The first, which I have covered previously, found that most US media has a significant liberal bias. It basically compared media’s references to thinktanks to references made by Members of Congress.
The second one looks at how often media use phrases used by Democratic and Republican politicians such as “estate tax” vs “death tax”. They find that the average newspapers language is similar to that of a left-of-center member of Congress.
In terms of why, they find media respond strongly to consumer preferences. By contrast the identity of a newspapers owner explains far less of the variation in slant, and also little evidence that media conglomerates homogenize news to minimize fixed costs in the production of content. So the concerns about owners of media dictating content seem unfounded.
Finally the third study looks at whether newspapers do influence voting behaviour and opinion. They signed random individuals up to the Washington Post (liberal) and Washington Times (conservative), plus a control group. Those receiving the Post were 8% more likely to vote for the Democratic candidate.