History Main / CannotTellALie

* In ''Literature/TheInheritanceCycle'', anyone speaking the Ancient language cannot lie. The elves are the only ones who speak it as their everyday language, and they're well-practiced in deceiving without speaking a literal untruth.

-->'''Ichabod''': George Washington? He was our Liar-in-Chief. He formed the Culper Spy Ring. That was a network of liars.\\
'''Abbie''': Thank you, colonial myth buster
* ''Series/StarTrekVoyager'': Seven of Nine is initially incapable of lying for a while after being separated from the Borg. {{Justified}} as it's pretty much impossible to hide the truth while part of a HiveMind.

* ''Series/TheWheelOfTime'': The first oath which the Aes Sedai take is to "speak no word that is not true", and they are bound by magic against it. However, they make judicious use of {{exact words}} when necessary to mislead others.

* In ''WebOriginal/LoomingGaia'', all fae (elves, goblins, gnomes, sirene, and cecaelia) are incapable of saying anything that they know isn't true. They can, however, twist the truth using ExactWords. For instance, you can see someone dead and ask a fae with a knife if they stabbed them, but if they actually slit their throat instead of stabbing, they can say no.

* The protagonist of Creator/WilliamSleator's ''Others See Us'' cannot lie, at least until he gains telepathy and realizes everyone around him is lying [[strike:even]] ''especially'' to themselves.

* The A!tol in Glynn Stewart's ''Duchy of Terra'' series are capable of lying but typically don't bother because, being similar to terrestrial squid, their changing skin coloration reveals their emotional state, making them their own LivingLieDetector. Individuals are fully capable of being underhanded, criminal, and treacherous, but keeping it a secret relies on not putting themselves in a situation where they'd be forced to lie to someone who understands their colour patterns.

* Most of Creator/IsaacAsimov's robots (the earlier ones, at least) cannot knowingly lie.

** There is one instance of a robot lying repeatedly. When, by accident, a robot develops telepathic powers, it lies to people when it knows the truth would hurt them (which would mean breaking the First Law). Unfortunately for the bot, humans have so many conflicting emotions, and lying to them can ultimately cause even more harm. Susan Calvin [[spoiler:destroys the robot with a LogicBomb after one of its lies indirectly wounded her]]. The title of that story (and [[TitleDrop end words]]) is in fact '''Liar'''.

** For that matter, the police in general are legally allowed to lie to you. To provide a few examples, they can claim to have evidence they don't have (generally to get someone to confess or provide evidence on another party), they can threaten property owners for not being allowed on the property even if they don't have a legal reason to enter, and they can provide false reassurances of safety to informants and witnesses.

* In the ''LightNovel/HaruhiSuzumiya'' novels, Kyon noted on more than one occasion that Yuki wouldn't lie. Who knows if this is WillNotTellALie or Cannot Tell A Lie, but he is convinced that it is this trope.

* ''The Riddler'', DependingOnTheWriter. A bit of belated backstory says that his father beat him for winning a contest, wrongly thinking he cheated. As a result, Ngyma has an outright compulsion to tell the truth, as expressed through his riddles. In one story he tries not to leave clues, but cannot stop himself, and when Franchise/{{Batman}} catches him Riddler says he needs to go to Arkham because there's something wrong with him.
* Franchise/WonderWoman is this DependingOnTheWriter. This wasn't the case originally when she had a secret identity, but after DC's [[ComicBook/PostCrisis first reboot]] in [[ComicBook/WonderWoman1987 the 80s]], the secret identity was discarded and she was depicted as being so honest she gained the sobriquet, Spirit of Truth. The reason she cannot lie also varies, with some writers attributing this to the Lasso of Truth preventing her from lying and others having her as too sweet to lie.

* Variation: immortals from ''LightNovel/{{Baccano}}'' are incapable of using aliases in the presence of other immortals, instinctively blurting out their real names if they try.

* Mephistopheles from ''Film/GhostRider'' claims so.

* UsefulNotes/GeorgeWashington was said to have remarked, "I cannot tell a lie", and admitting to chopping down his father's cherry tree. However, this is a myth. Even if it were true, it would be a matter of choice rather then inability (i.e. WillNotTellALie rather than Cannot Tell A Lie).

