Seena Fazel and colleagues investigate the association between schizophrenia and other psychoses and violence and violent offending, and show that the increased risk appears to be partly mediated by substance abuse comorbidity.
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 (I2 = 86%). In women, ORs ranged from 4 to 29 with substantial heterogeneity (I2 = 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).
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
Schizophrenia is a lifelong, severe psychotic condition. One in 100 people will have at least one episode of schizophrenia during their lifetime. Symptoms include delusions (for example, patients believe that someone is plotting against them) and hallucinations (hearing or seeing things that are not there). In men, schizophrenia usually starts in the late teens or early 20s; women tend to develop schizophrenia a little later. The causes of schizophrenia include genetic predisposition, obstetric complications, illegal drug use (substance abuse), and experiencing traumatic life events. The condition can be treated with a combination of antipsychotic drugs and supportive therapy; hospitalization may be necessary in very serious cases to prevent self harm. Many people with schizophrenia improve sufficiently after treatment to lead satisfying lives although some patients need lifelong support and supervision.
Why Was This Study Done?
Some people believe that schizophrenia and other psychoses are associated with violence, a perception that is often reinforced by news reports and that contributes to the stigma associated with mental illness. However, mental health advocacy groups and many mental health clinicians argue that it is a myth that people with mental health problems are violent. Several large, population-based studies have examined this disputed relationship. But, although some studies found no increased risk of violence among patients with schizophrenia compared with the general population, others found a marked increase in violent offending in patients with schizophrenia. Here, the researchers try to resolve this variation (“heterogeneity”) in the conclusions reached in different studies by doing a systematic review (a study that uses predefined search criteria to identify all the research on a specific topic) and a meta-analysis (a statistical method for combining the results of several studies) of the literature on associations between violence and schizophrenia and other psychoses. They also explored the relationship between substance abuse and violence.
What Did the Researchers Do and Find?
By systematically searching bibliographic databases and reference lists, the researchers identified 20 studies that compared the risk of violence in people with schizophrenia and other psychoses and the risk of violence in the general population. They then used a “random effects model” (a statistical technique that allows for heterogeneity between studies) to investigate the association between schizophrenia and violence. For men with schizophrenia or other psychoses, the pooled odds ratio (OR) from the relevant studies (which showed moderate heterogeneity) was 4.7, which was reduced to 3.8 once adjustment was made for socio-economic factors. That is, a man with schizophrenia was four to five times as likely to commit a violent act as a man in the general population. For women, the equivalent pooled OR was 8.2 but there was a much greater variation between the ORs in the individual studies than in the studies that involved men. The researchers then used “meta-regression” to investigate the heterogeneity between the studies. This analysis suggested that none of the study characteristics examined apart from co-occurring substance abuse could have caused the variation between the studies. Importantly the authors found that risk estimates of violence in people with substance abuse but no psychosis were similar to those in people with substance abuse and psychosis and higher than those in people with psychosis alone. Finally, although people with schizophrenia were nearly 20 times more likely to have committed murder than people in the general population, only one in 300 people with schizophrenia had killed someone, a similar risk to that seen in people with substance abuse.
What Do These Findings Mean?
These findings indicate that schizophrenia and other psychoses are associated with violence but that the association is strongest in people with substance abuse and most of the excess risk of violence associated with schizophrenia and other psychoses is mediated by substance abuse. However, the increased risk in patients with comorbidity was similar to that in substance abuse without psychosis. A potential implication of this finding is that violence reduction strategies that focus on preventing substance abuse among both the general population and among people with psychoses might be more successful than strategies that solely target people with mental illnesses. However, the quality of the individual studies included in this meta-analysis limits the strength of its conclusions and more research into the association between schizophrenia, substance abuse, and violence would assist in clarifying how and if strategies for violence reduction are changed.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000120.
The US National Institute of Mental Health provides information about schizophrenia (in English and Spanish)
The UK National Health Service Choices Web site has information for patients and carers about schizophrenia
The MedlinePlus Encyclopedia has a page on schizophrenia; MedlinePlus provides links to other sources of information on schizophrenia and on psychotic disorders (in English and Spanish)
The Schizophrenia and Related Disorders Alliance of America provides information and support for people with schizophrenia and their families
The time to change Web site provides information about an English campaign to reduce the stigma associated with mental illness
The Schizophrenia Research Forum provides updated research news and commentaries for the scientific community