evidence: If you want to know why Iowa imprisons a larger share of its black residents than almost any other state, mandatory minimum sentencing laws are one place to start, critics say. [...] During a four-decade period, 42 percent of Iowa inmates serving prison time for robbery were black, state data show. [...] In 2007, Iowa had one of the highest rates of incarceration of blacks in the nation, a study by The Sentencing Project showed. Blacks in Iowa were imprisoned at a rate 13.6 times greater than whites, double the national average, the study found.