I have a new review article in Perspectives on Politics critiquing Tim Groseclose’s book Left Turn that started as a blog post here way back in 2005 (gated; ungated). Here’s how it begins:

In 2005, University of California-Los Angeles political scientist Tim Groseclose and University of Missouri economist Jeff Milyo published a study in the Quarterly Journal of Economics (QJE) claiming to provide quantitative evidence of what they call “strong liberal bias” in the media. Their estimates place 18 of the 20 national news outlets to the left of the centrist US voter. Not surprisingly, this claim has received a tremendous amount of media attention, particularly after Groseclose published a book based on the QJE results titled Left Turn: How Liberal Media Bias Distorts the American Mind and made appearances on “The O’Reilly Factor” and other news programs.

Few scholars of the political media would deny that media organizations tend to have different slants on the news. These differences are often significant and appear to be driven in large part by economic factors such as consumer demand and media competition. It is also true, as Groseclose and Milyo correctly note, that most journalists in the United States tend to be liberal and to vote Democratic.

However, these two facts do not necessarily imply that American media outlets have an overwhelming liberal bias. The policy preferences of reporters are only one of many possible influences on the content of the news. Numerous other competing journalistic norms and practices exist that limit the extent to which reporters’ personal views influence their reporting. As a result, previous studies of partisan bias in reporting on presidential elections have generally not found consistent results. So why did Groseclose and Milyo (hereafter GM) reach such different conclusions? A closer examination of their method reveals that their estimates of media bias—and Groseclose’s extensive extrapolations from those findings in Left Turn—rely on questionable assumptions about the processes generating citations of think tanks and interest groups by reporters and members of Congress, respectively.

GM’s model is built on the assumption that the advocacy process in which members of Congress cite think tanks and interest groups in floor speeches somehow parallels the journalistic process by which reporters cite those groups in their reporting. This assumption is the basis for their mapping of media outlets onto a comparable ideological scale as members of Congress and the public (refer to their QJE article for technical details). If the press is unbiased, GM suggest, media outlets will cite think tanks in news reporting in a fashion that is “balanced” with respect to the scores assigned to the groups based on Congressional citations, which were measured during the 1993–2002 period. Any deviation from their definition of the political center (a composite based on a weighted average of House and Senate adjusted ADA scores) is thus framed by GM as bias.

Many objections can be raised to GM’s methodology, the significant extrapolations that Groseclose makes from those findings in Left Turn, and the ungenerous tone of his responses to his critics (whom he repeatedly dismisses as “left-wing bloggers”). In this contribution, however, I will focus on GM’s identifying assumption that the processes generating journalistic and Congressional citations to the think tanks and interest groups in their sample are identical. Specifically, I show how three plausible deviations from this assumption provide alternative explanations for GM’s finding that the media are overwhelmingly liberal.

Read the whole thing for more.