The US prison population fell in 2016 — for the 3rd year in a row

The number of people imprisoned in the US dropped for the third year in a row in 2016, according to a new report by the Bureau of Justice Statistics.

About 1.5 million people were in a US prison in 2016, down from a peak of 1.6 million in 2009. The imprisonment rate fell from a peak of 506 per 100,000 US residents in 2007 and 2008 to 450 in 2016.

The estimate does not include jails, so it undercounts the overall number of people who are incarcerated. More than 600,000 people are in jails in the US, according to an analysis by the Prison Policy Initiative.

The decline in imprisonment is a welcome shift for criminal justice reformers, who have long clamored against mass incarceration. The US has an incarceration rate that’s higher than the rates of authoritarian regimes like China, Cuba, and Russia, and in fact higher than any other country except the small island nation of Seychelles.

The research suggests that mass incarceration is not a good way to combat crime. A 2015 review of the research by the Brennan Center for Justice estimated that more incarceration — and its abilities to incapacitate or deter criminals — explained about 0 to 7 percent of the crime drop since the 1990s. Other researchers estimate it drove 10 to 25 percent of the crime drop since the ’90s. And a 2014 analysis by the Pew Charitable Trusts found that states that reduced their imprisonment rates also saw some of the biggest drops in crime, suggesting that there isn’t a hard link between incarceration and crime.

So the potential reversal of mass incarceration is good news for anyone interested in evidence-based crime-fighting policies. But there are some caveats to the overall trend.

On Twitter, Fordham Law School criminal justice expert John Pfaff pulled out some interesting findings from the new Bureau of Justice Statistics report. First, while the imprisonment rate has dropped, it’s dropped slower than the overall crime rate, “so in a way our punitiveness is RISING,” Pfaff wrote.

Second, much of the decrease in the prison population is being driven by a handful of states, such as California, Florida, New York, and Texas. In fact, while about 25 states have seen their imprisoned populations fall since 2009, about 24 have seen theirs rise. (Illinois was left out of Pfaff’s analysis due to data issues.)

This gets to one of the key issues in the criminal justice system: While we often view the criminal justice system from a top-down, national perspective, much of it works through a variety of state and even local policies, with about 87 percent of people in prison held at the state level. Pfaff has argued in his book Locked In: The True Causes of Mass Incarceration and How to Achieve Real Reform that the US criminal justice system in the US is actually more than 3,100 different systems — each representing a county or county equivalent in America.

While much of the attention to these systems has gone to drug offenses, Pfaff also points out that, in fact, mass incarceration is mostly about violent offenses. At the state level, where the great majority of US prisoners are held, about 54.5 percent of prison inmates were in for violent offenses in 2015. About 15.2 percent were in for drug offenses.

All of this greatly complicates the narrative about mass incarceration: It’s not just about one big federal policy, but about laws and policies implemented throughout thousands of systems. And it’s not just about the war on drugs, but about punitive punishments for violent offenses as well.

In 2016, America’s imprisonment rate dropped. But if lawmakers and reformers want that trend to continue, they will have to consider some of these broader issues too.

For more on mass incarceration and its causes, read Vox’s explainer.