* In ''Manga/UsotsukiSatsukiWaShiGaMieru'', despite her reputation as being [[CompulsiveLiar the exact opposite of this trope]], Satsuki herself. At first, it just seems that she dislikes lying, but it becomes clear when her friends try to goad her into at least telling a meaningless lie [[BigEater by eating her lunch until she does]] that she can't bring herself to be dishonest on any level, which kills their main strategy for how to deal with [[spoiler:student president Kai Mikami]]. The reason she's like this is later revealed to be a form of self-punishment: [[spoiler:the last time she lied was when she tried to explain to her father why she didn't do anything when she discovered her mother bleeding out on the floor, knowing he wouldn't accept the real reason (she confused her mother for a hallucination, as her [[{{Seer}} corpse-seeing abilities]] were just beginning to manifest), and he immediately calls his daughter out for the shallow attempt at deceiving him.]]

* In ''Manga/UsotsukiSatsukiWaShiGaMieru'', despite her reputation as being [[CompulsiveLiar the exact opposite of this trope]], Satsuki herself. At first, it just seems that she dislikes lying, but it becomes clear when her friends try to goad her into at least telling a meaningless lie [[BigEater by eating her lunch until she does]] that she can't bring herself to be dishonest on any level, which kills their main strategy for how to deal with [[spoiler:Mikami]].

** Ashley, as part of her "absurd goodness", finds it almost impossible to lie. It's not exactly WillNotTellALie, because there have been occasions when she's been perfectly ''willing'' to tell a lie, but blurted out the truth anyway. Since absurdly good characters never ''know'' they're absurdly good, she interpretes this as "not having a filter".

* In the ''[[VideoGame/HarukanaruTokiNoNakaDe Harukanaru Toki no Naka de -- Hachiyou Shou]]'' anime, after [[DarkMagicalGirl Ran]] loses her memories, the main characters decide that it will be better if she doesn't remember anything about her connections with the Oni Clan. Yasuaki, for [[TheStoic certain]] [[ArtificialHuman reasons]], fails to understand why they don't tell her everything, to the point of asking directly why they are lying. When he eventually ''does'' tell her the truth in order to figure out how the Oni Clan's curse works, [[NiceJobBreakingItHero all sorts of troubles proceed to happen]].
** The manga explores this even further. Yasuaki, who ''insists'' that he has no heart or emotions, wonders at one point why people tell lies, implying that, indeed, he doesn't understand the concept of lying ([[YoungerThanTheyLook yet]]). When [[SidekickCreatureNuisance Kotengu]] gets killed, however, Yasuaki [[CharacterDevelopment ends up lying to Akane]] that he is still alive just to make her stop crying. This event confuses him a lot, as he apparently believes that, not being human, he wasn't supposed to be able to lie.

* [[spoiler:Kazuo Tengan]]'s forbidden action in ''Anime/DanganRonpa3'' is this. If they lie, their bracelet will inject them with a deadly poison.
* In ''Manga/{{Pluto}}'', robots aren't ''supposed'' to be able to lie as part of being ThreeLawsCompliant. However, the more advanced an AI is, the more like a human they are...and humans can lie. [[spoiler:The "perfect" AI is even able to lie to ''himself'', deluding himself into thinking he's human.]]

* ''Anime/PuellaMagiMadokaMagica'': [[WeaselMascot Kyubey]] cannot technically lie, [[spoiler:as he comes from a purely rational alien species. This does not, however, prevent him from invoking his ExactWords and YouDidntAsk]].
** He also did not deem it necessary to tell them everything, becoming genuinely confused when all the girls got angry at him for "hiding the truth". Kyubey did intently hide some information to keep things in ''his'' favor though...

* ''Wiki/SCPFoundation'': Victims of [[http://scp-wiki.wikidot.com/scp-1082 SCP-1082]] are not only unable (and unwilling) to lie -- they won't even use euphemisms or non-literal language, and write and speak overly verbosely so as to leave absolutely no room for misunderstanding whatsoever.

%% These are all examples of Truth Serums and not this trope.
%%* The main character of ''Film/LiarLiar'' was made to be unable to tell a lie for one day, to the point where he couldn't speak one, write one, or even ask a question that he knew would be responded to with a lie. Furthermore, he couldn't even conceal the truth by not speaking or telling half-truths, which led to most of the film's humor, especially as the "no lying" thing interfered substantially with his occupation as an AmoralAttorney.
%%** ''Liar Liar'' is a partial remake of the 1941 Creator/BobHope comedy ''Nothing But the Truth'', which by contrast has a WillNotTellALie plot: Hope's character bets $10,000 (a huge sum in those days) that he can go 24 hours without fibbing.
%%* Inspired by ''Liar Liar'', Brazilian movie ''O Candidato Honesto'' ("The Honest Candidate") has a SleazyPolitician running for president being forced to tell the truth (along with other enforced honesty, such as refusing bribes) due to a dying wish.
%%* Sally in ''Film/PracticalMagic'' finds herself mystically incapable of lying to the detective who has come to investigate the disappearance of Jimmy, whom Sally and her sister had accidentally killed. She avoids confessing by giving a series of clever truthful-but-misleading answers.

