The federal prison population has declined thanks to a reduction in both the number of federal prosecutions and the severity of sentences for drug-related crime in recent years, according to the Judiciary, the U.S. Sentencing Commission (USSC), and the Bureau of Prisons (BOP).
The BOP reports an almost 14% reduction in federal prison population, from about 219,300 inmates in 2013 to 188,800 in April 2017. Consequently, overcrowding at BOP facilities has dropped from 37% overcapacity to 13%.
In 2011, the USSC implemented lower penalties for crack cocaine in accordance with the Fair Sentencing Act, and then cut the length of sentences again in 2014 for all drug-trafficking offenses. Nearly a third of all criminal filings in federal court are for drug crimes, so retroactive changes in sentencing guidelines meant that a significant number of prisoners who had already served their time under the new guidelines were released.
Federal judges across the United States are required to review the federal sentencing guidelines each time they contemplate sentences in a case.
The decline in the federal prison population has also been attributed to recent Supreme Court rulings that found that one of the definitions for violent felony under the Armed Career Criminal Act was unconstitutionally vague and later made that ruling retroactive.
In May, 81% of federal inmates were in BOP custody, while 11% were in privately managed facilities and 8% were in other types of facilities.
The Department of Justice has expressed interest in ending the federal government’s reliance on private prisons, and the reduced prison population will likely make it easier to shift inmates from private prisons to the federal government’s existing facilities.
 United States Courts, Policy Shifts Reduce Federal Prison Population, Apr. 25, 2017.
 United States Sentencing Commission, Map.
 See Johnson v. U.S., ___ U.S. ___, 135 S.Ct. 2551 (2015); United States Courts, Policy Shifts Reduce Federal Prison Population.
 See Welch v. U.S., ___ U.S. ___, 136 S.Ct. 1257 (2016); United States Courts, Policy Shifts Reduce Federal Prison Population.
 Federal Bureau of Prisons, Statistics, May 4, 2017.
 Sanburn, The U.S. Is Ending Private Prisons for Federal Inmates. So Where Will the Prisoners Go?, Time, Aug. 18, 2016.