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Decades of tough drug laws have contributed to a dramatic rise in the federal prison population, with inmates serving more time behind bars. The Obama administration recently encouraged nonviolent federal inmates — most of them drug offenders — to apply for early release. The announcement comes at a time of growing calls for drug sentencing reform, away from the severe punishment. Here are five things to know about the issue:

1. FEDERAL PRISON POPULATION ON THE RISE. The number and percentage of federal inmates serving time for drug offenses has skyrocketed over the decades. In 1980, slightly more than 20 percent of federal inmates — 4,749 of 22,037 — were serving time for drug crimes, according to The Sentencing Project, an organization that works on sentencing policies. By 2012, drug offenders represented more than half of inmates — 99,426 out of a federal prison population that had grown to 196,574.

2. MORE TIME BEHIND BARS. The "War on Drugs" with severe mandatory minimum sentences has helped increase the length of time federal inmates have served for drug offenses. In 1986, the average time was 22 months. By 2004, drug offenders were sentenced to serve nearly triple that time — 62 months, according to The Sentencing Project.

3. SHIFTING PUBLIC MOOD. An April 2014 Pew Research Center survey found that by a nearly 2-to-1 majority — 63 percent to 32 percent — those polled thought it was a good idea that some states have moved away from mandatory drug sentences for non-violent drug offenders. In 2001, opinion in the survey was about evenly divided.

4. MOMENTUM IN THE STATES. From 2009-2013, more than 30 states passed almost 50 bills involving the enforcement or definition of drug crimes, according to an April report by the Vera Institute of Justice. The changes include repealing or scaling back mandatory minimum sentences, increasing judicial discretion and adjusting penalty levels for certain drugs.

5. RACIAL DISPARITIES. For fiscal year 2013, nearly three-fourths of people sentenced for drug offenses in federal court were Hispanic or black, according to federal statistics. A 2010 law reduced the sentencing disparity between crack and powder cocaine. For decades, crack offenders — mostly black — had received far more severe punishment than those arrested with powder cocaine.