When a character or group of characters is by nature unable to tell any untruths, whether they be magical beings who are bound by that magic, speakers of a language that makes it impossible, or simply unable to grasp the concept of lying, that character Cannot Tell a Lie.
How restrictive this inability to lie is varies from character to character. For some, they are unable to deceive, following the letter of the law as well as its underlying meaning. For others, they are able to tell half-truths and omit important information, allowing for False Reassurance or even Malicious Slander and acting as a sort of Technical Pacifist Consummate Liar.
Children are prone to it, being too innocent to think of suppressing the truth — many truths have been blurted out by unwitting children — but this cannot be relied on; most children outgrow it (although some of them retain the tendency and grow up to be bad liars).
Some characters are very bad liars or broadcast some obvious signal when they lie. Those are not examples of this trope, although the results are effectively identical (the character can utter a lie, but can't fool anyone).
Characters that are temporarily forced to tell the truth, but otherwise can lie are under the effects of Truth Serums. Characters who are capable of lying, but choose not to Will Not Tell a Lie. If they're sworn to keep a secret, they will very quickly discover Keeping Secrets Sucks.
It should be noted that this trope is for characters who Cannot Tell A Lie as a character trait, which means that at all times they are incapable of lying. If the character is under the effect of a Truth Serum, it doesn't count. However, if said Truth Serum had a permanent effect, especially if it were applied before the main narrative, then that example is valid. Bad writers will use From a Certain Point of View to get away with "misunderstandings."
- Belldandy and other Goddesses First Class from Ah! My Goddess are absolutely incapable of telling lies. They're still able to simply say nothing when it's necessary to conceal information, but of course this can be quite revealing in its own right. After passing the Goddess First Class test, Urd declines the promotion (which would have allowed her full access to her immense power, far greater than even Belldandy's) and remains Second Class because she deems the ability to lie more useful than brute strength in protecting her family.
- Much of Death Note seems to rely on the idea that one of the rules for the death gods forbids them from lying to the people that hold their books. They are not, however, required to tell the user everything. Ryuk makes a living out of leaving out that last bit of information.
- Any augur in Fushigi Yuugi: Genbu Kaiden. Augurs must speak the truth, as they will lose their prophetic powers if they lie. Whether this actively means they are forbidden from lying or are incapable of lying is not explored. Seems to be the former, though, as there are two instances in the story where an augur has lied.
- In Princess Tutu, Mytho doesn't understand much because of losing his emotions, including not understanding the concept of lying... at least, until he begins to regain his emotions. The first time in the series he does tell a lie, Fakir reacts in shock.
- Variation: immortals from Baccano!! are incapable of using aliases in the presence of other immortals, instinctively blurting out their real names if they try.
- In the Harukanaru Toki no Naka de - Hachiyou Shou anime, after 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 certain 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, 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 (yet). When Kotengu gets killed, however, Yasuaki 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.
- Puella Magi Madoka Magica: Kyubey cannot technically lie, as he comes from a purely rational alien species. This does not, however, prevent him from invoking his Exact Words and You Didn't Ask.
- 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...
- Trueman from Yu-Gi-Oh! GX, sort of. He claims this is the case, which is how he got his name (he named himself, actually), but he does tend to use deceit and dishonesty many times in non-verbal ways, using illusion to prey on victims.
- From Magi: The Labyrinth of Magic, a person's rukh cannot lie. They will tell you exactly how they feel about you, and even if they shut up you'll hear their thoughts.
- Kazuo Tengan's forbidden action in Dangan Ronpa 3 is this.
- The former trope image, which depicts a scene from the English Civil War, invokes this (though hopefully not playing it straight). A Royalist household (mother with her two daughters and her son) is questioned by Parlamentarian soldiers about the patriarch's whereabouts. The moment presented is the one where the young son of the family is asked "When did you last see your father?" (hence the title). It is in fact never revealed whether he did the most sensible thing (to lie and to save his father's life) or actually Cannot Tell A Lie and, being the very incarnation of innocence (he even is Colour-Coded for Your Convenience), tell the truth and blow his father's cover. In the background, his two older two sisters and his mother can be seen anxiously sobbing, making this a Tear Jerker moment.
- Piffany from Nodwick is apparently so naturally pure that she feels constrained to blurt out the truth even when it would be dangerous.
- The Riddler, Depending on the Writer. 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 Batman catches him Riddler says he needs to go to Arkham because there's something wrong with him.
- In Courtney Crumrin and the Night Things, it's mentioned by the narrator that the Night Things are incapable of lying. Because of this, it tends to not occur to them that non- Night Things can lie to them.
- An interesting case with X-23. Laura is very well-known for her Brutal Honesty and being Innocently Insensitive. Because she was created as a Living Weapon covert operative and assassin, she certainly has the capability to lie in order to maintain her cover on missions, but outside of this context she consistently shows an inability to do so. It's a major bit of Character Development in her solo series when she lies to the child of one of her victims to spare him the pain of revisiting the loss of his parents.
