The risk of a serious crime being committed by someone who is mentally ill is similar to that of someone in their teens or twenties but according to the authors does not justify submitting them to increased institutional care.
A study published in the British Journal of Psychiatry today found that even when mentally ill patients did offend, the incident was more likely to be associated with drugs and alcohol abuse than their condition.
Any potential link between mental illness and violent crime has been fiercely argued between those who point to a number of "care in the community" tragedies where patients known to psychiatric services had harmed others and campaigners who have said that mental patients are far more likely to harm themselves than others.
While one-quarter of those convicted of serious crimes had had contact with the mental health services, the vast bulk of these were either drug abusers or alcoholics or suffered from personality disorders other than schizophrenia or serious mental illnesses. Most had begun their criminal career before having any contact with psychiatric services.
The study, which was carried out in Australia, linked two databases, the first being all convictions between 1993 and 1995 and the second a state-wide psychiatric case register.
Over the three years studied, 2,153 people were convicted for violent crimes of whom 70 had had treatment for schizophrenia. For men, this was a rate of some three to five times higher than for the general population. But when substance abuse was taken into account, the picture altered.
Those with schizophrenia but not substance abuse problems were only marginally more likely to receive convictions for violence and were no more likely to commit violent offences than young people in their teens and twenties. They were significantly less likely to offend in this manner than alcohol and drug abusers without mental illness.
The authors conclude that the increase in serious criminal offending in schizophrenia is "modest" and say that the relationship is so tenuous between the illness and the crime that prediction of serious violence is "virtually impossible".
"It does the mentally disordered a serious injury to confuse their behaviour - which may indeed be frightening and distressing - with murderous behaviour and call for measures which would only be justified to prevent the most serious forms of violence to be applied to large groups of the mentally ill," they say. The data reveals that those convicted of a violent offence are more than twice as likely to have had a primary diagnosis of substance misuse as of schizophrenia.
"This new research knocks on the head the idea that people with schizophrenia are generally dangerous," said Liz Sayce, policy director of the mental health charity Mind. "This assumption is deeply distressing to people who have schizophrenia, most of whom have never committed an act of violence in their lives."Reuse content