But having passed the First Step Act, lawmakers should be cautious about the next step. Progressives believe that the U.S. overincarcerates and needs to cut back sharply on the number of people in jails and prisons. But the "mass incarceration" claim doesn't withstand much scrutiny.

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What about overpunishment? Are penalties so harsh that American prisons are filled with men and women who are languishing there for decades because of some youthful indiscretion? That is the image the disincarceration movement wants to conjure up. But it is false.

First, 23% of felons convicted of violent crimes are sentenced to no incarceration whatever. Second, nearly 80% of state prison inmates are released before serving their full terms. If we measure punishment by actual time served, the picture is not disturbing. Some might argue that America has an underpunishment problem.

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The risk in the second step of criminal-justice reform is that it goes too far. The justice system, flawed though it is, provides incentives to desist from crime. The weaker it gets, the greater the risk of a significant crime increase. That happened in the late 1960s: When the crime tsunami began, the system caved in. Police arrested fewer offenders, and courts imposed fewer and lighter punishments. That contributed to the 20th century's worst sustained violent-crime wave.