The war on drugs tends to get the bulk of the blame for mass incarceration in America. President Barack Obama repeated this bit of conventional wisdom during a speech at the NAACP's 2015 national conference on Tuesday:

Over the last few decades, we've also locked up more and more nonviolent drug offenders than ever before, for longer than ever before. And that is the real reason our prison population is so high. In far too many cases, the punishment simply does not fit the crime. If you're a low-level drug dealer, or you violate your parole, you owe some debt to society. You have to be held accountable and make amends. But you don't owe 20 years. You don't owe a life sentence. That's disproportionate to the price that should be paid.

But as John Pfaff, a criminal justice expert at Fordham Law School, pointed out on Twitter, the criminalization of drugs isn't the sole cause of mass incarceration. Most people in state prisons, which make up a great majority of the prison system, are violent offenders. Only in the much smaller federal system is there a huge number of drug offenders.

This distinction is critical to understanding not just why mass incarceration happened, but how to solve it. It's not just increases in prison sentences for drug offenses that caused mass incarceration — it's also increases in sentences for all crimes, which were enacted in response to historically high violent crime rates from the 1970s through early 1990s. So if America wants to wean itself off mass incarceration, it will likely need to focus on reforming the punishment for violent — not just drug — offenses at some point.

The drug war does perpetuate some violent crime by creating a black market for drugs, which helps fund the activities of violent gangs and cartels. But emphasizing the drug war and nonviolent crimes like the president did can create the perception that nonviolent offenders are the sole victims of mass incarceration when that's just not the case.

As Pfaff put it in a tweet, "There's no way prison reform benefits from Obama completely misstating causes of prison growth in a major speech."

Most people are in prison for violent offenses

About 54 percent of people in state prisons — which house more than 86 percent of the US prison population — were violent offenders in 2012, and 16 percent were drug offenders, according to the Bureau of Justice Statistics.

This number is even higher in some states. In California, for example, 89 percent of inmates in 2013 were locked up for violent crimes, following several years of reforms that reduced the number of nonviolent offenders, according to the Urban Institute:

By comparison, 7 percent of inmates in the much smaller federal prison system in 2012 were violent offenders, while 51 percent were sentenced for drug offenses, BJS found. Part of that reflects a massive increase in federal prisoners for drug offenses over the past couple of decades, as a recent Urban Institute report pointed out.

Media outlets sometimes focus on the federal prisons, since that's the policy that Congress directly guides. But while these numbers mean reforms aimed at nonviolent drug offenders would go a long way in federal prisons, similar reforms will have a limited impact on state prisons, which, again, hold a great majority of US prisoners.

Criminal justice experts and reformers are beginning to draw this distinction in order to push for what they see as the next phase of reform: reducing extremely long prison sentences for violent offenses. Some advocates, under the banner of #Cut50, are now calling for a 50-percent reduction in the incarcerated population. But to do that, some violent offenders will need to be freed, and sentences will need to be low enough to ensure future convicts can't refill prisons, as this interactive from the Marshall Project shows:

How can policymakers do this without risking public safety? One idea is to make greater use of house arrest, probation, or parole instead of incarceration, which would still keep convicts under the government's watch. Another is to simply reduce prison sentences for violent crimes — since, as research shows, people tend to age out of crime, so letting them out of prison earlier likely wouldn't increase crime rates.

But it's likely some older inmates will come out of prison and commit more crimes if they don't get any support. That's why some reformers, like Mark Kleiman of New York University's Marron Institute, have proposed a "graduated reentry" system that eases inmates from prison to the outside world through strong oversight and incentives. For example, an inmate might be allowed more luxury time or autonomy if he can obtain and sustain a job for six months. This has two big benefits for the inmate: He's doing things — like keeping a job — that will help him get a foothold in the real world, and he'll learn how the real world works after so much time locked away. (Supporters of this concept acknowledge it's new and will need changes if it's implemented, but the first step, they say, is trying it out.)

Whether states actually take up these bigger changes remains to be seen, especially as the economy recovers and bigger budgets potentially reduce interest in cutting prison costs through criminal justice reform. But it will be necessary if America wants to truly quit mass incarceration.