The latest stats from the Federal Bureau of Prisons (BOP) shows drug arrests are the main source of incarcerations across the nation--a proportion that has been rising steadily over the past four decades. More than 50 percent of those in prisons are there because of drug offenses, of which 28 percent were arrested for marijuana-related charges. The BOP doesn't include include racial demographics in the most recent stats, but according to the Drug Policy Alliance 61 percent of those in prison for drug offenses are black or Latino, and the ACLU reports that black people are 3.7 times more likely to be arrested for marijuana related offenses than white people (even though both groups have similar drug usage rates).
Notably, the second largest source of prison populations is immigration. Combined, more than 60 percent of those currently in prison are there for nonviolent crimes.