Everyone thinks the media is biased -- too liberal if you are a conservative, and vice versa. This is because a neutral presentation of events is impossible -- a slant is always built into what is reported what is left out, what is emphasized, what is glossed over, and so on. So measuring bias against some measure of objectivity is als impossible, because no one can decide what objectivity would be what it would have looked like -- this is the kind of thing Derrida's philosophy was out to demonstate -- the lack of a stable transcendent objectivity, a Reality with a capital R that can be used to stabilize meanings. He (and Lacan, et al) argued that meaning was always in flux, being determined by establishing provisional relationships between slippery, ungrounded things -- aligning signifiers into sequence with the chain of them itself allowing them to mean anything at all. One can always find the dubious propositions that must be accepted as Reality without proof in order to ground any allegedly objective account of things and show that they are not beyond question at all -- that's the essence of deconstructive criticism.
So the idea that media bias might be assessed without recourse to objectivity is welcome. In this Slate piece media critic Jack Shafer summarizes the argument recently propounded by University of Chicago economists University of Chicago scholars Matthew Gentzkow and Jesse M. Shapiro.
"1) If a media outlet cares about its reputation for accuracy, it will be reluctant to report anything that counters the audiences' existing beliefs because such stories will tend to erode the company's standing. Newspapers and news programs have a visible incentive to 'distort information to make it conform with consumers' prior beliefs.'
2) The media can't satisfy their audiences by merely reporting what their audience wants to hear. If alternative sources of information prove that a news organization has distorted the news, the organization will suffer a loss of reputation, and hence of profit. The authors predict more bias in stories where the outcomes aren't realized for some time (foreign war reporting, for example) and less bias where the outcomes are immediately apparent (a weather forecast or a sports score). Indeed, almost nobody accuses the New York Times or Fox News Channel of slanting their weather reports.
3) Less bias occurs when competition produces a healthy tension between a news organization's desire to conform to audience expectations and maintaining its reputation."
This economic approach starts from the basic principle that information is a commodity whose price (its truth value) varies by the degree to which it is demanded. Demand is a matter of pre-exisitng audience expectation, and truth a matter of conforming to those expectations. Bias is an expression of the media consumer not getting what he's expecting when he's paying in time, money or attention. News is commercialized; there is no news outside of that fact -- no ideal, Real news that precedes its commercialization and dissemination. (Maybe in the Habermasian dream of the informed public sphere there was noncommercialized news; but it seems as though the concept developed with the development of a commercial, profit-driven media.)
I doubt proposition 3 holds, though, that competition corrects the bias of pleasing existing audiences. Reputation for objectivity is insignificant, for the reasons alrady discussed. Reputation is a matter of maintaining a certain ideological perspoective and showing the flexibility to fit facts to that framework.