How can such a grotesquely race-biased pattern of arrests exist? Professor Levine paints a sordid picture: young police officers are funneled into low-income black and Hispanic neighborhoods where they are encouraged to aggressively stop and frisk young men. And if you look for something, you’ll find it. So they find some of these young people with small amounts of drugs. Then these young people are arrested. The officers will get experience processing arrests and will likely get to file overtime, he says, and the police chiefs will get a measure of productivity from their officers. The young men who were arrested are simply pawns.

Professor Levine has documented an even more devious practice in New York City, where possessing a small amount of marijuana is just a civil violation (so is a speeding ticket), but having it “open to public view” is a misdemeanor.

According to a report he issued in September 2009: “Police typically discovered the marijuana by stopping and searching people, often by tricking and intimidating them into revealing it. When people then took out the marijuana and handed it over, they were arrested and charged with the crime of having marijuana ‘open to public view.’ ”

And these arrests are no minor matter. They can have very serious, lifelong consequences.

For instance, in 1998, President Bill Clinton signed a provision that made people temporarily or permanently ineligible for federal financial aid depending on how many times they had been arrested and convicted of a drug offense. The law took effect in 2000, and since 2006 lawmakers have been working to soften it. But the effect was real and devastating: the people most in need of financial aid were also being the most targeted for marijuana arrests and were therefore the most at risk of being frozen out of higher education. Remember that the next time someone starts spouting statistics comparing the number of black men in prison with the number in college.