A nation wide study about crimes committed by psychiatric patients was published in the current special issue of Canadian Journal of Psychiatry.
Canadian Journal of Psychiatry: Vol 60, No 3, March 2015 mars
Unfortunately, I am not a member of Canadian Psychiatric Association, so I could not read the full text of the articles. A summary of the results was introduced in the website.
Examiner.com: Mentally ill Canadians found 'not criminally responsible' are misunderstood
According to this, the researchers investigated the data of 1,800 people declared as not criminally responsible. They found that only 7% of the serious crimes was committed by persons who were irresponsible due to mental illnesses. In addition, the recidivism rate of them was 17%, lower than that in offenders who were responsible for the crime.
Canadian legislation is different from that in Japan. Therefore, it is not easy to interpret these results with my intuition. In Japan, the rate of mentally disordered persons in all criminal is 0.8%, according to the Ministry of Justice. However, public prosecutors in Japan tend not to prosecute the offenders who are suspected to be severely suffering from mental illnesses. Thus, there may be more crimes committed by mentally disordered patients.
Limited to violent offences, some mental illnesses raise the risk of perpetrating. Although there had been a wide range of discussion in the past, nowadays no specialists doubt this idea nowadays. For example, approximately 10% of murder cases are committed by mentally disordered persons, according to White Paper of the Crimes in Japan, whereas the prevalence rate of mental disorders is only 3%.
Notwithstanding this fact, we should not fear the patients with mental disorders excessively. It is extremely rare to encounter the crimes caused by them. In addition, 70% of the victims are the family members of the patients. The possibility that you will be involved in a crime by a serial killer is astronomical.
Recidivism is another complicated topic. It depends on the definition of mental disorders. In general, people who were identified as not criminally responsible are suffering from severe mental illnesses such as paranoid schizophrenia. In such cases, effective therapy can reduce the risk of reoffending dramatically. On the other hand, psychopathic patients and patients with substance misuse remain high risk of reoffending, even if intensive treatment was provided.
"Not Criminally Responsible" system seems to be frequently used in Canada, compared to in England and the US, given the data of this study. Some people are opposing this idea itself. In Japan, approximately 70% of the citizens think that offenders with mental disorders should be punished equally to the healthy people, as far as I have investigated. I understand this idea emotionally. However, it is not rational to punish those who did not understand the nature of their acts at the offence. Approving this matter can ignite an emotional conflict. We should be calm, neutral, and keep a scientific view in the argument of this matter.