© AP/Kiichiro Sato
Both in raw numbers and by percentage of the population, the United States has the most prisoners of any developed country in the world - and it has the largest total prison population of any nation. That didn't change in 2013. After several years in which the prison population dropped slightly, the raw number of inmates in United States custody went up again in 2013.

More than 1.57 million inmates sat behind bars in federal, state, and county prisons and jails around the country as of December 31, 2013. In the federal prisons, more than half of those sentenced to a stints of a year or longer are still there for drug crimes. In states including Louisiana, Mississippi, Oklahoma, Texas, Alabama, Arkansas, Arizona, and Georgia, at least 1 percent of male residents were in prison on December 31. And across the country, racial disparities persist. Black men are six times more likely than white men to be in prison. Hispanic men are 2.4 times more likely, according to a Sentencing Project analysis of the data.

This doesn't pain the full picture of the U.S. incarceration system. Many have estimated the total number of U.S. incarceration to be more than 2.4 million. This is in part because another estimated 12 million individuals cycle through the county jail systems in a given year for periods of less than a year, and are therefore not factored into a snapshot on December 31. There are also other mechanisms of incarceration not factored into this figure, including immigration detention, civil commitment, and Indian Country facilities, according to a Prison Policy Initiative briefing.

And a vastly greater number of Americans - 1 in 31 according to 2009 Pew figures - are under U.S. corrections custody either through parole, probation, or incarceration. One in three Americans have a criminal record, according to recent FBI estimates.

But it's not all bad news. In just the federal prisons, the population actually dropped for the first time since 1980. Some experts attributed this to decreased priority on marijuana arrests, as states move toward decriminalization or legalization, and federal authorities shift their resources elsewhere.

The federal prisons are where drug offenders are serving many of the most onerous sentences for drug offenses carrying mandatory minimum sentences. And despite the first decrease in more than two decades, its population remains expansive.

"These figures challenge premature and overly optimistic forecasts of the end of mass incarceration," said Marc Mauer, Executive Director of The Sentencing Project.

Just this week, a Congressional Budget Office analysis found that passing a bipartisan bill in Congress to reform mandatory minimum sentences, the Smarter Sentencing Act, would reduce prison costs by $4 billion in just the first decade. The Justice Department projected savings of at least an additional $7.8 billion in the second decade. The Smarter Sentencing Act would roll back required sentences for drug offenses that start at 5 or 10 years in jail and ratchet up quickly from there. It would instead allow judges to use their judgment to set an appropriate sentence, as well as retroactively eliminate an antiquated and racist disparity between sentences for crack cocaine and powder cocaine offenses. And some recent changes coming from the executive branch this year to attempt stopgap sentencing reform were too recent to have affected the 2013 population.

At the state level, populations saw a drop over the past few years in large part due to court-ordered population reductions in California, where the U.S. Supreme Court deemed prisons so overcrowded that they constituted cruel and unusual punishment under the Eighth Amendment. Now that California's reductions have leveled off, the state population went up again in 2013, despite reforms in some states that are widely considered models for the federal prisons. Even some red states cited by U.S. Attorney General Eric Holder as having been leaders in some areas of criminal justice reform, such as Texas, saw slight increases in their prison populations.

"The existing reforms can only take us so far," University of California, Berkeley, criminal justice scholar Steven Raphael told the New York Times. University of Missouri - St. Louis criminologist Richard Rosenfeld noted that just thwarting the exponential increase in the population was an achievement, particularly at the federal level. Prior to this year, the federal prison population had spiked more than 790 percent since 1980.

Local jails, which typically house inmates arrested but not yet sentenced and those sentenced for less than a year, also saw their daily population rise this year.

The proportion of inmates held in private prisons actually decreased 3 percent in 2013, after a few states ended their relationships with private prisons entirely. Private prisons now house some 8 percent of the U.S. prison population. The industry, however, has more than made up for its loss in the prison industry with its share of federal immigration detention, and its entry into other criminal justice industries like rehabilitation.