Confirmation Bias

Confirmation Bias is the fallacy of lending extra weight to information and arguments that confirm your own beliefs while disregarding or downplaying evidence that disputes them.

This comes in several forms:

Seeking evidence for a belief one already holds, or eagerly accepting it, while disregarding or downplaying contradictory evidence.
Interpreting ambiguous information with a focus on how it favors one's own beliefs.
Rationalizing contradictory evidence in a way that still affirms one's own beliefs.

In fandom, the first of these three types of Confirmation Bias is the most common. Essentially, if a book or any other media product follows a distinct philosophical, political, or religious slant, then people who agree with that slant will often like it despite any flaws it has.

Conversely, people who disagree with its message will often reject it out of hand, regardless of how well written it is.

Keep in mind, however, that many people neither agree or disagree with a message before being presented with an argument that they find convincing. The purpose of many of the examples of this trope can legitimately be said in at least a small part to convince the undecided, rather than change someone's mind. Even so, the Confirmation Bias effect is noticeable.

It is worth noting that even if a work may be not be aimed at converting ideological outsiders, that doesn't necessarily mean it's not designed to convert people.

Also, this does not mean that everyone with a given viewpoint will like a work just because it follows that same view. Some may criticize it for not doing a good enough job persuading those who are undecided or who have the opposite view. Contrast Don't Shoot the Message, which is about disliking a work because of its style even if one may agree with its message.

Examples of Confirmation Bias include:

Advertising

Any pro-choice/pro-life commercial assumes the message it's giving. IE, an unborn baby is a person and thus abortion is murder = Pro-Life; or they're not a person, and part of the woman thus her decision = Pro-Choice. It's doubtful anyone is ever converted by these, as both very rarely attempt to convince the viewer of the (non)person-hood of the unborn baby.
Those Mac vs. PC ads that depend entirely on Ad Hominem and misconceptions to sell their points to the audience, which in most cases either already agrees, disagrees and is already aware of what is untrue about the ads, Took A Third Option, or is very easily Distracted By the Shiny, which admittedly usually tends to work in Apple's favor.
- The older ads weren't quite as preachy. The newer ones, though...well, they really rely on Viewers are Morons and what people already hate about Windows (one recent commercial featured all the PC's walking away and just Justin Long staying after a potential user asked for a computer that didn't have any error messages or other problems).
- These actually managed to make Microsoft Windows look like a put-upon underdog. As has been pointed out many times, which of these two men would you hire if they were competing for a job?
- The T-Mobile parody commercials weren't much better... though T-Mobile was lampooning Apple iPhone, which was at the time an AT&T exclusive.
- The UK versions of the ads are even worse. The Mac and PC are played by Robert Webb and David Mitchell, respectively. They play basically the same characters as they do in Peep Show, where Webb's character is stupid, lazy, and unreliable, and Mitchell is hard-working and serious-to-a-fault. Which qualities would you rather have in your computer?
There are several PSAs that often don't bother persuading those outside of a particular group. It's almost as if the producers are told to produce PSAs like this.

Film

Pretty much any movie Michael Moore made. Fahrenheit 9/11 may be the best example.
Expelled: No Intelligence Allowed is an anti-evolution film that claims the theory of evolution influenced the Holocaust and the Cold War.

Literature

Any book that attempts to prove or disprove a certain philosophical or political viewpoint. Reviews on amazon.com for these books tend to be either one star or five stars, due to the reviewers' opinion of the book being so heavily influenced by whether it clashes or not with their personal thoughts. The fact they appealed to people who differed not only in opinion but in the basic ways they see the world and make judgments probably has something to do with it as well.
- The God Delusion by Richard Dawkins (whose thesis is exactly what it sounds like) is a notable recent example. Naturally, when Alister McGrath wrote The Dawkins Delusion (a book that argues against Dawkins' book), almost anyone who agreed with Dawkins' book automatically disliked McGrath's book, whereas those who disagreed with Dawkins' book were almost always immediate fans of McGrath's book. Similar happened with Christopher Hitchens' book God is Not Great: How Religion Poisons Everything.
- As with many of these examples, very few actually read them both as noted in the reviews themselves.
C. S. Lewis' The Chronicles of Narnia series and Philip Pullman's His Dark Materials series, which are author tracts for Christianity and anti-theism, respectively. Notable in that Lewis didn't originally intend his work to turn into one, while Pullman very much intentionally wrote his series with promoting anti-theism in mind (as well as making an anti-religious rebuttal to Lewis). As a result, His Dark Materials is a lot more heavy-handed about its message. Justified in the case of The Chronicles of Narnia, as it was never intended as a tool for conversion, but for an already-Christian readership to explore their own beliefs in a fantastic setting.
Left Behind is really only liked by people who already believe in the Rapture. And not even all of them.
Speaking of Left Behind, Michael D. O'Brien's Children of the Last Days series has a similar effect, being essentially a Catholic version of the former (sans Rapture). Regardless of the writing, it comes strongest to those who tend to share the author's "traditional Christian" views.
Much of the writing of libertarian anarchist Science Fiction author L. Neil Smith falls under this. In fact, he has explicitly stated that he writes mainly to entertain those already having an anti-state worldview.
Anything by Ayn Rand.

