Is cable news polarizing American politics? A clever new experiment suggests that the answer is "probably not." (Which is a relief given that I'm hosting MSNBC's 8 p.m. hour this week. America, it's not my fault!)

Temple University's Kevin Arceneaux is the expert on this question. The key, he writes, is to remember that "people choose to watch partisan news." The people watching Fox News are not a random sampling of Americans. They're the people who come home after a long day and say, "You know what sounds absolutely great right now? Listening to Bill O'Reilly for an hour."

They're the people, in other words, choosing to watch Fox News. Same for the people watching MSNBC or CNBC or CNN. To understand the effect cable news is — or isn't — having, you need to understand the people who affirmatively decide to watch it:

The type of person who gravitates to partisan news shows is more politically and ideologically motivated than those who choose to watch mainstream news or tune out the news altogether, partisan or otherwise. People are not passive or particularly open-minded when it comes to political controversies. Not only do they choose what to watch on television, but they also choose whether to accept or reject the messages they receive from the televisions shows they watch.
In short, two forces simultaneously limit and blunt the effects of partisan news media. First, partisan news shows cannot polarize — in a direct sense — the multitude of Americans who do not tune into these shows. Second, the sort of people who actively choose to watch partisan news are precisely the sort of people who already possess strong opinions on politics and precisely the sort of people who should be less swayed by the content they view on these shows.

Saying anything rigorous about the effect of cable news thus means somehow figuring out the counterfactual: "What would Fox News viewers know and believe about politics if we lived in a world without Fox News?"

Arceneaux and his co-author, Martin Johnson, came up with a clever experiment to do just that. They forced different kinds of people to watch different political news programs, or to stop watching political news programs. What they found was that opinionated cable news programs had a big effect on people who don't like watching the news but a small effect on people who like watching the news and already agree with the program (the "proattitudinal" group):

So it can matter a lot for people who don't like watching cable news. But you know what's true about people who don't like watching cable news? They don't watch cable news.

Meanwhile, the "counterattitudinal" group — the people who disagreed with the show they were watching — showed the strongest reaction of all: They hated what they were hearing, and polarized sharply in the other direction. The result "undermines the Pollyanna notion that if people would just listen to the other side, the country would be a more tolerant and moderate place," writes Arceneaux.

That's not to say that polarized news channels have no effect on polarization, of course. They energize their own viewers, and they arguably have a significant effect on political elites, who respond with outsized concern to criticism from the media outlets they perceive as "their side".

But they are, at best, contributors to polarization rather than causes of it. The people who watch them are watching them because they are already polarized. If Fox News didn't exist, Fox News's viewers would have to invent it.