People with mental illness are overrepresented in the ranks of those in the U.S. behind bars. A new report (PDF) out today from the Urban Institute compiles the numbers, as well as documents how little is known about the most efficient and productive ways to treat mentally ill people who are locked up.

More than half–56 percent–of those held in state prisons, 45 percent of those in federal prison, and 64 percent of people locked up in local jails have a mental health problem, according to the report. One in five people suffers from a depression-related disorder, making that illness the most common mental health issues inmates face. Manic depression, and bipolar disorder come in second.

The report authors stress that their inquiry focuses on those with severe mental health illnesses, acknowledging that mental health and symptoms of mental illness exist on a spectrum, but leaving off "general mental health maintenance" and problems like sleep disorders to focus instead on "clinically significant disturbance in an individual’s cognition, emotion regulation, or behavior that reflects a dysfunction in the pyschological, biological, or developmental processes underlying mental functioning." 

This graph, from the report, shows the scope of the issue:

However, people with mental health problems often receive inadequate care, with just one in three state prisoners receiving some form of mental health care and one in six inmates in jails receiving mental health care. The report doesn’t include breakdowns by race.

"With such high numbers, their care and treatment is not just a humanitarian concern, it is a critical economic, societal, and public safety issue," the report authors write.