Butler, Daniel M. and Emily Schofield (2010) “Were Newspapers More Interested in pro-Obama Letters to the Editor in 2008? Evidence from a Field Experiment.” American Politics Research 38(2): 356-371.
During the 2008 presidential election, the authors submitted letters to the editor at 100 major U.S. newspapers as part of a field experiment to test whether interest in the letter depended on which candidate the letter supported. The authors find, contrary to what charges of a liberal media bias would suggest, that newspapers expressed more interest in pro-McCain letters than pro-Obama letters. Furthermore, it was found that papers were most likely to be interested in letters supporting the candidate they did not endorse, a result that is consistent with the idea that editors seem to be using their gatekeeping powers to allow dissenting opinions to be heard.