For Republican presidential contenders challenged by the media, the go-to answer has become a claim of victimhood: You are biased against us. As Marco Rubio put it at the CNBC debate last month, "The Democrats have the ultimate super PAC. It’s called the mainstream media." Are media outlets really biased against Republican candidates? One of the most careful studies, by Matthew Gentzkow and Jesse Shapiro of the University of Chicago, doesn’t find much evidence of that. Its central conclusion is that readers...