This is a very interesting statement, by Scott Althaus, political science professor and director of the Cline Center for Advanced Social Research at the University of Illinois Urbana-Champaign:

Josh Zeitz: "Scott, you noted that partisan media and media polarization have been growing since the 1970s. ... Is it getting worse in your mind, or has it always been this way?"

Scott Althaus: "It’s not new. It’s unclear if it’s worse than in the past, because there has been very little systematic research that goes all the way back 240 years to assess levels of negativity. From the 1780s all the way through the mid-19th century, the dominant model of news coverage was a partisan model, an advocacy style of news coverage. The idea of an objective journalism wouldn’t come up really until after World War I and it wasn’t the dominant mode of reporting in the United States until probably after World War II. But what came after the partisan mode and was competing with it for a long, long time is this kind of marketplace model of give people whatever they want. If they want silly stuff, if they want funny stuff — whatever entertains. And that model, along with the partisan press model, were the dominant ways that news reporting was produced in the United States up until the middle of the 20th century.

"So what we’re seeing today is in many ways a regression to the mean. We are going back to where we used to be, and the mystery then is why do we get this strange bubble that starts in the late 1940s and begins to decline very clearly in the 1980s where the Walter Cronkite School of Journalism — just the facts — becomes the thing that we expect. This is the outlier in our history, for sure."

The history of media polarization in the US

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