President Donald Trump tweeted Saturday that “Real Fox News is doing great” while “Fake News CNN is dead”, but a 2012 study found that people who heavily consume the former are less informed than those who watch no news at all.
In 2012, a Fairleigh Dickinson University survey reported that Fox News viewers were less informed about current events than people who didn't follow the news at all. The survey had asked current events questions like "Which party has the most seats in the House of Representatives?" and also asked what source of news people followed. The Fox viewers' current events scores were in the basement. This finding was immediately trumpeted by the liberal media—by Fox, not so much—and has since become known as the Fox News effect. It conjures the image of Fox News as a black hole that sucks facts out of viewers' heads.
Fox trades in stories about the venality of big government, liberal overreach and little-guy heroes of the heartland. A large share of Fox stories deftly push emotional buttons (lest the viewer push the buttons on his or her remote…)
This format has been successful, but it has drawbacks. There's a lot that goes on in the world that doesn't easily fit the Fox template. There are important stories that don't make anyone angry, prove liberals are evil or otherwise carry an emotional punch. Fox viewers get less of them. Fox News is like an all-you-can-eat buffet, serving up red meat. A more balanced diet might be healthier in the long run.
Author William Poundstone encountered similar results in his own surveys, though he cautions that correlation does not equal causation:
I have done similar surveys, both of current events and more general knowledge. In my research too, Fox News viewers scored the lowest of over 30 popular news sources (though Fox viewers did at least score better than those saying they didn't follow the news). The chart’s horizontal black lines with tick marks indicate the margins of statistical error. Last Week Tonight with John Oliver, a news satire, had the best-informed viewers.
The Fox News effect is a correlation. It doesn't prove that watching Fox News causes people to be ill-informed.
Causes are much trickier to establish. I will give my guesses (along with the warning that they're only guesses). The first thing to realize is that every news medium has its own audience demographics. It's no secret to advertisers that the average Fox News viewer has less formal education and income than the average New York Times reader.