Loyola University law professor Justin Levitt tried to quantify the epidemic of voter ID fraud that’s forcing so many states to pass restrictive voter ID laws. He looked for not only cases where someone was convicted, but tracked “any specific, credible allegation that someone may have pretended to be someone else at the polls, in any way that an ID law could fix.”

Out of roughly a billion votes cast, he found 31 credible cases of voter ID fraud. And that is, he thinks, an overestimate. At the same time, thousands of people really are being turned away from polling places because they don’t have the right ID. So voter ID laws fix a fake problem by creating a very real one.

Which isn’t to say voter fraud isn’t real. Last night on MSNBC, Levitt explained why the voter ID laws don’t do anything to address the main kinds of voter fraud: