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.
I've blogged about this before, way back in February about the tragic story of Jennifer San Marco (here) - that posting includes several more links about this topic.