Where America’s jail population exploded. (Hint: It’s not just big cities.)

America’s jail population has exploded over the past few decades — and much of the growth has come from small, rural counties instead of big, urban ones, a new report by the Vera Institute of Justice found.

The report shows that jail populations grew in all types of counties, but small counties (population-wise) experienced the biggest growth.

As a result, the jail incarceration rate was way higher in midsize and smaller counties in 2014: 271 jail inmates per 100,000 residents in the 40 largest counties, 325 per 100,000 residents in the 212 midsize counties, and 446 per 100,000 in the more than 2,600 small counties.

The report takes a different kind of look at mass incarceration, which is often examined through a national lens. It suggests that much of the growth in incarceration over the past few decades — jails make up about one-third of the incarcerated population — is a result of policy changes at the local and state level, particularly in small counties.

A quick point of clarification: Jails are where people are held while waiting for trial or after they’re convicted of typically minor charges, and they’re generally run by local police departments. Prisons are where someone is sent after being convicted of more serious crimes, and they’re usually overseen by state and federal agencies.

So what’s behind the big population increase in jails? One answer is more people are being sent to jail: The rate of admissions grew by 1.75 times between 1978 and 2014. But a bigger driver was length of stay, which on average increased by 2.5 times — from nine to 23 days. All of this led to a fourfold increase in the jail population since the 1970s.

But Vera researchers aren’t completely sure why there’s a big disparity between small, midsize, and large counties. They have a few theories: It’s possible that jails in cities are more constrained by space, since there’s less of it in metropolitan areas. It’s also possible that large metro areas have more people to draw on for taxes, so they don’t need to resort to the criminal justice system — mainly fines and court fees, enforced through jail time — for revenue. Or perhaps rural and small counties simply built jails beyond their needs, and they opted to use the space once the capacity was built.

Whatever the case, the increase was likely driven not by crime but by policy changes — specifically, local and state policies, not federal policies that are so often the focus of public and media attention.

The report reinforces an often underappreciated point of mass incarceration: Though much of the media and public attention falls on the federal government, it’s often the local and state governments that push more and more people to jail and prison. Only about 6 percent of the jail population is held by federal authorities, and just about 13 percent of people in prison are held at the federal level, according to Vera and the US Bureau of Justice Statistics.

So what’s behind the increase in jails? It’s not crime: Since 1991, all types of crime in the US have plummeted — yet jail populations continued to increase, Vera found.

The answer, Vera argues, instead lies in policy changes. With the onset of “tough-on-crime” policies over the past several decades, police now take even petty crimes more seriously — through policies like “broken-windows policing” — and therefore have pushed more people to jail. And more people have been jailed for convicted crimes, including many cases that might not have resulted in incarceration at all before.

Rolling back these types of tough-on-crime policies would put a big dent in mass incarceration. But doing that would require reforms at the local and state levels — especially, as Vera found, in the thousands of smaller, rural counties that are often ignored by media coverage of mass incarceration.