It’s a fact that may surprise many liberals: Mass incarceration is a result of way more than the war on drugs.

Over the past few years, long prison sentences for low-level drug offenses have gotten a lot of attention in the media and the public for contributing to higher incarceration rates. But a new report by the Urban Institute suggests it's not these low-level sentences that really helped cause higher imprisonment rates in the US, but rather sentences for violent crimes like murder.

The report is just the latest in a growing body of evidence that mass incarceration has been caused far more by rising punishments for violent offenses than drug offenses — and it complicates the traditional liberal narrative about how the US became the world leader in incarceration.

The report’s big finding is summed up by the following map:

Let’s break this down. First, there are two categories the map above is tracking: the top 10 percent longest time served in prison and the bottom 90 percent. The top 10 percent is marked by the light blue lines, while the bottom 90 percent is marked by the black lines.

This effectively compares prison sentences for the most extreme violent crimes with lower-level crimes, including drug offenses. Among people sentenced before age 25 and serving the longest prison sentences, 94 percent were convicted for violent offenses, and 69 percent of those violent offenders were convicted of murder.

The map shows that time served for the bottom 90 percent didn’t increase much, if at all, in most states — with the important exception of California, given that it’s the most populous state. Instead, the much bigger increase was seen in the top 10 percent.

What this shows, essentially, is that prison sentence length for lower-level offenses did not increase much, while prison sentence length for some of the worst offenses vastly increased.

“Longer sentences are stacking up,” Ryan King, the lead researcher for the Urban report, told me. “And in many states, the data suggest that they’re stacking up at a rate significant enough that it can offset reforms for the less serious offenses.”

The report includes various other findings. It found there are vast racial disparities in the top 10 percent of prison sentences, just as there are for lower-level offenses. The people locked up also tend to be fairly young, which robs communities — particularly black neighborhoods — of people who could grow up to be productive citizens instead of serving out disproportionately harsh sentences. It also told the stories of a few people who suffered through some of these long sentences. You should really read the whole thing.

But I want to home in on the big finding because it shows what the traditional story about mass incarceration has gotten wrong. Much of the attention has gone to harsh mandatory minimum sentences for drug offenses, but they seem to have had a fairly small impact on overall incarceration rates. What seemed to change, instead, is that the system enforced longer prison sentences for some of the worst offenses — and that led to a lot more imprisonment.

Mass incarceration is about much more than the war on drugs

The findings really give more credence to the growing body of evidence that prison sentences for violent, not drug, offenses have led to a sharp rise in US incarceration levels.

Perhaps the best source for all the evidence so far is criminologist John Pfaff’s book Locked In: The True Causes of Mass Incarceration and How to Achieve Real Reform, for which you can read my book review as well.

Here’s the short version: Much of the attention to mass incarceration, including from reform efforts, has gone to low-level offenses, especially for drug and property crimes. In large part, this is likely a result of the media focusing too much on the federal prison system instead of the state prison systems: While about half of federal prisoners are in for drug crimes, only about 16 percent of state prisoners are — and more than half of state prisoners are in for violent crimes.

This is notable because the great majority — 87 percent — of prisoners in the US are housed at the state level, not the federal level. So to greatly reduce incarceration, the country will need to focus on the state level. And to do a lot at the state level, the US will need to reduce the incarceration of violent offenders.

This obviously gets a lot trickier, politically, than addressing low-level drug offenses. A poll conducted by Morning Consult for Vox last year, for example, found that nearly eight in 10 US voters support reducing prison sentences for people who committed a nonviolent crime and have a low risk of reoffending. But fewer than three in 10 backed shorter prison sentences for people who committed a violent crime and have a low risk of reoffending.

“It’s one of the spaces where the policy and public safety arguments are going to have the least impact,” Pfaff acknowledged, “because many will view it as the right thing to do to lock them up forever.”

But there are ways to cut prison sentences for violent offenders without leading to more crime.

For one, incarceration is simply 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.

