I am a bit mystified over the growing literature on "media bias", but KPC friend Jim Synder and his co-author Riccardo Puglisi have a new NBER working paper (ungated version here) which uses data on newspaper coverage of recent political scandals to show that it's the partisanship of the newspaper rather than the partisanship of the readers that drives which scandals are covered more intensively.
Here, let them tell you:
We analyze the coverage of U.S. political scandals by U.S. newspapers during the past decade. Using automatic keyword-based searches we collected data on 35 scandals and approximately 200 newspapers. We find that Democratic-leaning newspapers (i.e., those with a higher propensity to endorse Democratic candidates in elections) give relatively more coverage to scandals involving Republican politicians than scandals involving Democratic politicians, while Republican-leaning newspapers tend to do the opposite. This is true even when controlling for the average partisan leanings of readers.
So they are basically saying media bias is a supply side story, not a demand side story, which is lovely and believable, but to me the big question left hanging here is so what? Why should we care that media outlets have a political agenda?