Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I even say who wrote them. If you have more recent summaries to add to this collection, send them my way I guess. Sorry for the ads; they cover the costs of keeping this online.
Nie, Norman H.; Miller, Darwin W., III; Golde, Saar; Butler, Daniel M.; Winneg, Kenneth. 2010. The World Wide Web and the U.S. Political News Market. American Journal of Political Science 54 (April): 428-439.
No serious observer of American politics would be surprised if you made two basic claims: (1) Small-circulation media outlets (websites, cable channels, independent newspapers) can be far more ideologically extreme than large-circulation outlets (network news) that need to appeal to a large audience to remain profitable, and (2) people prefer media sources that confirm their existing biases.
In the most recent issue of AJPS, Nie and his colleagues have an article that makes those two claims. The claims seem perfectly plausible. And they present well-executed research backing them up. Their findings are consistent with a string of previous work making the same argument and coming to the same conclusion (they list several such studies along the way). The main difference: Previous studies have operationalized "small-circulation outlets" as talk radio or cable television, but Nie et al look at internet news sites. They find that more ideologically extreme folks are more likely to visit online news sites.
At the risk of sounding ungenerous, my reaction was "well, duh." I suppose it's important to have precise measurement, so it's worth looking specifically at who is viewing online news sites even though we would expect to find that the same people who consume other niche media would also consume online news. And I suppose that's a purpose of journals--to look carefully at specific questions. I'm not criticizing them for writing the article by any means. In fact, a "well duh" reaction means they thought to test something that the rest of us assumed was true without bothering to check for sure. Good for them.
So, in sum: People who visit online news and political sites are more politically extreme.
Research on similar subjects
Nie, Norman H. (author) • Miller, Darwin W., III (author) • Golde, Saar (author) • Butler, Daniel M. (author) • Winneg, Kenneth (author) • American Journal of Political Science • American Politics • Cable Television • Ideology • Media and Politics • Perceptual Bias • Polarization • Voter Information • Voting • Websites Blogs and New Media