Media matters

Economics of Contempt makes the obvious, common sense argument for why liberal media bias almost has to exist:

[N]eurological studies have shown that people's feelings toward a political party dramaticallyThe Political Brain, Drew Weston described a great study he and two colleagues conducted during the 2004 election. Westen and his colleagues studied the brains of 15 self-identified Democrats and 15 self-identified Republicans as they were presented with a series of slides that showed undeniably inconsistent statements by John Kerry. The partisans were asked to consider whether Kerry's two statements were inconsistent, and were then asked to rate the extent to which Kerry's statements were contradictory, from 1 (strongly disagree) to 4 (strongly agree). They then repeated the process with undeniably inconsistent statements by George W. Bush, and again with inconsistent statements by politically neutral males. Here's how Westen described the results:
They had no trouble seeing the contradictions for the opposition candidate, rating his inconsistencies close to 4 on the four-point rating scale. For their own candidate, however, ratings averaged closer to 2, indicating minimal contradiction. Democrats responded to Kerry as Republicans responded to Bush. And as predicted, Democrats and Republicans showed no differences in their response to contradictions for the politically neutral figures.
The results showed that when partisans face threatening information, not only are they likely to "reason" to emotionally biased conclusions, but we can trace their neutral footprints as they do it.

When confronted with potentially troubling political information, a network of neurons becomes active that produces distress. ... The brain registers the conflict between the data and desire and begins to search for ways to turn off the spigot of unpleasant emotion. We know that the brain largely succeeded in this effort, as partisans mostly denied that they had perceived any conflict between their candidate's words and deeds.

Not only did the brain manage to shut down distress through faulty reasoning, but it did so quickly -- as best we could tell, usually before subjects even made it to the third slide [which asked them to consider whether the statements were inconsistent]. The neural circuits charged with regulation of emotional states seemed to recruit beliefs that eliminated the distress and conflict partisans had experienced when they confronted unpleasant realities. And this all seemed to happen with little involvement of the circuits normally involved in reasoning.

But the political brain also did something we didn't predict. Once partisans had found a way to reason to false conclusions, not only did neural circuits involved in negative emotions turn off, but circuits involved in positive emotions turned on. The partisan brain didn't seem satisfied in just feeling better. It worked overtime to feel good, activating reward circuits that give partisans a jolt of positive reinforcement for their biased reasoning.
This is basically the root of the well-known "confirmation bias."

So given that (1) journalists are overwhelmingly Democrats, and (2) party affiliation dramatically affects the way our brains interpret political news, is it really possible that there isn't a liberal media bias?

No empirical study of newspaper stories or talking heads on TV is ever going to be able to objectively determine whether there's a liberal media bias, because what people think constitutes "liberal bias" depends on their party affiliation also. I don't perceive a liberal media bias, but then again, I'm a Democrat, so my brain would presumably interpret political news the same way a biased liberal media would.

But if we know that the inputs are heavily biased, it's very likely that the output is biased as well.

affect the way their brains interpret political news. In his book
One of the more interesting results of current neuropsychological research is that some scientists think that, at least for hot-button issues, we reason backwards:  we decide what we believe based on our emotional needs, and then figure out a reason that we should believe it.  Regardless, I think EofC makes an excellent point:  based on what we know about journalists and political cognition, it's very unlikely that there isn't substantial bias in both academia and the media.  I also think there's no way to develop any direct test of media bias that will satisfy people who want to disbelieve in it.