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Keith Humphreys is a professor of psychiatry at Stanford University.

Congress failed this year (again) to pass criminal-justice reform legislation, despite strong bipartisan interest in curbing mass incarceration. Nevertheless, a Bureau of Justice Statistics report released Thursday shows that the U.S. imprisonment rate dropped for the seventh straight year in 2015, reaching its lowest level since 1997. How is the country sustaining progress on de-incarceration, congressional paralysis notwithstanding?

The answer is twofold.

First, states, rather than the federal government, control most of U.S. correctional policy. California, for example, has reduced its prison population by tens of thousands in recent years through legislative reforms and ballot initiatives. Many other states have also implemented reforms that cut prison populations — not incidentally while reducing crime, as well — including Alaska, Colorado, Kentucky, Maryland and South Carolina. Because state facilities house more than 85 percent of U.S. prison inmates, states can roll back mass incarceration even when Congress is dithering over parallel federal reforms.

Second, as in many other public policy areas, the Obama administration responded to congressional gridlock on criminal-justice reform by taking executive action instead. Most notably, President Obama has granted clemency to more than 1,300 federal prisoners. His two attorneys general (Eric Holder and Loretta Lynch) have also played a role, for example by pushing the Justice Department to pursue mandatory minimum sentences more sparingly and to allow compassionate release of aging prisoners more often. Holder also urged the U.S. Sentencing Commission to make Congress’s one recent reform success — the 2010 Fair Sentencing Act, which reduced crack-cocaine penalties — apply retroactively to thousands of federal inmates.

Because the federal prison system is a small part of the U.S. correctional system, even all these Obama administration actions accounted for less of 2015’s drop in incarceration than did state-level reforms. But by becoming the first president in at least 40 years to leave office with a smaller federal prison system than he started with, Obama can deservedly claim to have finally gotten the federal government on the de-incarceration path blazed by many governors and state legislators across the country. Together they have brought the country to a generational low in imprisonment, despite the ongoing inability of Congress to get off the dime.

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Poor white kids are less likely to go to prison than rich black kids

The states that spend more money on prisoners than college students

How mass incarceration is spreading to rural counties and the suburbs