Recent research, which has studies the effect of drug and alcohol abuse on mental health, has confirmed the association between schizophrenia and violence.
Published in the open access journal PLoS Medicine, the study has also found that the risk of violence from patients with psychoses, who also have substance use disorder, is no greater than those who have a substance use disorder, but who do not have a psychotic illness.
This finding suggests that schizophrenia and other psychotic illnesses are not responsible for any additional risk of violence above the increased risk associated with substance abuse.
Seena Fazel, of the Department of Psychiatry at the University of Oxford, and colleagues conducted a systematic review of all previous studies examining psychotic illness and the risk of violence to try and resolve their varied conclusions.
While some of the previous studies concluded that there was no increased risk of violence from patients with schizophrenia, others reported that there was a marked increase in the risk of violence in individuals with schizophrenia.
In their systematic review, the researchers identified 20 studies that compared the risk of violence among people with psychotic illness with those in the general population.
They used statistical tools to allow for the differences between the studies, and found that the risk of violent outcomes did increase for individuals with schizophrenia or other psychoses.
Men with schizophrenia or other psychoses were typically four to five times as likely to commit a violent act as a man in the general population.
For women with schizophrenia or other psychoses, there was an eight times greater risk of violence than those in the general population, although the researchers suggest this might be explained by the lower prevalence of alcohol and drug use in the general female population.
The researchers also found that substance abuse was the only factor causing variation between the studies.
According to them, substance abuse greatly increased the risk of violence for those with a psychotic illness, but this increased risk of violence was similar to those in the general population with substance abuse but no psychotic illness - suggesting that most of the excess risk of violence in psychotic patients appears to be mediated by the abuse of drugs and alcohol.
The researchers admit that further research is needed to clarify the relationship between schizophrenia and other psychoses, substance abuse, medication adherence and violence.
They, however, suggest that their findings could help redress the stigmatisation of patients with schizophrenia and other psychoses.
The authors write: "As substance use disorders are three to four times more common than the psychoses, public health strategies to reduce violence could focus on the prevention and treatment of substance abuse at an individual, community and societal level."