Occasional advertising boycotts of Rush Limbaugh’s program notwithstanding, political talk radio has been wildly successful in recent years—in terms of both revenue and ratings. Of course, political talk radio generally means conservative political talk radio, especially since the demise of the liberal Air America network in 2010. The most popular political talkers, like Limbaugh, Sean Hannity, and Glenn Beck, are all conservative. So you might be inclined to think that political talk radio’s recent success reflects increasingly conservative values among the general public. However, Jeffrey M. Berry and Sarah Sobieraj of Tufts University caution us: Not so fast.

As they argue in the October 2011 issue of Political Science & Politics, the surging popularity of political talk radio stems from changes in the industry over the last two decades: deregulation and the decline of music-based stations. The 1996 Telecommunications Act’s deregulation of the industry allowed businesses to own more radio stations in individual markets, and across the country as a whole. That allowed large companies like Clear Channel to buy more stations, many of which it converted to talk formats. Why talk? In Clear Channel’s case, it’s because it already owns Premier Networks, a subsidiary that syndicates some of the biggest talk programs, including Limbaugh, Hannity, and Beck. Programming new stations with shows it already owned made good business sense.

Meanwhile, many music stations found making a profit a growing struggle because listeners increasingly have ditched radio in favor of digital music technology. Why listen to commercial radio when you can listen to your mp3 player commercial-free? Or stream Internet radio on your computer? Fewer people listen to music on the radio, so radio stations attract less advertising, so station owners flip formats from music to talk.

Much of the recent growth in political talk radio listenership simply stems from the fact that more people find it on their dial. Availability breeds listenership. According to Arbitron, the number of news/talk radio stations more than doubled between 2007 and 2009. But news/talk includes not just conservative talk—it’s also all-news, sports talk, and, yes, even a handful of liberal talk stations. And doubling the number of radio stations does not necessarily double the size of the overall news/talk audience, if the new stations are located in small markets. Limbaugh’s audience growth between 2007 and 2009 was in fact a modest 11 percent, according to the Pew Project for Excellence in Journalism. Hannity’s grew by 12 percent. Beck’s audience growth was far more impressive: 80 percent.

So Berry and Sobieraj favor a supply-side rather than demand-side explanation for the growth of talk radio, a structure-of-the-market rather than an attitude-of-the-consumer approach. But why the conservative tilt in talk? Because, they suggest, liberals see viable alternatives on radio while conservatives do not. African Americans and Hispanics, predominantly liberals, are often already attached to ethnic-oriented stations. Liberals generally are more likely than conservatives to find mainstream commercial radio news and NPR trustworthy.

As Berry and Sobieraj point out, there has to be a saturation point for political talk radio. It stops making sense for radio stations to flip their formats to political talk when other stations in town already have it. And in fact, more recent figures from Pew suggest that the audiences for the top political talkers have grown little since 2009, even with a Democrat in the White House.

A development to watch is Arbitron’s transition to a new ratings method. Selected listeners in larger markets already wear the Portable People Meter, a device that looks like a beeper and detects automatically which radio stations are within a listener’s earshot. The ppm is considered to be more reliable than Arbitron’s diary method, which depends on listeners to write down which stations they listen to and when. The diary system has been thought to favor formats with especially loyal listeners—formats like political talk.

Michael Schudson and Katherine Fink are contributors to CJR.