Austan Goolsbee writes (no permalink yet) of Matthew Gentzkow and Jesse M. Shapiro, and their paper “What Drives Media Slant? Evidence From U.S. Daily Newspapers“. The non-gated version is here.
In essence the authors measure how much newspapers use key partisan phrases like "personal accounts" for social security privatization and compare the vocabularies of those newspapers to the vocabularies of partisan politicians. The political slant of newspapers is then matched to campaign contributions in the zip codes those newspapers serve. Shapiro sums up the result:
The data suggest that newspapers are targeting their political slant
to their customers’ demand and choosing the amount of slant that will
maximize their sales.
It also turns out that the political views of the paper’s owner have no effect on the slant of the paper.