A lot of supernatural beings are described as having certain compulsive ways of behaving, which can be used against them. There are several varieties:

!!Magical:
This is when there is either something magical that forces them to act compulsively, or some magical consequences for not doing so. Examples include a MagicallyBindingContract or [[RestrainingBolt geas]] that is in some way related to the nature of that creature.

!!Obsessive-Compulsive:
This type, similar to the RealLife condition known as Obsessive-Compulsive Disorder, is when there are unpleasant emotional consequences to breaking the compulsion, such as extreme anxiety. HeroicWillpower may make it possible for them to overcome this.

!!Happy Compulsion:
This is when they act compulsively because they ''want'' to act that way. Maybe it's really fun to [[EvilLaugh laugh maniacally]] or answer riddles or whatever, to the point where it's hard to restrain yourself from doing so.

!!Stupid Compulsion:
This is when the character is apparently unaware of the consequences of following the compulsion. In these cases, it can be hard to distinguish from [[TooDumbToLive ordinary stupidity]], but a highly patterned way of outsmarting a certain kind of creature suggests some sort of compulsiveness. If this is present in AI, then it goes under ArtificialStupidity.

Compare {{Idiosyncrazy}} and PolitenessJudo. Subtrope of BatmanGambit. Often a subtrope of SanityHasAdvantages.
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!!Examples:

