President Bernie Sanders couldn't stop mass incarceration by himself. But this one reform would be a very good start.

Most people know by now that America is the world leader in throwing its own citizens in cages. What they probably don't know is that reforming the way local jails set bail is the quickest and easiest way to begin getting rid of that grotesque trophy.

Case in point: Mass incarceration was a major flash point in last week's Democratic debate in Wisconsin, but neither candidate mentioned local jails at all. Even while Bernie Sanders was staking out an extremely ambitious goal, saying, "Here's my promise: At the end of my first term as president we will not have more people in jail than any other country" (he clearly meant prisons), he didn't mention bail reform.

On the policy merits, Sanders went too far. While his goal is by no means unattainable, the federal government simply does not have enough control over the prison system — the vast majority of which is in the hands of state and local governments — for any president to cut the incarcerated population by 600,000, which is what it would take to deliver on Sanders' promise. A presidential candidate should not make promises that are literally impossible.

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However, the sentiment of Sanders' goal is right on. What's more, in their rush to chide Sanders for being naive and irresponsible, critics badly understated the extent to which federal policy affects incarceration outside of federal prisons — particularly jails. It's a great opportunity for Sanders to clarify his message and seize on bail reform — a vastly overlooked part of the mass incarceration problem. While it probably wouldn't move the U.S. from the top spot by itself, bail reform could make an enormous difference.

First, some background. Federal prisons are the part of the prison system under the direct control of the federal government, and they contain about 196,000 prisoners (which has fallen from 210,000 only a year ago, by the way). State prisons contain about 1.4 million people, while jails have about 646,000. That makes a total of 2.3 million people incarcerated, compared to 1.7 million prisoners in China.* That yawning discrepancy leads Leon Neyfakh to argue the following:

This leaves much out. First, federal crime policy exerts a strong gravitational pull on state behavior. Federal sentencing guidelines heavily influenced the state versions; state-level lawyers, judges, and policymakers tend to look to the higher-status federal system for cues and ideas, and there is much back-and-forth staff movement. Hence, if the federal criminal justice system were to make a sharp turn against harsh punishments, it's virtually certain that would percolate through some if not most of the state systems and thus reduce the prison population over time. Federal leadership matters here.

This effect also holds for bail policy, which is the primary determinant of the size of the jail population. As I covered extensively last year, about 62 percent of the people in jail are legally innocent. A major reason why is the Bail Reform Act of 1984, which made it dramatically easier to keep people locked up before federal trials; most of the states followed suit. Today, roughly two-thirds of the people in jail are there either because they are too poor to make bail, or because they've just been arrested and will make bail in the next few days. Over the last 15 years, fully 99 percent of the growth in the jail population is due to increased incarceration of the legally innocent.

This is a human rights atrocity for many reasons, but perhaps the biggest one is that the first 48 hours or so in jail is extremely traumatic for people with no experience in the prison system. It's why the suicide rate in jails is 2.5 times greater than in actual prisons — witness Sandra Bland, an ordinary middle-class person who apparently committed suicide very soon after being thrown in jail.

Now, it would be unconstitutional for Congress to simply force states to change the way they do bail. But there are four less direct avenues to pursue: First, pursue reform for federal prisoners, to take advantage of the percolation effect mentioned above. Second, put conditions on the many grants the feds dole out for the states' criminal justice systems, requiring bail reform as a condition of getting the money. Third, pass a law declaring current use of money bail a violation of the 14th Amendment's due process protection, which Congress has power to protect. Fourth, there is a very strong case that current bail policy is a violation of the 8th Amendment, so the Department of Justice could pursue a lawsuit and attempt to get a Supreme Court ruling allowing the feds to step in. The last two of these are a bit of a long shot, but taken together this would be a powerful package.

But what would bail reform look like? There are two basic principles: First, work to make sure arrestees are processed as fast as possible — ideally within 24 hours, as many jurisdictions are moving towards. Second, very sharply reduce the use of money bail. If used, it should never be beyond a person's ability to pay. No person should ever rot in jail waiting for a trial because he can't scrounge up the cash to make bail — poverty should not be a crime. Besides, research from the Vera Institute of Justice shows that bail is largely worthless for making sure that accused criminals show up to trial. In most cases, it simply isn't needed — basic pretrial supervision works much better.

There is tremendous churn in and out of the jail system — 11.4 million people were admitted in 2014. Bail reform would thus be more about diverting the flow of prisoners rather than releasing lots of long-term ones. A new federal law mandating speedy processing of arrestees, and sharply restricting the use of money-bail, would erode the jail population from two directions at once. It could be combined with incentives to use alternatives to arrest, like citation-and-release or pre-booking diversion, to further slow the rate of jail entry. At a very rough guess, such a reform done well could knock about a third — perhaps 200,000 people — off the jail population.

At any rate, even very aggressive bail reform wouldn't get us to the Chinese figure of 1.66 million prisoners quoted above, and it would require congressional action. But bail reform would be a gigantic step in the right direction. When it comes to fighting mass incarceration, it's the easiest and most obvious first step.

*Interestingly, the Chinese number excludes pre-trial detainees, while the U.S. one does not. It also does not include "administrative detainees," or political prisoners. According to the Institute for Criminal Policy Research, there has been no official release of the number of such prisoners since 2009, but if the figures have stayed steady over time, that would add some 650,000 to the Chinese total, bringing it about even with the current U.S. number. Though Sanders surely didn't mean it this way, the easy way to slip into the number 2 spot would very likely be to convince China to release a survey of its detainees.