The ratio of mentally ill people held in the state prison system compared to the number being treated at hospitals is a staggering 10 to 1, says Mother Jones, reporting on new research by the Treatment Advocacy Center, a nonprofit organization that promotes psychiatric care for the mentally ill.

This skewed ratio is being driven by two main factors. One is a drop in the number of patients in mental health facilities. At its peak in the 1950s, says TAC, more than 550,000 patients were in mental hospitals. Now, the number is down to around 35,000. Another factor is a resumption of the practice of sending mentally ill patients to prison. Prior to the 1970s, the group says, mentally ill people were routinely sent to mental hospitals. But, in the 1970s this changed, and people were increasingly spending time in prisons and jails. That uptick is supported by a separate 2007 study:

Those with mental disorders have been increasingly incarcerated during the past three decades, probably as a result of the deinstitutionalization of the state mental health system. Correctional institutions have become the de facto state hospitals, and there are more seriously and persistently mentally ill in prisons than in all state hospitals in the United States.

The 10-to-1 ratio is actually an underestimate, too, TAC suggests: the research does not count the number of mentally ill people in private or federal prisons.

According to Mother Jones, putting mentally ill people in prison cannot be regarded as a cost-saving measure. And, because they're in conditions often not equipped to properly address their needs, mentally ill people often fare worse than regular prisoners.

The issue is not a uniquely American one: in Canada, the U.K., and elsewhere in Europe, people will mental illnesses, particularly psychosis-related issues, says the World Health Organization, are overrepresented in prisons.