Almost a million mentally ill people in the US are in prison or released on probation, according to the first comprehensive study of the problem.
The report by the Justice Department statistics bureau also says that 40 per cent of those in prison are receiving no treatment for their illness.
Some 283,800 prisoners are estimated to be mentally ill. An additional 547,800 mentally ill offenders have been released into the community on probation.
The study reveals that the mentally ill prisoners are more likely to have committed a violent offence than other inmates.
The author of the report, Ms Paula Ditton, says however this does not mean they are more violent as other causes could produce these figures. For example, police might find it easier to catch violent criminals who are mentally ill or judges might be more inclined to sentence them to prison than other violent offenders.
The mentally ill serve on average 15 months longer than other inmates because their delusions or paranoia make them more likely to get into fights and lose privileges.
Compared to other prisoners, twice as many of the mentally ill prisoners had been homeless before their arrest or had suffered from sexual or physical abuse in childhood.
The researchers defined the mentally ill as those who reported their own illnesses and, to compensate for under-reporting, those who had ever spent a night in a psychiatric hospital.
The study looked at four groups: those in state prisons, federal prisons, local jails and probationers.
In state prisons, the highest rate of mental illness was among the white female inmates where the rate was 29 per cent. The rate was 40 per cent among white females aged 24 and under, the report says.
The problem is attracting increasing attention since the large-scale closure of state mental institutions beginning in the 1970s, when it was decided that patients would be better living in the community and being treated with new anti-psychotic drugs.
From 599,000 in 1955, the number of patients in state mental hospitals has fallen to 69,000 in 1995.
A leading expert in the field, Prof Linda Teplin, says jails have now become the mental hospitals of the poor.
Her pioneering research in the Cook County jail in Chicago showed 9.5 per cent of the male inmates there suffered from severe mental disorder, or four times the rate in the general population.
She has welcomed the Justice Department report but queries the method whereby the prisoners were asked if they suffered from mental disorder. People with such disorders often are not aware of them or do not want to report them, she says.
The Justice Department is holding a major conference on the problem next week.