A report by Dan Kopf of the website Priceonomics shows mixed results, with incarceration actually rising rather than declining in almost half the states. More than half the total decline in prison population in the US was the result of California’s response to Brown v Plata, the 2011 court case that ruled conditions in California prisons to be unconstitutional and forced a release program.The report quotes John Pfaff of Fordham Law:

“While federal reforms—such as the Fair Sentencing Act of 2010, which decreased the punitiveness of some drug sentences and the elimination of private federal prisons—receive a great deal of media attention, Pfaff argues that they have a limited impact on incarceration. Of the nearly 1.6 million Americans incarcerated in prisons, only about 200,000 are in federal prisons. “If we freed every single federal prisoner today,” says Pfaff, “we would still have the highest incarceration rate in the world.”

Pfaff’s research on the causes of mass incarceration shows that decisions made by local prosecutors and police are the most important drivers of the increase in country prison population. Specifically, prosecutors are much more aggressive in charging individuals with felonies today than they were thirty years ago—even for the same crime.

“I feel like the most important way to reduce incarceration would be to restrict the ability of prosecutors to file felony charges,” Pfaff said. “We need legally binding charging and plea bargaining guidelines like the ones judges operate under.”