For a long time people have talked about the problem of low-information voters, those who typically don't care all that much about politics, rarely watch the news, and rely on general impressions. To varying degrees this has probably been true of most voters in most places throughout the history of voting.
Lately I've been thinking about another group, people care a great deal about politics, devote a lot of time to talking and reading about it, and consider themselves very well-informed indeed despite the fact that a large part of what they think they know is demonstrably wrong at the level of objective fact. There have always been a lot of these folks as well, especially on the political extremes, but there may be cause to worry that this problem is getting a lot worse. Here's a two-and-a-half-minute example:
As multiple fact-check organizations across the political spectrum (and even Newt Gingrich) have pointed out, evidence shows that it's extremely unlikely (to put it charitably) that millions of non-citizens voted in last month's U.S. elections. It's also not true that California "allows it," and of course President Obama didn't tell illegal immigrants to vote. But clearly some people believe such nonsense, and they haven't made it up themselves; they're getting it from somewhere. Some of the misinformation even comes from president-elect's Twitter feed.
Of course, politicians have been known to lie, people to repeat rumors, and news media to get things wrong. But lately we've been seeing deliberately made-up, highly sensational fake new stories published on the professional-looking websites of what appear to be real news outlets. Some of the sites are owned by Americans, some of whom claim to be engaged in satire or shenanigans or social experiments meant to see just what people can be made to believe. (One admitted he targeted Trump supporters because they don't bother to fact-check.) Interestingly, a fair number of sites are owned by a group of well-educated entrepreneurial teenagers in Macedonia. (For a flood of articles on that, just do a web search for "fake news Macedonia.")
And we're not talking about some esoteric little backwater of the Internet. In the run-up to the latest election people on Facebook shared more links to deliberately fake news stories than to real ones from actual newspapers and networks,
Facebooks and Google have promised to look into doing something about fake news. (I would hope this means flagging the links as questionable rather than blocking them outright.) Legitimate news sources have also devoted time to informing people about the existence of fake news, and more generally, the media seem increasingly willing to label false assertions made by politicians that demonstrably aren't true.
By the way, this subject may put you in mind of a supposed quotation popularized by the film The Big Short: "It ain't what you don’t know that gets you into trouble. It's what you know for sure that just ain't so." The movie misattributes it to Mark Twain (whose birthday was yesterday, November 30, by the way), but the line doesn't appear in Twain's published works. Quote Investigator traces a similar line to another 19th-century humorist, Josh Billings.
On the other hand, as Alex Shepard pointed out in The New Republic (or its website, anyway), the book the movie was based on (The Big Short by Michael Lewis) accurately quotes Leo Tolstoy as saying something along similar lines:
The most difficult subjects can be explained to the most slow-witted man if he has not formed any idea of them already; but the simplest thing cannot be made clear to the most intelligent man if he is firmly persuaded that he knows already, without a shadow of a doubt, what is laid before him.
(Updated 2016 January 04 to add the last paragraph and to note that the European center of fake news entrepreneurship is for some reason Macedonia, or what the UN calls "the former Yugoslav republic of Macedonia" to avoid pissing off Greece.)