Live-Action TV

The word "clapter" was coined to describe the latter effect in TV - when an audience applauds a joke more than actually laughing at it. It's an accusation often levelled at the more recent years of The Daily Show. (Its synonym "clappy humor" has an entry in the Urban Dictionary.)
- There are great honking buttloads of "comedians" who feed on clapter. Not surprisingly, they tend to disappear whenever the political winds shift in their favor.
M*A*S*H started with a noticeable anti-war stance, but was still entertaining enough to be enjoyed by someone who disagreed with the show's views. As the series went on, it seemed to become, at least within the show, increasingly required that viewers agree with every line of the Alan Alda Book Of Morality.
This is undoubtedly one of two reasons that The O'Reilly Factor and Countdown with Keith Olbermann exist. By extension, any show (or book, or radio program, or whatever) with a severe political slant is going to fall into this at some point. There's almost no pundit who doesn't.
Seth MacFarlane's Star Trek homage show The Orville does this with its anti-religious stance to the point that some non-religious viewers have called it out as waging The War on Straw against religion.
Panel shows try to avert this by having a variety of guests of different viewpoints (mainly because it would be boring to see a bunch of people sitting around agreeing on everything). Of course, there are plenty of ways to make a biased panel show (e.g. get a Fox News Liberal, stack the deck by putting aggressive "strong" debaters against lesser-experienced ones, unevenly stack the panel on one side so everyone can gang up on the minority opinion, etc.)
The TV series Years and Years. The way one views the series almost entirely depends on whether or not the viewer agree with the political opinions of Russell T Davies, making it either a terrifying look into the future or a laughable tantrum against people Davies disagrees with.
Two examples crop up in the crime series Bones:
- "The Archaeologist In The Cocoon" does this against Christian creationists. In the episode, an anthropologist is the murder victim and one his biggest customers - and a suspect in the murder - is the curator of a creationist museum, which purports to disprove human evolution and supports intelligent design. In addition to being an abrasive person who denies that prehistoric fossils are fossils and calls them mere rocks, the curator also gets correctly accused by Sweets of buying the fossils to destroy or hide them as they contradict his religious views.
- "The Tiger in the Tale", in addition to any other episode featuring Brennan's outrage against exotic pets and dislike of zoos. (Since Emily Deschanel is a known animal rights advocate, there's a possible element of Author Filibuster as well).

Music

Certain genres of music may fall into this trope, particularly those that began as underground movements and became mainstream later on; artists and songs would be judged primarily on their subject matter, perceived attitude, or whether or not a message is present, rather than being enjoyed/loathed for the music itself. Hip hop music, due to its lyric-heavy, melody-sparse nature, might be the most prominent example. Particularly the artists of the Political Rap sub genre. The main polarizers being Public Enemy, The Coup, and the extremely controversial rappers Paris, and Immortal Technique. And to a lesser extent, Ice Cube, Ice T, and Tupac Shakur.
- Also affected are genres which rely on a particular philosophy: Things like Punk and Black Metal come to mind.

Newspaper Comics

Politically themed comic strips, from Doonesbury to Mallard Fillmore to Prickly City to The Boondocks: if you agree with them, they're hilarious; if you don't, they're poison to the mind. If you have no bias one way or another, then you're probably just skipping over them all to see what Frazz is up to today.
- Ditto for The Brilliant Mind of Edison Lee. It's questionable if even the people it's allegedly preaching to find it funny. Comments on the comics Snark Bait blog The Comics Curmudgeon seem to indicate not...
- And I Drew This, by the Ozy and Millie lady.
- The Bad Reporter wasn't originally this—a cartoon from the early 2000s mocked the "comic-book-ization" of the media and argued that both sides of the political aisle were engaging in Demonization, without particularly insulting the followers of either side so much as their leaders. A cartoon a few years later parodied I Am Sam with undecided voters in place of the mentally retarded. Not trying to win any converts now, are we, Mr. Asmussen?
- Same goes for "alt-comic" strips like This Modern World, which are placed on weekly/monthly free newspapers for a certain city that have a significant liberal population.
Minimum Security was mostly only published in alternative newspapers where extreme views are more commonly. As such, it is no surprise to see a comic strip where everyone ranging from religious people, scietists, businessmen, and people who simply eat meat are depicted as evil and stupid. The main character doesn't eat meat and advocates the destruction of society, thus, she is smarter.

Radio

Most political radio shows—of both stripes—tend to be this by default. Neal Boortz does his best to encourage dissenting opinions, for certain values of "encourage" and "dissenting" and "his best", because (as he said in his book Somebody's Gotta Say It) shows where everybody agrees with the host tend to be extremely boring and sycophantic.

Recorded and Stand Up Comedy

Jeff Dunham's schtick with "Walter", a puppet who often voices Midwestern conservative views, is partly made up of "clappy humor," though mostly done to appeal to audience members who "know someone like this".
George Carlin was a master of clappy humor too, and often used it to express his negative views on society, politics and religion. His act came to focus more and more on his general misanthropy and criticism of optimistic people regardless of political affiliation. His last tour especially played up the "grumpy old man" angle.
Wanda Sykes's standup routine is a fairly painful example of this.
Much of Lewis Black's material pre-2009 was essentially a liberal critique of the Bush administration, individuals within it, and cultural conservatives paired with over the top facial expressions and screaming in reaction to those things. Hilarious IF you are a culturally liberal person.

Other Media

The works of Jack Chick, although most of those with the same slant think he was extreme.
Tony Kushner not only admitted to, but defended this in a 1997 essay:

The converted need preaching to as much as the unconverted, and will usually prove far more responsive and interested in change ... Those who are involved in the struggle to change the world need art that assists in examining the issues at hand, which are usually incredibly complex.

Some critics have argued that the internet would result in "echo chambers" where everyone would just view news sites, blogs, and forums that didn't challenge their views. However, according to one study, the "echo chamber" isn't any worse than any other media.
Played with in regards to YouTube videos about conspiracy theories. There will be a few instances where the title points in one direction, but the content points another direction, and sometimes there will be an admission that this is done to draw in the opposing crowd.