The federal inmate population fell to to 184,000 by the end of 2017. (iStock)

When states began shrinking their prison populations almost a decade ago, the federal prison system was still growing each year and thereby undermining progress in reducing mass incarceration. But in the past four years, the federal system has cut its inmate population by one-sixth, a decrease of over 35,000 prisoners.

Because criminal justice is mainly the province of the states, the federal prison system holds only about 13 percent of U.S. inmates. Yet that is still a significant number of people in absolute terms: The system held 219,300 inmates at its peak in 2013. Four subsequent years of significant contraction dropped the federal inmate population to 184,000 by the end of 2017.

Obama-era changes to drug crime prosecution and sentencing coupled with a historic level of clemency grants to federal inmates by President Barack Obama helped bring the federal prison system to its lowest population size since mid-2004 and its lowest incarceration rate (i.e., adjusted for population) since the end of 2002.

Given President Trump’s penchant for “tough on crime” rhetoric, some observers may find it surprising that the federal prison population kept dropping under the first year of the Trump administration. The most likely cause is also the most obvious. When a nation is blessed with two decades of falling crime rates, this eventually translates into lower incarceration rates because there just aren’t as many offenders to arrest, charge and imprison.

Whether the federal prison population continues to decline will depend in part on Trump administration policies. Attorney General Jeff Sessions recently reversed the Obama-era policy of avoiding mandatory minimum sentences in low-level drug cases, which could result in some future growth in the federal inmate population even if crime continues to fall.

The other key determinant of the federal prison population’s future is whether Trump will make use of his powers to pardon or commute the sentences of federal inmates. He only did so for one inmate this year, but that doesn’t necessarily mean he won’t grant more clemencies later.

Beginning around the mid- 1970s when people suggested (mostly in jest) that Jimmy Carter should pardon Gerald Ford for pardoning Richard Nixon, presidential pardons have become more controversial, and presidents of both parties have therefore usually avoided them early in their term of office. Trump may, like his predecessors, become more inclined toward clemency grants in the coming years. If so, many federal prisoners will be hoping his mercy extends to people who haven’t worked for him.