America’s incarceration rate fell to a two-decade low in 2016, according to a recent report by the US Bureau of Justice Statistics (BJS).

In 2008, the incarceration rate hovered at an all-time high — with a rate of 1,000 per 100,000 people. Since then, the incarceration rate has fallen to 860 per 100,000. That still means that 2.2 million people were incarcerated at any given period in 2016, but that’s a big drop from the 2.3 million just a few years before.

The Pew Research Center charted the numbers, showing the result of a decades-long drop in the crime rate and sentencing reforms, particularly at the state and local levels, that have tried to pull back punishments for low-level offenses such as drug crimes and theft:

This trend could continue despite President Donald Trump and US Attorney General Jeff Sessions’s “tough on crime” rhetoric and policies — promising to, for example, lock up more low-level drug offenders.

That’s because as much as Trump and Sessions may try to make the federal system harsher, most incarceration happens at the local and state levels — where the federal government has no jurisdiction. According to other BJS data, more than 87 percent of people in prison are held in state facilities.

Even on the federal level, though, the incarceration rate has fallen post-Trump, from more than 192,000 in 2016 to below 184,000 so far this year.

Still, maybe hold off a bit before celebrating too much. As Pew noted in its own map based on data from the World Prison Brief, the US is still the world’s leader in incarceration, ahead of authoritarian regimes like Cuba, Russia, and China:

(The numbers in the map differ from the charts above due to methodological differences between the data sources, Pew explained: “While BJS emphasizes the number of inmates per 100,000 adults ages 18 and older, for example, the World Prison Brief measures each country’s incarceration rate as the number of inmates per 100,000 people of any age. It also counts jail inmates in a slightly different way.”)

Getting the incarceration rate down much further is going to be tricky. Almost all the discussion about US prison sentencing reform has, to this point, focused on pulling back the war on drugs and harsh sentences for low-level offenders. But the bulk of people in prison are convicted of violent crimes, not low-level offenses.

According to the Prison Policy Initiative, around 21 percent of people in jail or prison are in there for a drug crime. About 42 percent of people in jail or prison are in there for violent crimes — making violent offenses the single biggest driver of incarceration out of all offense categories.

There are ways to bring down the incarceration rate, even by pulling back punishments for violent offenses, without causing more crime. For one, the data suggests that people actually age out of crime — they’re more likely to commit a crime in their late teens or early 20s than they are in their 30s, 40s, 50s, and so on. So letting people out of prison 10 or 20 years down the line — instead of 40 or 50 years, or never — likely wouldn’t pose a threat to public safety.

One reason to do this: Incarceration is a pretty inefficient way to keep the public safe. 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, though other researchers estimate it drove 10 to 25 percent of the crime drop since the ’90s.

And other studies have shown that more incarceration can even lead to more crime. As the National Institute of Justice concluded in 2016, “Research has found evidence that prison can exacerbate, not reduce, recidivism. Prisons themselves may be schools for learning to commit crimes.”

Previous polls, however, have found that the American public has little appetite for reducing punishments for violent offenders. If the incarceration rate is to come down further, though, it will be necessary.

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