In nearly every campaign and policy debate, at least one of the sides (and often both) make the claim that the news media is biased toward the other side. As previously discussed here, some political scientists measure media bias by comparing the citations of think tanks and advocacy groups in different media outlets with mentions of the same organizations in Congress. The idea is that reporters and legislators reveal their ideology by citing liberal or conservative groups.
In my new book, The Not-So-Special Interests: Interest Groups, Public Representation, and American Governance, I find that the patterns of media citations of interest groups reflect the composition of the advocacy community in Washington. One side in a policy debate gets more of an airing for their views when there are more mobilized groups on that side. Both the Washington print media and the national television news tend to rely most on the largest, oldest, and broadest organizations, whether they are liberal or conservative.
How does this help explain claims of liberal media bias? There are simply many more public interest groups representing liberal issue perspectives in Washington than those speaking on behalf of conservative issue perspectives (by my count, four or five times as many). The liberal groups are also, on average, larger and older. On some issues, there is a liberal issue group but no equivalent conservative one (although some see the cacophony of specialized voices as a weakness for liberals).
The large ideological difference in the population is reflected in organizational citations in media coverage. Liberal issue groups (including environmental and consumer groups) account for one-quarter of all advocacy group mentions on the television news; the conservative equivalents account for only 3.5% of the mentions. (Groups that represent occupations, identity groups, or other issue perspectives account for the remainder).
My counts do not include corporations and their associations, which vastly outnumber public interest groups and often have conservative views. When corporate interests are included, they represent a large share of media citations in policy debates (although one comparison found advocacy groups more prominent).
To see how this works in practice, look back at a USA Today story previewing 2011. The story cites Third Way, the Brookings Institution, the Heritage Foundation, the U.S. Chamber of Commerce, the AFL-CIO, the Committee for a Responsible Federal Budget, Environmental Working Group, and the Bipartisan Policy Center. The Groseclose/Milyo measure of media bias would compare its citations to those of members of Congress, perhaps finding it more likely to cite liberal groups.
Yet looking for differences across media and policymakers misses the larger picture: some organizations consistently gain more attention from both. I find that policymakers and reporters both amplify the voices of the same types of large, institutionalized organizations. A previous study of think tanks also found that organizational size and resources, rather than their ideology, determines their prominence.
The population of available groups is also consequential. Reporters call a cross-section of the types of groups that are accessible to them. If the organizations that they cite are indicators of media bias, the largest source of biases in coverage are differences in the relative mobilization of each constituency. Some social groups, economic interests, and issue perspectives generate more organized representation and they receive more media coverage.