The fact that Republicans are more likely to watch Fox and Democrats to watch MSNBC is a chicken-and-egg problem. To be sure, people prefer to watch anchors and commentators whose views they already agree with, but do the channels actually make their viewers more liberal or conservative as well?

To solve this riddle, the researchers, Emory University's Gregory Martin and Stanford University's Ali Yurukoglu, took advantage of a surprising pattern among cable subscribers: People are more likely to watch any station with a lower channel number...

Fox's and MSNBC's numbers are more or less random across the country, and in towns where MSNBC has a lower a number, cable subscribers tend to be more liberal -- even compared to people who get their television through a satellite dish.

These viewers watch a few more minutes of MSNBC a week on average, but not because they agree with the hosts' politics. They're watching MSNBC because they're more conveniently placed in the line-up. The same is true of Fox.

Martin and Yurukoglu found that watching four more minutes of Fox a week makes you 0.9 percentage points more likely vote Republican, while watching MSNBC for four more minutes makes you 0.7 percentage points more likely to vote Democrat.