Generally when people consider media bias, they point to reporters who slant the news in a certain way, or newsrooms that face political pressures from on high, or consumers who demand news that confirms their worldview. Much-trodden terrain, sure. But a recent study (pdf) by Matthew Gentzkow and Jesse M. Shapiro gets at this from a different direction, arguing that bias could emerge as the natural result of media firms who try to build a reputation for truthful, accurate reporting. That is, bias can arise even when: a) consumers care only about the truth, b) media firms care only about maximizing profits, and c) eliminating bias would make everyone better off. Here's the main mechanism:

Our first set of results shows that firms will tend to distort information to make it conform with consumers' prior beliefs. To see why, consider that a noisy or inaccurate signal is more likely to produce reports that contradict the truth. An agent who has a strong prior belief about the true state of the world will therefore expect inaccurate information sources to contradict that belief more often than accurate ones. Suppose, for example, that a newspaper reports that scientists have successfully produced cold fusion. If a consumer believes this to be highly unlikely a priori, she will rationally infer that the paper probably has poor information or exercised poor judgment in interpreting the available evidence. A media firm concerned about its reputation for accuracy will therefore be reluctant to report evidence at odds with consumers' priors, even if they believe the evidence to be true. The more priors favor a given position, the less likely the firm becomes to print a story contradicting that position.

In the real world, of course, bias does result from slanted reporters or political pressure from on high (David Schuster's tell-all about his time at Fox offers one example), or canny politicians gaming the "he-said she-said" system. But I take it the point here is that even if you removed all these factors and assumed benign motives, bias could still occur. On Gentzkow and Shapiro's model, more competition will cut away at this problem, since media firms who care about truthfulness will worry about competitors exposing their distortions. That I doubt. The world of blogs is an extremely competitive, even if imperfectly, and yet distortions and bias happen all the time. Even—*gasp*—on the site you're currently reading...

Perhaps that's because readers aren't only looking for accuracy when they read blogs (they're also looking for good writing or humor or passion or interesting thoughts or whatever), so bloggers can get away with distortions and biases that media firms can't. Or perhaps media consumers in general just don't value the truth all that much, and instead want their worldviews confirmed, or enjoy the aesthetic properties of bullshit, or whatever, in which case no amount of media competition will ever eliminate bias. That seems extremely likely.