There is a growing literature on media bias and, more generally, on how does the so called  marketplace of ideas really work. By far, this is not a new topic but it is only recently that mainstream economists became interested in the topic. Matthew Gentzkow and Jesse M. Shapiro make a very clear and comprehensive introduction to this literature in their article: “Competition and Truth in the Market for News” (Journal of Economic Perspectives 22(2), Spring 2008). The piece has a number of interesting quotations:

[T]he best test of truth is the power of the thought to get itself accepted in the competition of the market . . . That at least is the theory of our Constitution. — Oliver Wendell Holmes, Abrams v. United States (250 U.S. 616 [1919])

The press was protected so that it could bare the secrets of government and inform the people. Only a free and unrestrained press can effectively expose deception in government. — Hugo Black, New York Times Co. v. United States (403 U.S. 713 [1971])

Let [Truth] and Falsehood grapple; who ever knew Truth put to the worse, in a free and open encounter? — John Milton, (1644 [2006]), The Areopagitica

So long as popular truth is one-sided, it is more desirable than otherwise that unpopular truth should have one-sided assertors too; such being usually the most energetic, and the most likely to compel reluctant attention to the fragment of wisdom which they proclaim as if it were the whole. — John Stuart Mill, (1859 [2006]), On Liberty

It is so difficult to draw a clear line of separation between the abuse and the wholesome use of the press, that as yet we have found it better to trust the public judgment, rather than the magistrate, with the discrimination between truth and falsehood. And hitherto the public judgment has performed that office with wonderful correctness. — Thomas Jefferson (1803 [2007])

No one on the White House staff is to see anybody from the Washington Post or return any calls to them… Just treat the Post absolutely coldly… I want a policy in which the Washington Star, the Washington Daily News… and others who may be competitive with the New York Times and Washington Post continue to receive special treatment. — memo from President Nixon (quoted in Graham, 1997, p. 478)

More related papers:

• Besley, Timothy, and Andrea Prat. 2006. “Handcuffs for the Grabbing Hand? Media Capture and Government Accountability.” American Economic Review, 96(3), 720–36.
• Gentzkow, Matthew, and Jesse M. Shapiro. 2006. “Media Bias and Reputation.” Journal of Political Economy, 114(2): 280–316.
• Gentzkow, Matthew, and Jesse M. Shapiro. 2006. “What Drives Media Slant? Evidence from U.S. Daily Newspapers.” National Bureau of Economic Research Working Paper 12707.
• Mullainathan, Sendhil, and Andrei Shleifer. 2005. “The Market for News.” American Economic Review, 95(4): 1031–53.