* ''Literature/TheWheelOfTime'':
** The Aes Sedai from ''Literature/TheWheelOfTime''. They ''cannot'' speak a lie, even when that lie would save millions of lives. This was a magically-created prohibition to make people trust them. Unfortunately, because most Aes Sedai have become masters of being MetaphoricallyTrue, ExactWords, and [[YouDidntAsk lies of omission]], they have arguably earned ''more'' of a reputation for deception than they might have otherwise.
** This also leads to a WhamLine later in the series, when an Aes Sedai blatantly lies.

* [[spoiler:Achakura]] in ''FanFic/KyonBigDamnHero'', as Nagato programmed her. She even [[LampshadeHanging complains she can't lie about her weakness]].

* In ''[[https://www.fanfiction.net/s/3205105/1/And-the-Truth-Shall-Set-You-Free And the Truth Shall Set You Free]]'' Harry, due to accidentally swallowing a bottle of Veritaserum as a baby, is utterly unable to lie. An escaped ''Bellatrix Lestrange'', of all people, teaches him how to give [[{{Mathematicians Answer}} Mathematician's Answers]] and half-truths before he starts Hogwarts.
* In ''FanFic/MegaManDefenderOfTheHumanRace'', robots generally can't lie, except for ones with advanced programming, like [=ProtoMan=]. As of episode 7 this also includes Guts Man, Cut Man, and Magnet Man.
* Discord in ''FanFic/RomanceAndTheFateOfEquestria'' is trapped in this state. Mostly it leads to a lot of needless rambling and DidISayThatOutLoud moments. Being Discord, he's good at invoking ExactWords.
* The main character of ''FanFic/TheTwilightChild'' is this mixed with WillNotTellALie. She's near-incapable of lying in any form, and should she actually attempt it she'll either immediately backspace or her vocal cords just shut down on her. It also thwarts her StepfordSmiler tendencies as well.
* In ''FanFic/{{Sight}}'', Zanpakutou spirits can't lie due to being the manifestation of their wielder's soul. As a result, it's easy for Ichigo to tell when their wielder is lying because the zanpakutou will look extremely guilty for the lie.

* In the Literature/HeraldsOfValdemar series, Tremane has a spell put on him by the Son of the Sun, Solaris, that makes him unable to lie. The Heralds can cast a spell that compels people to tell the truth as well, which they use when hearing cases as {{circuit judge}}s.

* In the Literature/{{Lensman}} series, one of the first aspects demonstrated of using the Lens is that one cannot telepathically lie with it. This was mostly seen in ''First Lensman'', when the Lens is first introduced to Civilization and the Galactic Patrol used that aspect as a selling point for prospective entrants; insist on a Lensman using telepathy and you'll always get the truth from them, even if it becomes BrutalHonesty at times. This becomes one of several aspects (alongside being of exceptional mental character and incapable of being imitated) that gains the Lensmen complete trust throughout Civilization: setting the stage for the remaining books.

* The Truth Vessels in ''Videogame/DeadlyRoomsOfDeath'' have this to improve reliability, as they were created to collectively serve as a living search engine. They also speak a language that was specifically designed to make misinterpretations impossible.
* In ''Videogame/FalloutNewVegas'', Yes Man is programmed to be forthcoming with all information to anyone who asks for it. He acknowledges that this was probably rather short-sighted in hindsight from Benny, who was the one who had him reprogrammed.

* Dr. Wily ''thinks'' that robots Cannot Tell A Lie in the animated ''WesternAnimation/MegaMan'' series. Rock proves him wrong.

* ''WesternAnimation/ThePowerpuffGirls'':
** Episode "Lying Around The House" has a little figure that grows every time the girls tell a lie. To get rid of it, they must tell the truth about their transgressions, which they eventually do. First done as issue #21 of the comic, "Big Fish Story."