An officer is seen through the door window from inside a holding cell at a rural jail in Colorado. (Hugh Carey/Associated Press)

When policymakers talk about problems with the nation’s jails and over-incarceration, they often are discussing concerns that center on the largest cities.

But a study released Tuesday shows that rural facilities are growing the fastest and are driving a national increase in jail population. That growth is especially surprising, researchers say, because it comes even as the crime rates in those rural areas remain much lower than their urban counterparts.

“This is a phenomenon that has been overlooked,” said Laurie Garduque, director of justice reform at the MacArthur Foundation, which helped fund the study conducted by the Vera Institute of Justice. “We’ve seen a modest decline in big metro jails, but the numbers have continued to go up in small and rural jails.”

Out of the 740,000 people in U.S. jails, 20 percent are in rural areas, and 33 percent are in medium and small metro areas. Large urban metro areas have 27 percent of the prison population and suburbs of large metro areas hold the remaining 20 percent.

Researchers at the Vera Institute arrived at their conclusion after analyzing newly released data from the Bureau of Justice Statistics.

Between 1970 and 2013, researchers found, pretrial incarcerations — when people charged with a crime are held in jail — have increased by more than 425 percent in rural counties. By comparison, Vera researchers found that since 2000, pretrial incarcerations in big cities have leveled off. And in some cities, such as Atlanta and New York, the number of people incarcerated has declined.

The jail incarceration rate in New York City is now almost the same as it is in its less-urban neighbor Westchester County, according to the study. The rate also is lower in Los Angeles than in Orange County.

The growing urban-rural jail divide is most acute in the South and the West.

One reason for the significant growth of rural jails nationwide is the increasing financial incentives that such facilities receive for renting out beds to federal, state and other local agencies, the study authors said.

According to the study, 84 percent of jails hold people on behalf of other local jails or state and federal authorities. In the 1970s, fewer than half of U.S. counties reported holding someone for another agency.

“These jails have created jobs and revenue that some of these counties are using to offset other costs,” Garduque said. “In some places, you fund your criminal justice system essentially by leasing out your beds in jail.”

The study’s authors made clear that crime is not driving the growth in rural jails, as crime rates are generally down nationwide. Crime rates remain substantially lower in rural counties than in urban areas.

“People often think jail incarceration is connected to crime, but this is happening in areas that haven’t seen increase in crime,” said Jacob Kang-Brown, a senior research associate at the Vera Institute.

Instead, the study’s authors pointed to the lack of resources for justice systems in rural areas as another likely reason jail populations are increasing.

Ram Subramanian, the Vera Institute’s editorial director, said, “The one thing that we saw that united these counties [is] the fact that they are very under-resourced places.” There are fewer judges to quickly hear cases, a lower level of pretrial services and fewer alternatives to incarceration in rural areas, he added.