The federal prison in Butner, North Carolina on Nov. 20, 2015. Getty

Attorney General Jeff Sessions has rescinded an Obama administration memo that directed the Bureau of Prisons to phase out the use of private correctional facilities to house federal prisoners.

In his letter rescinding the memo Thursday, Sessions said the original guidance issued by former Deputy Attorney General Sally Yates "changed long-standing policy and practice, and impaired the Bureau's ability to meet the future needs of the federal correctional system."

Under the Obama administration, the federal prison population started to shrink for the first time since Jimmy Carter was president. Yates' August 2016 memo said the Obama administration was seeking to phase out federal use of private prisons in part because they do not provide "the same level" of "correction services, programs and resources" as well as "safety and security," as federal facilities. She also said they don't provide cost savings.

President Barack Obama visits the El Reno Federal Correctional Institution outside Oklahoma City, July 16, 2015. Reuters

But most importantly, Yates cited the shrinking federal prison population as a reason why the Bureau of Prisons no longer needed to contract prison space to private companies.

Given bipartisan efforts to reduce the prison population, the largest in the world both by total and per capita size, Sessions' comments about needing private prisons to meet "future needs" would seem to run contrary to prevailing political winds. It could imply that President Donald Trump, who declared himself the "law and order" candidate in last year's presidential election, will pursue policies that will increase the federal prison population.

Before Obama accomplished the feat, the last president to preside over a decline in the federal prison population was Jimmy Carter. The federal prison population expanded by 78 percent under Ronald Reagan, 39 percent under George H.W. Bush, 56 percent under Bill Clinton and 32 percent under George W. Bush, according to Pew Research Center. Between 1980 and 2013, the federal prison population expanded from 24,640 to 219,298 before falling for the first time in nearly 40 years in 2014. The current federal prison population stands at 189,078, according to the Bureau of Prisons, with just 12 percent of that population — or 21,366 inmates — held in private prisons.

It was not clear what policies Trump could implement, or rescind, that would increase the federal prison population. The vast majority of incarcerated people in the U.S. are in local jails and prisons (the federal prison population is less than 10 percent of the 2,173,800 people incarcerated in the U.S. in 2015). So while the federal prison population could increase, it wouldn't mean that mass incarceration would expand.

Nearly half of prisoners in federal prison are there for violating drug laws, which could receive renewed enforcement under a Trump administration and result in an small uptick in the number of federal prisoners. On Thursday, White House Spokesperson Sean Spicer said he expects "greater enforcement" of federal laws covering marijuana under the Trump administration, while linking recreational marijuana use to the nation's opioid crisis.