People with severe mental illness are responsible for one in 20 violent crimes, researchers say.
Early intervention "is key to helping the mentally ill"
UK experts studied 13 years of data from Sweden, where population data on mental health and crime is kept.
It was found 18% of murders and attempted murders were committed by people with a mental illness.
Campaigners are split as to whether the number, published in an American Journal of Psychiatry study, is less or more than would have been expected.
The study, carried out by researchers from Oxford University's department of psychiatry and Sweden's Karolinska Institute, looked at data from 1988 to 2000.
They found there were 45 violent crimes committed per 1,000 inhabitants.
Of these, 2.4 were attributable to patients with severe mental illness, which also includes bipolar disorder (manic depression) and other psychoses.
This means that 5.2% of all violent crimes over the period were committed by people with severe mental illness.
When the figures were broken down, it was also found that 15.7% of arsons were committed by people with such illnesses, as were 7.5% of threats and harassment.
Just under 7% of cases of assaulting an officer, 6.3% of aggravated assaults, 5% of sexual offences, 3.6% of robberies and 3% of common assaults were also carried out by this group.
When the researchers looked specifically at violent crimes committed by women aged 25 to 39 - a much lower number than are committed by men - they found 14% were committed by those with serious psychiatric disorders.
Dr Seena Fazel, the forensic psychiatrist who led the research, said: "The figure of one in 20 is probably lower than most people would imagine.
"Many see those with serious psychiatric disorders as significantly contributing to the amount of violent crime in society.
"In many ways the most interesting aspect of our findings is that 19 out of 20 people committing violent crimes do so without having any severe mental health problems."
A spokesman for the Sainsbury Centre for Mental Health said: "Having a severe mental health problem does not make a person violent.
"People with conditions like schizophrenia are in fact more likely to be the victims of violence than others in the population.
"This study shows clearly that people with severe mental health conditions commit a very small proportion of violent crimes and that the widely held prejudices about schizophrenia are inaccurate and unfair.
"It is now time to stop this stale debate about mental health and violence and start looking at how to overcome the prejudice and consequent discrimination that stop people with severe mental health conditions from having an ordinary life in our society."
But Michael Howlett, director of the Zito Trust, said: "This is a very high figure.
"And I think it is an under-estimate, which does not include those people with personality disorders who commit crimes.
"But the figures do suggest it is possible to intervene early in people's lives, before crimes are committed."
However Mr Howlett added: "It's very important to stress that the majority of people with mental illnesses do not commit crimes."