[[folder:Anime & Manga]]
* In ''SoulEater'', Death the Kid has a crippling obsession with symmetry, resulting in him stopping mid-battle to chew out one of his [[EmpathicWeapon Weapons]] for being off center, going home in the middle of a mission to check whether the picture in his room was centered properly (it was), refusing to attack a symmetrical opponent and going berserk and obliterating an asymmetrical one.
** He manages to ignore such obsessions (specifically not freaking out about the line in his [[MarkedChange hair]]) when he needs to deal with Mosquito. But that arguably is a case of one compulsion being overridden by another - his need to create order between life and death.
* The Contractors of ''DarkerThanBlack'' have to fulfill a "remuneration", which is a compulsion to do something after using their powers-- and the compulsion is so powerful that they ''must'' do it (though it's never really explained what would happen to a Contractor who's unable to fulfill his or her remuneration). This can vary from folding the corners of every page in a book to drinking beer to [[{{Fingore}} breaking one's own fingers]]. The only exception is for someone whose mind is no longer in their original body; for example, Mao, who has the ability to possess animals and whose human body was killed, doesn't have a remuneration. [[spoiler:This is why Hei doesn't have to pay any price for his ability: it comes from his dead sister's SoulFragment.]]
* In ''OnePiece'', an entire species of birds (known as South Birds) compulsively face south and thus are used as compasses. At one point, the crew captures one and makes fun of it. The bird threatens to throw them off course and turns its head North...until it gets tired and turns back to south. It's also worth noting that there are North Birds as well, and will produce East or West birds through mating with South Birds.
* Teru Mikami of ''Manga/DeathNote'' always uses exactly one page of the Death Note each day - no more, no less. This means that Near can replace the page that he uses on the day of the confrontation at the Yellow Box Warehouse weeks in advance. All of the other pages work, so the criminals that Mikami writes down keep dying and he doesn't get suspicious, but when he tries to kill Near and the rest of the SPK, he only confirms his guilt and Light's.
** He also follows a strict schedule without fail, so any weird action that he would do on a particular day would be a sure sign of something suspicious, Case in point, he went to the bank twice in one day, allowing the SPK member tailing him to find the real death note that was used to kill people.
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[[folder:Comics]]
* With a RoguesGallery rife with mental instability, many are the times Franchise/{{Batman}} has won the day simply by playing on his foe's idiosyncrasies. ComicBook/TwoFace is probably one of the most consistent examples, with his need to consult his trademark coin: in theory, the results of the toss are equally as likely to turn up in the hero's favor. ([[TheoryofNarrativeCausality In practice...]])
** In ''Comicbook/ArkhamAsylumASeriousHouseOnSeriousEarth'': The doctors, attempting to "cure" Two-Face, try to show him that life has more than two outcomes by replacing his coin with dice and eventually a deck of cards. This reduced Dent to a complete wreck, unable to even go to the bathroom. In the end of the comic, Batman returns the coin and Two-Face immediately flips it to decide whether to kill Batman. He looks at the coin and lets him go...[[spoiler:despite the coin coming up scratched, meaning he ''chose'' to let him go. Or not, since WordOfGod claims he did it as an AprilFoolsPlot.]]
** A currently-running story has Two-Face completely unable to control himself after a rival villain stole his coin. Dent's dialogue and internal monologue makes much of his frustration with his lack of control.
** In an old storyline Batman secretly replaced Two-Face's coin with a gimmicked duplicate that always came up heads. While this prevented Two-Face from making some nasty decisions, his fury when he discovered the trick kind of canceled out the benefit.
** Which is kind of ironic since Two-Face's coin ''is'' a two-headed trick coin: he simply scratched one of the sides.
** ComicBook/TheRiddler had a nervous breakdown when he realized that his OCD made it so that he couldn't stop leaving clues for the police and Batman to find.
-->"You don't understand... I ''really'' didn't want to leave you any clues. I really planned never to go back to Arkham Asylum. But I left you a clue anyway. So I... I have to go back there. Because I might need help. I... I might actually be crazy."
* Canonically {{Thanos}} has been defeated because he has an uncontrollable compulsion to ultimately let himself lose. Seriously. He's not called the "Mad Titan" for nothing.
* A character called ''Fatman'' had the power to morph himself into any size and shape, but his WeaksauceWeakness was that he would irresistibly turn back if he ever saw something tasty to eat.
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[[folder:Film]]
* In the second AustinPowers movie, Mustafa feels compelled to answer any question that he's asked three times in a row.
* In ''Film/WhoFramedRogerRabbit'' the titular hero is goaded into revealing his hiding spot when the BigBad begins tapping out 'shave and a haircut' a gag that no toon can resist. This apparently isn't hyperbole as Roger fails to physically restrain himself and leaps through the wall and answers with 'Two Bits!'
* The evil leprechaun in the first ''Film/{{Leprechaun}}'' had a compulsion to shine shoes. The characters are able to escape him a few times by throwing shoes in his path.
* Memorably subverted in the otherwise [[InNameOnly forgettable]] ''Dracula 3000''. They've got the counter-intuitively named Orlock chained up, covered with knotted string and surrounded by spilled rice. When he gets released he reveals that he'd already undone the knots and counted all of the rice, so there's nothing to stop him from murderizing them all.
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[[folder:Literature]]
* Played with a fair bit in Creator/TerryPratchett's ''Literature/{{Discworld}}'', where methods to defeat vampires included mixing up their sock drawer (they would then have to pair and sort the socks), and forcing other obsessive-compulsive type behaviour. Another example would be the werewolves in The Fifth Elephant. Saying the word 'bath' actually gets them to wince, and throwing something has them automatically run over and grab it in a doggy-esque manner.
* In the first ''Literature/ArtemisFowl'' book, [[VillainProtagonist Artemis]] uses the fairies' inability to enter a human dwelling without permission, compulsion to obey a human while in their dwelling, and inability to come back later to try get revenge to extract a lot of gold from them. [[spoiler: They get around this by LoopholeAbuse and sending in a non-magical troll.]]
* ''Literature/SheepsClothing'' uses the vampiric compulsion to count things to try to keep the three antagonists contained. Unfortunately, the head vampire has an army of crows, and he sets them to eat up all the seeds thus scattered.
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[[folder:Live Action TV]]
* In ''Series/TheXFiles'' episode "Bad Blood", Mulder diverts an attacking vampire by flinging a bag of sunflower seeds. The vampire is compelled to stop and pick them up. Mention is also made of the alleged vampiric compulsion to untie knots, such as tied shoelaces.
** In actual vampire folklore, the creatures ARE compelled to untie knots, or to count things. Putting knots in the grave or strewing newspapers about a victim's house, are ways to distract one. So yes, [[Series/SesameStreet The Count]] is quite accurate in that respect.
* In ''Series/{{Supernatural}}'' they distract a leprechaun (played by Robert Picardo) by spilling salt on the ground and forcing him to count them, giving them time to figure out how to banish him.
