Fox News is, by far, America’s dominant TV news channel; in the second quarter of 2017, Fox posted 2.35 million total viewers in primetime versus 1.64 million for MSNBC and 1.06 million for CNN. Given that Fox was founded by a longtime Republican Party operative and has almost exclusively hired conservative commentators, talk radio hosts, and the like to host its shows, it would stand to reason that its dominance on basic cable could influence how Americans vote, perhaps even tipping elections.

The effects of CNN and MSNBC on centrist voters are mostly negligible; MSNBC, in 2000 and 2004, modestly increased odds of voting Republican, before it turned left in time for 2008. But Fox News increases Republican voting odds for centrists, for Democratic viewers, and even, in 2004 and 2008, for Republicans already strongly inclined to vote that way. Watching three minutes more of Fox News per week in 2008 would have made the typical Democratic or centrist voter 1 percentage point likelier to vote Republican that year.

“Fox is substantially better at influencing Democrats than MSNBC is at influencing Republicans," the authors find. While most Fox viewers are Republican, a sizable minority aren't, and they're particularly suggestible to the channel's influence. In 2000, they estimate that 58 percent of Fox viewers who were initially Democrats changed to supporting the Republican candidate by the end of the election cycle; in 2004, the persuasion rate was 27 percent, and 28 percent in 2008. MSNBC, by contrast, only persuaded 8 percent of initial Republicans to vote Democratic in the 2008 cycle.

These are big effects, with major societal implications. The authors find that the Fox News effect translates into a 0.46 percentage point boost to the GOP vote share in the 2000 presidential race, a 3.59-point boost in 2004, and a 6.34-point boost in 2008; the boost increases as the channel's viewership grew. This effect alone is large enough, they argue, to explain all the polarization in the US public's political views from 2000 to 2008.

What's more, they find that Fox isn't setting its ideology where it ought to to maximize its viewership. It's much more conservative than is optimal from that perspective. But it's pretty close to the slant that would maximize its persuasive power: that would result in the largest rightward movement among viewers. CNN, by contrast, matched its political stances pretty closely to the viewer-maximizing point, showing less interest in operating as a political agent.