The police reaction to the protests in Ferguson, MO, underscores a basic truth about the criminal justice system in America: it's racist. As, in fact, does the broader history of protest in America. It turns out that, for decades, protests with black participants were more likely to meet with a police presence. And at events the police do show up for, they're more likely to arrest or beat protesters when at least some of the protesters are black.
These findings come from a 2011 paper coauthored by professors at Notre Dame, Stanford, and the University of Wisconsin. The professors — Christian Davenport, Sarah Soule, and David Armstrong — took a database of 15,000 American protests from 1960 to 1990 and used newspaper reports to identify which protests had at least some black protesters. After controlling for some confounding variables, they compared those protests to ones with no black protesters.
The first major finding is that police were more like to show up when black people were in the crowd. This chart shows how that looks over time:
Then, the researchers looked only at protests where police showed up, comparing policed protests with some black protesters with those where were none. Davenport, Soule, and Armstrong found that "once police are present, they are more likely to make arrests, use force and violence, and use force and violence in combination with arrests at African American protest events."
For a more thorough explanation of the paper's methodology and findings, you should definitely read The Monkey Cage's Kim Yi Dionne, who first wrote up the study. One important addendum she adds: "It's important to note that the study also shows that the effect — what the authors call 'Protesting While Black' — has varied over time. Their study suggested that the impact of 'Protesting While Black' was historically bounded to the earlier period, prior to the enactment of civil rights legislation." For more, read the whole thing.