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[[folder:Myth And Legend]]
* Japanese {{kappa}} traditionally have a depression on the top of the head, which must contain water for them to have any power. Fortunately, they are compulsively polite and so if you bow to them, they will bow back - spilling the water and rendering them helpless.
* The Jiang Shi (ChineseVampire) can be defeated by strewing anything small, such as grains of rice, in its path. It will be compelled to stop and count them.
** According to legend, this worked quite well against other vampires too. A vampire would get so caught up in counting the grains that by the time he got them all, the sun would be up and it would be "bye-bye vamp."
** [[Series/SesameStreet "One! One grain of rice! Two! Two grains of rice! Three! Three grains of rice! AH AH AH AH AH!"]]
** According to Hebrew myth the same applied to certain demons, most notably Asmodeus.
* In Britain and Germany, you could often trick a changeling into [[ReallySevenHundredYearsOld revealing their true age]] by doing something really strange, usually boiling water in eggshells. Then they'd say something like 'I'm as old as the Eastern Woods, but I've never seen anyone boil water in eggshells before!' Some stories claimed that they'd give up the trick and be defeated just from saying something like that, others used it merely as confirmation before they tortured the changeling to get the true child back.
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[[folder:Tabletop Role Playing Game]]
* In the ''TabletopGame/OldWorldOfDarkness'' splatbook for were-cats, each tribe of cats had three or four unbreakable compulsions. As most of these compulsions were significant tactical disadvantages to the were-cats, they strongly discouraged their members from revealing these secrets to ANYONE. For example, one tribe of cats had to follow a line of salt. The effectiveness of this tactic when the cat in question was on large natural salt-deposits was not explored.
** In the new [=WoD=], [[TabletopGame/ChangelingTheLost changelings]] who grow powerful enough suffer from "frailities", similar to the old stories about TheFairFolk. One may be unable to drink anything but alcohol, while another may have to dance when she hears a clock chime.
* The furry RPG ''TableTopGame/{{Ironclaw}}'' has things known as flaws, physical or personality-based quirks that hamper one's abilities elsewhere. While external (physical) flaws don't fit into this category, internal (personality-based) flaws do, and ''are'' expected to be followed. Considering that said flaws are usually LawfulStupidChaoticStupid stuff, depending on how strongly you have that flaw, it can be very easy for opponents to exploit them to their advantage and overwhelm you.
* [[BloodKnight Khornates and orks]] in ''TabletopGame/{{Warhammer 40000}}'' are murder machines in close combat, but easy to defeat if you outnumber and outgun them: [[AttackAttackAttack just shoot as they charge into your guns.]]
* The ''TabletopGame/DungeonsAndDragons'' supplement "Tome Of Magic" introduced the Binder PrestigeClass. Binders make pacts with unique spirits called Vestiges to bind them to their body and use their abilities. Each Vestige has it's own "Influence" based on their personality and history which forces the Binder to act a certain way or take certain actions, such as killing or not killing certain types. For example, the Vestige Otiax gets agitated whenever it sees a closed door, and if it finds a key it has to find the lock the key opens. Some of the influences can be ignored, but you'll take a penalty to your stats until you release the Vestige.
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[[folder:Video Games]]
* ''[[StarControl Star Control 2]]'' has The Words: "Hold! What you are doing to us is wrong! Why do you do this thing?" [[spoiler:Use this [[DialogueTree dialogue option]] on the [[BigBad Ur-Quan]], and they will explain their history and [[MotiveRant the motives]] for their actions.]] However, they'll still attack you afterwards. The reason it works is [[spoiler:that phrase is a MeaningfulEcho from a race they tried to exterminate in their {{Backstory}}, which initiated the EnemyCivilWar between the two factions of the Ur-Quan, the Green Kzer-Kza, and the [[OmnicidalManiac Grey Kohr-Ah]].]]
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[[folder:Webcomics]]
* ''Webcomic/TheKingfisher'' features vampire "progenitors" with mental frailties along these lines. Theodore is a younger vampire who learns the secret weaknesses of the progenitors and uses them for revenge. His first victim, Inka, was compelled to count beans while he attacked her.
* [[LeftHanging Had it ever been continued past chapter three,]]''Webcomic/HannaIsNotABoysName'' would've featured this tactic with the SuperOCD VampireHunter, Abner. According to the author's sketches, he would somehow be defeated by the [[TrashOfTheTitans masterfully disgusting]] [[BackAlleyDoctor Doc Worth]]; using Worth's repulsiveness to drive the mysophobic (that's 'fear of germs') hunter away.
* TheAdventuresOfDoctorMcNinja provides [[SufficientlyAnalyzedMagic empirical evidence]] that [[OurVampiresAreDifferent this one is true about its vampires]] on [[http://drmcninja.com/archives/comic/27p10/ this page]].
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[[folder:Web Original]]
* Part of containment procedures for Wiki/{{SCP|Foundation}}-[[http://scp-wiki.wikidot.com/scp-578-arc 578-ARC-3]] is covering the containment area with sand and painting a labyrinth on the floor, since the undead wight in question compulsively counts salt grains and traces labyrinth lines, slowing it down when it inevitably manifests. There was apparently some resistance to this kind of "folkloric strategy" at first among the scientifically-minded SCP researchers.
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[[folder:Western Animation]]
* In an episode of ''WesternAnimation/DannyPhantom'', Desiree the Wishing Ghost is compelled to grant any wish she hears. Sam takes advantage of this to undo her mischief so that Danny can defeat her. In her first appearance, Danny ends up wishing her back into her bottle, lampshading that if he were smarter, he would have just done that in the beginning.
* Disney's ''WesternAnimation/AladdinTheSeries'' had a Greek inventor character who compulsively revealed weaknesses to his inventions... tied to a compulsion to dictate notes to himself not to do that again.
** [[NeatFreak His obsession with cleanliness]]. Splash a bit of mud on his tunic and he'll be incapable of anything but whining until he scrubs it all out.
* Bruce teaches Terry this in an episode of ''WesternAnimation/{{Batman Beyond}}''.
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[[folder:Real Life]]
* In ancient Egypt, cats were considered sacred animals, and to harm one meant bringing down a severe punishment on the offender, up to and including death. When the Romans invaded, rumor has it that one of their tactics was to have their front line carry cats out in front of them, gambling that this compulsion would prevent the opposing Egyptian army from striking through the cats to get at the Roman troops. As we all know from history, it apparently worked. This compulsion against harming the sacred animals shows up, of all places, in the first ''[[Film/TheMummy1999 Mummy]]'' movie, where Branden Frasier's character scares off [[ThePunishment Imhotep]] with a cat.
* Autistic people of all kinds may exhibit this behavior, helpful or not.
** Ironically, holding cats (other animals work, but they like cats most, for obvious reasons) tends to make these tendencies less severe.
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