Are America's jails used to punish poor people?

At this moment, more than 730,000 people around the U.S. are locked up in local jails, including many non-violent offenders who are simply too poor to post bail.

The findings come from a new study by the nonprofit Vera Institute of Justice, with support from the John D. and Catherine T. MacArthur Foundation, as part of the foundation's Safety and Justice Challenge initiative. Although violent crime has declined almost 50 percent in the past two decades, annual admissions to jails have almost doubled to 11.7 million, the study found.

Along with that surge in jail admissions has come longer stays behind bars, with the average length of stay stretching to 23 days from 14 days three decades ago. About three-quarters of pretrial detainees and sentenced offenders are languishing in local jails for nonviolent crimes, and many of those are too poor to pay court-imposed fines.

"We are punishing people for their poverty," Nick Turner, executive director of the Vera Institute, told CBS MoneyWatch. "When we looked at the data, we found 75 percent of the population of people in jail are awaiting trail and are there for nonviolent offenses. That is mind-boggling when you think the country spends $22 billion on jail populations."

That spending has resulted in little improvement to public safety, the report said. At the same time, the $22 billion spent each year by local jurisdictions to jail people amounts to a financial burden on taxpayers, who end up paying for health care for inmates or pensions for jail staff, for example.

Local jails have a much broader reach than state and federal prisons, with the study finding that jails admit 11.7 million people annually, almost 19 times the 631,000 who go to state and federal prisons every year.

Those who end up in local jails are hit with a range of penalties that have far-reaching consequences, including loss of of income or forcing children to go into foster care if the jailed person is a parent. Once out of jail, men who were behind bars tend to earn less than their counterparts, the study noted. Formerly incarcerated men, two-thirds of whom held jobs before their arrests, saw their hourly wages decline by 11 percent.

"Jails have lost sight of their intended purposes," said Laurie Garduque, director of the MacArthur Foundation's Justice Reform program. "It's not supposed to be a place where people languish because they can't make low-level bail or can't pay their traffic tickets."

Along with its support for the Vera Institute's study, the MacArthur Foundation announced a $75 million initiative to reduce over-incarceration. The Safety and Justice Challenge will fund 20 jurisdictions to create more effective plans for how America manages its jails, Garduque said.

Some communities are already using new strategies to more effectively deal with non-violent offenses, such as King County, Washington's LEAD program, which diverts low-level offenders to community-based services.

"We know there are alternatives starting at the point of arrest and moving to pretrial and how quickly they go through the court system," Garduque noted. "It's because of these promising alternatives that we are making this investment. Local problems need local solutions."

For many jail inmates, mental illness and substance abuse are also issues, which can make it less likely that they can afford court fines. For instance, about one-third of people in jail with mental illness didn't have a job a month before their arrest, the study found, citing a 2006 study from the Bureau of Justice Statistics.

Jails tend to lock up the disadvantaged, with 47 percent of inmates lacking either a high school degree or having passed the GED. It's also racially skewed, with blacks jailed at almost four times the rate of white Americans, the study found. While almost everyone is offered a chance to get out of jail if they can post bail, these groups are often less likely to be able to afford it.

"If you look at the New York City jail system, 31 percent of non-felony defendants are held because they can't pay $500 bail," Turner said. "That quick dip in jail has ramifications, and we are investing in a process that doesn't produce results."