Living in a poor neighborhood can dramatically increase someone's chances of getting arrested for drug violations — and until last year, a major federal program actually encouraged police to target these neighborhoods.

Project Know, a drug addiction resource center, compiled a series of maps showing drug arrests at the neighborhood level in eight major US cities between 2012 and 2014. Most striking is how the number of drug-related arrests correlates with a neighborhood's poverty level, as this map from Chicago shows:

While some research has found a correlation between poverty and drug use, police may also have been targeting low-income neighborhoods for easier arrests that would net them more federal funding. Neill Franklin, a retired Maryland police major who's now executive director of Law Enforcement Against Prohibition, a criminal justice group that opposes the war on drugs, previously explained to me:

One of the requirements for completing a federal grant application for funds to combat drugs was showing how many drug arrests we made. The thinking was that the more drug arrests you have, the more significant your drug problem. If you have a significant drug problem, the federal government will give you more funds. So what did we do? We had our officers go out and make as many drug arrests as they could. Where did we do that? We did that in communities of color. Yes, it was that easy. Most of the people in these impoverished communities are always in the streets. They sell on the street corner. They have no political power or capital and no financial power, so there's also very little pushback. Doing these evening and afternoon sweeps meant 20 to 30 arrests, and now you have some great numbers for your grant application.

As Franklin describes, a cop is a lot more likely to catch someone selling drugs on the streets while policing an impoverished area, where some dealers sell drugs outside, than a wealthier neighborhood, where people are more likely to buy drugs indoors from friends and peers they already know. And since more arrests would lead to more federal funds, police forces often had a financial incentive to pick up easy targets in these impoverished neighborhoods.

The federal government has taken steps to eliminate this perverse financial incentive. Last year, the Department of Justice changed a major law enforcement funding program, the Byrne Justice Assistance Grant, to no longer use number of arrests as an accountability measure. This change could give police less of a reason to focus so much on drug arrests — and maybe let them put resources in other areas.

Watch: Sen. Cory Booker (D-NJ) discusses racial disparities in the criminal justice system

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