Mass incarceration has made the United States the world leader in inmates per capita. A  few decades ago, urban counties were the epicenter of that phenomenon, incarcerating the largest proportions of their residents. Yet new data suggests the population of jailed or imprisoned Americans is increasing most rapidly outside major cities. In fact, people living in suburban and rural areas are more likely to see the inside of a cell than those in urban centers.

The Vera Institute of Justice in New York compiled the data using federal statistics. The organization found that, since 1970, the jail population has expanded sevenfold in small counties, more than twice as fast as it has in large counties. On a typical day in 2014, those large counties had an average of 271 inmates in jails per 100,000 people between the ages of 15 and 64. In small counties, the figure was 446 inmates.


This map, courtesy of the Vera Institute of Justice, shows how many people lived in each county in 2014 and the share of them who were in a local jail. Data for Alaska and Hawaii were not available.

These days, not only are more people being booked in jails, they are also staying longer. In 1978, according to the Vera Institute, the average inmate waited just nine days in jail in 1978. Today, the average stay in jail lasts more than three weeks.

The report from the Vera Institute focused on locally administered jails rather than prisons. While policymakers and the press discuss prisons more frequently, local jails are where the vast majority of Americans who are locked up go.

Jails accounted for a little more than 11 million admissions annually, while state and federal prisons recorded just 627,000 admissions last year. The institute's preliminary analysis of figures from New York and California indicates that the pattern of increasing rates of incarceration in suburban and rural counties holds when prisons are included, too.

[Read more: The U.S. has more jails than colleges. Here's a map of where those prisoners live.]

Though far more people are admitted to jails over the course of a year, there are far more in prison on any given day, since inmates in prison typically serve a couple of years after conviction. On the other hand, most jail inmates stay for only a few weeks while waiting for a trial.

It wasn't clear from the data why jail populations have increased so dramatically in sparsely populated areas, said Nancy Fishman, who directs the Vera Institute's work on jails.

Part of the explanation could be that the demographics of America's suburbs have changed, with many poor households emigrating out of the city. In the country's 100 largest metropolitan areas, there are now fewer people living in poverty within the city limits than outside them, according to research conducted at the Brookings Institution.

Because of their poverty, members of those households might be less likely to make bail when they are arrested, with more people spending time behind bars as a result.

Fishman also suggested that building and maintaining jails is expensive for local governments, especially in sparsely populated areas, and that some jurisdictions might use the threat of jail to extract fines and fees from residents to cover the cost of their expanding criminal-justice systems in a vicious cycle.

As an example, she cited Ferguson, Mo., where Michael Brown Jr., an unarmed young black man, was shot and killed by a police officer last year. Residents and advocates for criminal justice reform said police in Ferguson and neighboring communities relied on revenue from traffic tickets and other fines, then jailed residents when they didn't pay, even those who couldn't afford to do so.

"As soon as you build beds, the beds get filled," Fishman said. "As these systems have gotten bigger, they have become oddly self-perpetuating."

Whatever its causes, the increase has been most drastic among women. The share of women in jails nearly tripled from about 5 percent in 1970 to 14 percent last year. A stretch of counties in Appalachian states, notably Tennessee and Kentucky, has especially high rates of female incarceration in jails.

You can explore the Vera Institute's map yourself and examine trends in jail populations over time by clicking here.

More from Wonkblog:

The best way to deal with America's incarceration problem is hiding in plain site

How “prisons without bars” could cut the cost of prison and keep people from coming back

A shocking number of mentally ill Americans end up in prison instead of treatment