Federal statistics show that from 2010 to 2017, crimes, arrests, and resulting jail admissions fell by 14, 20, and 18 percent, respectively. In fact, there were 2 million fewer admissions to jails nationwide in 2017 than seven years earlier. Still, despite these positive trends, the total number of people in county and municipal jails remained virtually unchanged—at about 750,000.

As opposed to prisons, jails are locally operated correctional facilities that hold diverse populations, including people awaiting trial, serving sentences of a year or less for minor crimes, being held pending transfer to another facility, or who have violated probation or parole supervision. Many people are booked into or released from jail on any given day and the facilities are often densely populated.

Maintaining large jail populations at a time of falling crime and arrests can have unnecessary and negative consequences for those incarcerated, jail staff, and broader communities. That includes potential exposure to and transmission of infectious illnesses, such as the new coronavirus rapidly spreading across the globe. Crowded places where people often cannot keep a safe distance from one another, such as jails, provide opportunities for COVID-19 and other communicable diseases to proliferate.

So what might account for this apparent disconnect between falling crime and arrest numbers and stagnant jail populations? Given the drop in admissions, the place to look would likely be the other key driver of correctional populations: how long people stay in a facility. An analysis by Pew of data from the federal Bureau of Justice Statistics shows that the average length of stay for all jails in the U.S. increased 22 percent between 2010 to 2017—from approximately 21 to 26 days.

This increase effectively counteracted what would have been a significant decrease in the jail population because of the drop in admissions. In fact, the Pew analysis indicates that with the number of admissions in 2017, the national jail population would have been about 129,000 smaller—a more than 17 percent drop—if average time in jail had remained at the 2010 level.

But these numbers don’t paint a nuanced picture of what may be behind the relative stability of jail populations. It could be that there are more people serving long periods of time, that the number of people serving shorter stays has decreased, or some combination of the two.

In our next analysis, Pew’s public safety performance project will delve into possible explanations for the increase in average length of stay. That will include a breakdown of jail stays and space by varying lengths of time—particularly a week or shorter and a month or longer—that people spend in jail. This research can help provide greater clarity into the stubbornly high U.S. jail population and guide policy changes intended to ensure that falling arrest and crime rates are matched by dropping numbers of people in American jails.

Jake Horowitz is the director and Tracy Velázquez is a research manager with The Pew Charitable Trusts’ public safety performance project.