Hillary Clinton has criticized Bernie Sanders as a candidate who's making promises he can't keep — although usually this criticism is leveled at his plans to enact a single-payer health care system and free public colleges, or tear apart Wall Street banks.

But at a New Hampshire rally on Friday, Sanders added another policy area where he's promising more than he can likely deliver: mass incarceration.

He said, while talking about high youth unemployment (emphasis mine):

So here's a promise that I make to you: Number one, at the end of my first term, we will not have more people in jail than any other country. And one of the reasons for that is that instead of having high rates of youth unemployment, we are going to have education and jobs, not jails and incarceration. I want kids to be in school. I want kids to have the education they need. I want kids to have jobs, not just hang out on street corners with the potential to get into trouble.

This is not the first time Sanders has made this claim — he's tweeted it before, and he repeated it at the Democratic debate on Thursday, February 11.

Asked about that tweet, the Sanders campaign told Mother Jones that Sanders would, in his first 100 days as president, appoint "a commission of criminal justice experts, leaders in the African American, Hispanic, and Native American communities, and others who have had success on the local level in reducing the number of young adults going to jail and in transitioning people out of prison to other settings."

This is a noble goal. It's true that America incarcerates more people than any other country in the world. And politicians on both sides of the aisle have shown interest in changing that, particularly as more and more research shows mass incarceration isn't even significantly cutting crime anymore.

But there's practically no chance Sanders could fix this all in his first term — and his claims, frankly, call into question his knowledge on the subject, despite his good intentions.

Sanders misstated the cause of mass incarceration

The big problem in Sanders's idea is the implication that there's a clean correlation between unemployment, crime, and incarceration.

Let's assume that Sanders can, somehow, bring down the youth unemployment rate to zero. It's a very dubious assumption — it's, to say the least, uncertain Sanders could pass a jobs bill at all. But for the sake of generosity, let's assume this is possible.

There is a genuine dispute over how much of an impact this would have on crime, but generally, better economies are believed to reduce crime to some extent.

But what about incarceration? This is where Sanders gets into trouble: Mass incarceration is not the sole result of higher crime rates. America's prison population exploded over the past few decades even as the crime rate plummeted. (And criminal justice experts widely believe that more incarceration played a small role in the crime reduction.)

This applies equally to youth: There's no correlation between youth unemployment, youth crime, and mass incarceration. Youth unemployment wildly fluctuated in good and bad economies since the 1970s. Meanwhile, youth crime plummeted. Yet all this time, the prison population actually grew by more than 400 percent.

So Sanders seemed to overstate the connection between youth unemployment and mass incarceration.

Sanders's comments, then, mischaracterize why incarceration rose so much over the past several decades. It wasn't that crime kept increasing, so there were more crimes to lock people up for. Instead, policymakers and law enforcement made a conscious decision to incarcerate more people even as crime fell, because they believed it was the best crime-fighting strategy at a time when crime was unusually high.

In other words, mass incarceration is a conscious policy decision largely separated from crime and the economy. So to really significantly cut incarceration, what needs to be done is to pull back on incredibly harsh prison sentences — including, by the way, for violent offenses.

If someone doesn't get this, he or she won't understand the root of mass incarceration — and will have an incredibly hard time understanding how to fix it.

Still, Sanders did say that much lower youth unemployment is just "one of the reasons" that "at the end of my first term, we will not have more people in jail than any other country." But there's reason to doubt even significant federal criminal justice reform could accomplish what Sanders promised.

Federal criminal justice reform couldn't end mass incarceration all by itself, either

The Sanders campaign said that Sanders would set up a commission to take on criminal justice reform and reduce incarceration. It's of course possible that even if he doesn't understand the root cause of mass incarceration now, this commission will, and can guide him toward good, sensible fixes.

But whatever the commission proposes, it likely would not reverse incarceration in the way Sanders is describing.

For one, Sanders, as president, would only have direct control of the federal system, which houses about 13 percent of the overall prison population. Even if Sanders pardoned every single person in federal prisons — a ludicrous idea, since it would free convicted terrorists — that would only reduce America's incarcerated population to about 2 million from around 2.2 million.

In comparison, the country with the second-largest incarcerated population, China, has about 1.7 million people in jail and prison, based on 2013 estimates. (One major caveat: There's always good reason to doubt China's numbers, since the government notoriously massages statistics to make itself look better. But these are, for now, the best numbers we have — and the numbers Sanders seems to be going off of.)

Now, Sanders could influence state incarceration policy through a federal law that, for example, gives states money for plans to cut incarceration without increasing crime.

But this would likely cut the states' jail and prison populations at the margins — since most state prisoners are violent offenders (at least 54 percent), it's unclear just how far states would be willing to go before getting into some very politically risky territory.

Also, that's not something Sanders has said he supports. The closest thing in his racial justice platform is vague promises to stop locking up nonviolent offenders — but, again, those make up a minority of prison inmates at the state level, where more than 86 percent of US prisoners are housed.

There's also the question of whether states even want to reduce incarceration much further, especially if it involves releasing violent offenders or punishing them less.

States have a self-interest in continuing the reductions: It saves money, and studies show higher incarceration is no longer effective for fighting crime. So it could be a win-win.

Yet if you look at the chart above, the reforms that states enacted in the past few years — after big political battles and to much celebration — have barely reduced the incarcerated population, and it's still historically high. And there's little to no sign most states are willing to go much further, even under pressure from a President Sanders.

And, of course, there appears to be no chance that Congress would be willing to pass a law that encourages states to go further. Congress will, after all, likely remain at least partly in Republican hands, and the current Republican Congress isn't even willing to pass a milder form of criminal justice reform today.

Appreciating the limits of federal power

More broadly, Sanders's claim is problematic because it suggests that the president has a tremendous amount of power over the national criminal justice system. Experts, particularly John Pfaff at Fordham Law School, have long worried that this type of messaging from the media and politicians may cause people to expect too much from federal officials and too little from their local and state counterparts.

A recent example is when petitions asked President Barack Obama to pardon Steven Avery, who was depicted in Making a Murderer as perhaps wrongfully convicted. But Avery was convicted of a state crime, not a federal one, so Obama doesn't have the power to pardon him — a fact literally hundreds of thousands of petition signers seemed to miss.

This is a crucial distinction. For better or worse, the bulk of the criminal justice system is driven by state policies. And that means no president and no Congress can really put an end to mass incarceration — not without state cooperation. But if people don't get that, how can they know whom to put pressure on for serious criminal justice reform?

Sanders's claim neglects that fact. Ultimately, that's why his goal to totally reverse mass incarceration just doesn't add up.

At the same time, Sanders's commitment at least places him among the politicians who genuinely want to reverse mass incarceration. For criminal justice reformers, that's certainly a welcome trend from the tough-on-crime politics of old — even if Sanders doesn't get some of the details quite right.

Watch: How mandatory minimums contributed to mass incarceration