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THERE’S GOOD news on a subject usually associated with the social ills of the United States: incarceration. According to newly released Justice Department statistics, the prison population fell 1.4 percent in 2016, to 1,505,000. That is a decline of almost 7 percent since the prison population hit an all-time high of 1,615,500 at the end of 2009. The overall incarceration rate — the number of people in state prison or municipal jails per 100,000 population — has declined since 2009, too, and stood at 670 by the end of 2016, marking lowest rate in two decades. Perhaps most significant, the broadest indicator of the public’s involvement with the criminal-justice system, the share of the population that is either in prison or in jail, on parole or on probation, has declined substantially as well. In 2016, 1 in 38 adults were under one of these forms of “correctional supervision”; 10 years earlier, the figure was 1 out of every 31.

The usual caveats apply: The U.S. incarceration rate remains high, relative both to levels in the 1970s, before the “age of mass incarceration,” and to levels in comparable societies elsewhere in the industrial world. A disproportionate share of the recent decline in the prison population is accounted for by a single state — California — which faced a Supreme Court ruling in 2011 requiring it to reduce prison overcrowding. And racial disparities in incarceration remain, with blacks and Latinos overrepresented compared with their share of the overall population. Yet that problem, too, in less bad than it was. The Pew Research Center reported this year that the number of African Americans incarcerated fell faster than the number of whites between 2009 and 2016. The number of blacks in federal and state prisons exceeded the number of whites by 47,100 at the end of this period, down from a difference of 94,800 at the beginning of it.

The big picture is that a seemingly in­trac­table problem has stopped growing and begun to shrink. Some 35 states cut both crime and incarceration between 2008 and 2017, according to the Pew Charitable Trusts (separate from the Pew Research Center). Much credit is due to state-level efforts to provide alternative forms of accountability for nonviolent offenders, streamlining of release for parole-eligible inmates — and other reforms. President Barack Obama’s efforts to reduce the federal prison population, while perhaps mostly symbolic (the federal system accounts for a small share of all incarceration), focused public attention on the issue. The Trump administration, unfortunately, has brought back a more simplistic attitude, exemplified in its resistance, led by Attorney General Jeff Sessions, to Republican Sen. Charles E. Grassley’s (Iowa) proposals to reduce certain mandatory minimum sentences for nonviolent drug offenders.

But, as we said, the federal government has direct power over only a small minority of the correctional system. The states have the power to do more, there is more for them to do, and, as the numbers prove, they can do it.