schizophrenia and other psychoses.
Methods and findings: Bibliographic databases and reference lists were searched from 1970 to February 2009 for studies that reported on risks of interpersonal violence and/or violent criminality in individuals with schizophrenia and other psychoses compared with general population samples. These data were meta-analysed and odds ratios (ORs) were pooled using random-effects models. Ten demographic and clinical variables were extracted from each study to test for any observed heterogeneity in the risk estimates. We identified 20 individual studies reporting data from 18,423 individuals with schizophrenia and other psychoses. In men, ORs for the comparison of violence in those with schizophrenia and other psychoses with those without mental disorders varied from 1 to 7 with substantial heterogeneity (I(2) = 86%). In women, ORs ranged from 4 to 29 with substantial heterogeneity (I(2) = 85%). The effect of comorbid substance abuse was marked with the random-effects ORs of 2.1 (95% confidence interval [CI] 1.7-2.7) without comorbidity, and an OR of 8.9 (95% CI 5.4-14.7) with comorbidity (p<0.001 on metaregression). Risk estimates of violence in individuals with substance abuse (but without psychosis) were similar to those in individuals with psychosis with substance abuse comorbidity, and higher than all studies with psychosis irrespective of comorbidity. Choice of outcome measure, whether the sample was diagnosed with schizophrenia or with nonschizophrenic psychoses, study location, or study period were not significantly associated with risk estimates on subgroup or metaregression analysis. Further research is necessary to establish whether longitudinal designs were associated with lower risk estimates. The risk for homicide was increased in individuals with psychosis (with and without comorbid substance abuse) compared with general population controls (random-effects OR = 19.5, 95% CI 14.7-25.8).
Conclusions: Schizophrenia and other psychoses are associated with violence and violent offending, particularly homicide. However, most of the excess risk appears to be mediated by substance abuse comorbidity. The risk in these patients with comorbidity is similar to that for substance abuse without psychosis. Public health strategies for violence reduction could consider focusing on the primary and secondary prevention of substance abuse. Please see later in the article for Editors' Summary.