These figures explain the cause against mass incarceration: Not only does it deprive a lot of people of their rights and take a lot of people out of their communities, but it also isn’t even particularly effective at stopping crime.

The research also shows that people age out of crime. A 60-year-old is simply much less likely to attack or rob someone than is a 20-year-old. That means it’s possible to sentence violent offenders to five, 10, or 20 years — instead of 30 or 40 years, or life — without dramatically increasing the chances that they’ll reoffend.

There are also new ideas for reintegrating people into society without the threat of long prison sentences. Researchers Mark Kleiman, Angela Hawken, and Ross Halperin, for example, suggested a “graduated reentry” system that would support people who are released from prison and then slowly give them back their rights as they hit certain milestones, such as getting and keeping a job.

This kind of policy, along with reductions in legislative mandates for lengths of prison sentences, could help cut how much time even violent offenders serve in prison. And that would help address a major contributor to mass incarceration, based on Urban’s analysis.

Cutting long prison sentences isn’t enough to end mass incarceration

Still, there are limits to how far simply cutting prison sentences, particularly at the legislative level, could go.

Typically, much of the attention in the criminal justice reform world goes to cutting lengthy prison sentences for drugs — particularly mandatory minimums that require judges to impose a lengthy punishment even if they don’t want to. Similarly, reformers might think it’s a good idea to focus on cutting the length of long prison sentences for violent crimes as well.

Pfaff argues this would only go so far — because the problem goes much deeper than what state law says is an appropriate sentence for a certain crime. He points to how the sentences are implemented at the local level, particularly by prosecutors.

Looking at California county-level data, Pfaff highlighted that some counties have much higher median sentences for their worst offenders than others. So while the state median for time served among the 95th percentile was 23 years in 2014, it was 33.5 years for San Francisco County, nearly 25 years for Los Angeles County, and 21 years for Sacramento County. The differences between high-population counties suggest there is a lot of variation in how prosecutors enforce the laws that state policymakers create for them.

“There’s an incredibly different story across counties, even among counties with similar populations,” Pfaff argued. “We don’t want to lose sight of trying to regulate the prosecutors, the plea deals they make, and how they charge people.”

Pfaff’s finding dovetails with some of his earlier work, which found that prosecutors — not more arrests or crime — drove much of the increase in incarceration since the 1990s. Analyzing data from state judiciaries, he compared the number of crimes, arrests, and prosecutions from 1994 to 2008. He found that reported violent and property crime fell, and arrests for almost all crimes also fell. But one thing went up: the number of felony cases filed in court. In short, prosecutors were filing more charges even as crime and arrests dropped, throwing more people into the prison system. Prosecutors were driving mass incarceration.

So to really crack down on mass incarceration, policymakers will need to find a way to reel back prosecutors — on top of the kind of policy recommendations that Urban makes for reducing prison sentences and time served.

This also shows just how complicated the problem really is. We often talk about the US criminal justice system as if it’s just one system. In reality, it’s more than 3,100 systems, representing every county and county equivalent in America. As Pfaff wrote in his book, “[T]he term ‘criminal justice system’ is a misnomer; criminal justice is, at best, a set of systems, and at worst it is a swirling mess of somewhat antagonistic agencies.”

To really address the problem of mass incarceration, then, it’s not enough to just focus on drug crimes; it’s also important to focus on violent offenses. It’s also not enough to just focus on the laws guiding prison sentences; it’s also necessary to look at how those laws are enforced in the real world. And addressing all of these issues will require a truly systemic effort — from addressing what the local prosecutor is doing to what laws state policymakers pass to what the president and his attorney general are asking the US Department of Justice to do.

It will be a long, arduous effort. After years of lawmakers building up incarceration at every level of government, it will likely take years of more policymaking at every level of government to unwind what previous generations of leaders have done.

“This is a long-term project,” King of Urban said. “But we do see it as one that’s ringing a bell saying, look, we’re going to have to deal with this.”