Yeah... fascinating study.
And it's the fault of both parties, Democrats and Republicans.
Along these lines, a highly recommended interview with Matt Taibii on why Wall Street executives can launder $850 million dollars for drug cartels and just pay a fine (no jail time), but a black man caught with a couple grams of marijuana (less than $30 worth) goes to jail.
http://www.democracynow.org/2014/4/15/who_goes_to_jail_matt_taibbi