Theory Tunnelvision

Everyone has their own view on the world and what happens around them. Maybe Bob believes that Humans Are Bastards. Maybe Alice thinks Chris is going to betray her. Given enough time though, these views and opinions can change, allowing Bob to see the good in man, and Alice to start to trust Chris. This trope is when that never happens; A character suffering from Theory Tunnelvision will stick with whatever idea he or she has for way longer than what would be normal. Even if Bob meets hundreds of Good Samaritans, he'll keep stating that Humans Are Bastards, and Alice will never trust Chris even though he saved her life several times now.

There are several kinds of Theory Tunnelvision, and several reasons for someone suffering of it:
 * Pride/stubbornness: Bob is just too proud or stubborn to say that they've been wrong, instead keeping up their own version of reality.
 * Familiarity: Bob has been told every day of his life that group X is evil. As a result, nothing group X will ever do can change Bob's vision. Even if someone from group X would rescue Bob from a burning building, all he'd say is to let go of him.
 * Hostility: Bob has a certain view of the world. Alice, his nemesis, states that he is wrong and has a lot of evidence to support her. However, since Alice is Bob's nemesis, he has no reason to believe her, even if the evidence is groundbreaking.
 * Prejudice: Bob has been betrayed so many times in his life that he simply cannot believe that someone would ever do something good. Even if Alice helps him pay his monthly bills, he sees it as a way to get him in debt with her.
 * Trust: Bob simply cannot believe that Alice would ever do something bad; even when she steals money right out of his hands, he deems it an accident. This one pops up very often with parents towards their children.
 * Mental disorder: Due to a handicap, Bob believes that whatever he thinks is true, is in fact true; anyone who states otherwise is deemed a liar. In these cases, Theory Tunnelvision is always justified.

Tunnelvison Theory can be both positive and negative, and the person suffering from it can be proven right in the end. The key point is that someone who suffers from it will discard any and all evidence that proves him or her wrong, no matter how solid it is. If someone suffering from Theory Tunnelvision at some point does break, expect a Heroic BSOD, along with muttering about how this should be impossible. A fair warning though: Do not try to break Theory Tunnelvision on purpose, as there is a good chance that you'll lose your friendship with the sufferer... or worse.

Selective Obliviousness is the supertrope of this, Flat Earth Atheist and The Scully are subtropes. A staple technique of the Tautological Templar. The Fundamentalist is the logical extreme of this, building everything around a few notions held this way. What develops into Theory Tunnelvision may start out as Aggressive Categorism.

Truth in Television, even normal non-neurotic people will take considerable evidence to overwhelm a strongly held theory, let alone those in psychological denial who'll get angry at people pointing out reality.

No Real Life Examples, please.

Anime and Manga

 * In Monster, detective Lunge initially believes protagonist Dr. Kenzo Tenma to be the serial killer. Over the course of the series, evidence that Johan is the real killer is practically thrown at him, from close acquaintances of Johan to criminal psychiatrists. However, Lunge continues to believe that Johan is a pseudonym for Tenma. It isn't until.

Live Action TV

 * In the Buffy the Vampire Slayer episode "I Only Have Eyes For You," Giles is convinced that a haunting is trying to communicate, despite all evidence to the contrary.
 * Dexter: Multiple times throughout the show, a character will suspect something is off (to the point of a Cassandra Truth) and Dexter will have to invent evidence to turn them away. It often works. One extreme example when it doesn't is when Laguerta adamantly believes that Doakes is innocent and no matter what evidence Dexter plants to the contrary, she still stands by this belief.
 * In Monk, there was an episode with a nudist. Adrian's theories got crazier and crazier as he tried to explain how the nudist was guilty

Literature

 * In Harry Potter, the eponymous character and his friends seem to be dead set to blame anything and everything that goes wrong at Hogwarts on Snape
 * Really just Harry. The others swing between finding his prejudice annoying, believing him and humouring him.
 * In The Last Battle, Dwarves are in paradise eating a feast but they believe they are in a hut eating slops.

Video Games

 * In Super Paper Mario, upon hearing that Mushroom Castle was attacked and Princess Peach was kidnapped (AGAIN!), Luigi immediately decides that Bowser must've been behind it.
 * Played for laughs in the Hordes of the Underdark expansion for Neverwinter Nights -- if you have Deekin with you in Hell, he interprets the signs of battle between Demons and Devils as the remnants of something long and complicated involving dragons.

Web Comics

 * In Schlock Mercenary, during the Mahuilotu arc the local police just cannot get over the idea that the killer shark was some kind of "murder-sub" built by John der Trihs. (They knew that their terraformed planet didn't have any sharks. Why blame John? They probably figured that he who smelt it dealt it.)
 * The Toughs later threaten to sue the idiots for what their lawyer calls "impersonating a police force".
 * In Homestuck, Terezi finds the corpse of and immediately concludes that Vriska is the most likely suspect. As she searches the crime scene for clues, Terezi admits to herself that she's only pretending to not already know who the murderer was. Then she finds the bodies of  and concludes that Vriska's on some kind of killing spree. She's a bit taken aback when Vriska freely admits to killing, but isn't even aware that the others died.
 * El Goonish Shive got those two alien enthusiast nerds annoying people. Hilariously enough, they never noticed real aliens and half-aliens in their town.

Web Original

 * In There Will Be Brawl, Mario immediately assumes Bowser is behind Peach's latest kidnapping, and refuses to hear any other explanation... not that you could blame him, of course. He only relents when.