Ideology in the news media.

• Author: Gentzkow, Matthew
• Position: Research Summaries

In many traditional models of politics, such as the pioneering work of Anthony Downs, voters lack private incentives to become informed. (1) The news media therefore play a crucial role in any democracy, amortizing the costs of gathering and filtering news across many citizens, lowering the costs of acquiring political information, and strengthening private incentives to become informed.

Democracy might function poorly without the news media, but the special role of the media in providing information relevant to voting and other political decisions also endows it with significant power to shape how events are perceived. Anecdotal evidence suggests that the news media do, indeed, exercise significant discretion in how they present events. Consider, for example, the following three reports on a December 2, 2003 battle in the Iraqi city of Samarra: (2)

Fox News: "In one of the deadliest reported firefights in Iraq since the fall of Saddam Hussein's regime, U.S. forces killed at least 54 Iraqis and captured eight others while fending off simultaneous convoy ambushes."

New York Times: "American commanders vowed Monday that the killing of as many as 54 insurgents in this central Iraqi town would serve as a lesson to those fighting the United States, but Iraqis disputed the death toll and said anger against America would only rise."

Al Jazeera: "The U.S. military has vowed to continue aggressive tactics after saying it killed 54 Iraqis following an ambush, but commanders admitted they had no proof to back up their claims. The only corpses at Samarra's hospital were those of civilians, including two elderly Iranian visitors and a child."

These accounts are based on the same facts. But through selective omission, choice of words, and varying credibility ascribed to the primary source, they convey very different impressions of what transpired.

What drives the variation we see in how a given event is presented by different news outlets? Does the diversity of perspectives reflected in the quotes above strengthen democracy, or undermine it? We have explored these questions in a series of theoretical and empirical papers about the role of ideology in the news media.

Theories of Media Bias

The model of media bias we develop was motivated by three empirical observations. (3) First, consumers tend to choose news outlets whose slant agrees with their own political beliefs. Second, they do not perceive themselves to be trading off quality in doing so, but rather judge these like-minded outlets to be more accurate and trustworthy than those they disagree with. Third, media firms appear to cater to this demand for like-minded news.

Our model begins with the observation that a Bayesian consumer, who is uncertain about the quality of an information source, will infer that the source is of higher...