- Loki: Agent of Asgard hits Loki of all people with this. After the inversion is undone they just can't take falsehoods any more, not even benign ones or jokes. They're not literally unable to tell them but all come out incredibly weak and make them feel so bad about them that they correct themself immediately. They theorize that this is either an after effect of the inversion, or being in the middle of the truth wave, or maybe they're just sick and tired of untruths or some combination of these. This prompts them to be suicidally honest with Verity and Thor.
- Achakura in Kyon: Big Damn Hero, as Nagato programmed her. She even complains she can't lie about her weakness.
- In 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 Mathematician's Answers and half-truths before he starts Hogwarts.
- In Mega Man: Defender of the Human Race, 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 Romance and the Fate of Equestria is trapped in this state. Mostly it leads to a lot of needless rambling and Did I Say That Out Loud moments. Being Discord, he's good at invoking Exact Words.
- The main character of The Twilight Child is this mixed with Will Not Tell a Lie. 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 Stepford Smiler tendencies as well.
- In 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.
- A Boy, a Girl and a Dog: The Leithian Script: In the Halls of Mandos nobody can lie. It's physically impossible. A soul can refuse to answer or tell something what they believe is the truth, but nobody can intentionally tell a lie.
- In Brother on Brother, Daughter on Mother, Rachel Connor is just a lousy liar. She tries to cover the fact that she's an illegal genetic augment after her Healing Factor blocks Borg assimilation, but Alicia Gantumur doesn't buy it, saying whatever the real explanation is, she won't push it "as long as the Captain knows." Later, Eleya's time-traveling future daughter comments that she was sent back in time instead of Connor, her superior, because Connor still hasn't learned to lie convincingly several decades on.
Reshek Taryn: I clean her out every time we play poker.
- In Cuckoo Bird, faeries like Izuku cannot tell lies because they suffer an excruciating slashing and burning sensation in their throat whenever they do. Because of this, they specialize in misleading and tricking others with Metaphorically True statements to get around this.
- The Thermians from Galaxy Quest cannot grasp the concept of lying (or fiction) — at least at first. Big Bad Sarris painfully forces them to learn about deception. They now know that such a concept exists, but they are terrible at recognizing it, and certainly not experienced enough to concoct their own lies.
- The Invention of Lying takes place in a world where no one can lie, except for Ricky Gervais's character. Despite no one being able to lie, it's actually not a very nice place - there is no religion, no fiction, and because people are brutally honest (or at least incapable of blurting out inconvenient truths) everyone is cruel, crass or prone to over-sharing.
- In the first Superman movie, Supes tells Lois he never lies. Not once.
- Mephistopheles from Ghost Rider claims so.
- Jackson Rippner in Red Eye never lies. This has led to some fans theorizing that his joke about killing his parents was actually true.
- The titular character in I Am Sam is mentally challenged and doesn't understand the concept of lying which causes problems with his lawyer who wants him to "tweak" the truth a little in court.
- The newer generation of replicants in Blade Runner 2049 are implied to always tell the truth due to their obedience programming. This ultimately proves untrue, as we see K tell Lieutenant Joshi that he killed the replicant child; Luv makes it explicit right before she murders the lieutenant.
- George Washington 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. Will Not Tell a Lie rather than Cannot Tell A Lie).
Ichabod: George Washington? He was our Liar-in-Chief. He formed the Culper Spy Ring. That was a network of liars.
- In a Paperinik New Adventures short they have fun with this story. It ends up with a time traveler coming back in time to prevent little George from chopping down the tree (so that his son's teacher won't have an example to quote when she'd say truth is to be rewarded), only to find out that he landed on the tree and little George decided it was easier to 'confess' that he, George, chopped it down rather than trying to explain the truth.
- The Austrian satirist Roda Roda (1872-1945, born Sándor Friedrich Rosenfeld) parodied this in a story "from an American school primer": Young Abraham Lincoln and a playmate together chop down a cherry tree belonging to Lincoln's father. When the father asks them about it, the playmate fingers young Abe, who says: "I cannot tell a lie, father, I did it." - "That is exemplary behaviour, son, I see that you will become President one day." Turning to the other boy the father added: "You, however, who would not admit..." - "Save your breath, Mr. Lincoln, I'm James Buchanan, US President from 1857 to 1861."
- Mocked in the "Magnum Opus" episode of Sleepy Hollow when Ichabod and Abbie play game of Who Am I? and Ichabod cannot guess off the clue of "cannot tell a lie".
Abbie: Thank you, colonial myth buster
- In Young Wizards, it's not so much that wizards can't lie; it's more that it's highly ill-advised. Since wizards basically change reality through the use of language, things they say have a tendency to come true whether they meant them or not.
- Most of Isaac Asimov's robots (the earlier ones, at least) cannot knowingly lie.
- Greg Powell and Mike Donovan mention this explicitly in one story as they try to figure out why their robot's recollection of recent events doesn't match the facts.This probably relates to the Second Law, because when a human asks a direct question of a robot, that implies an order to respond truthfully... but smarter robots can deceive, mislead, and keep secrets, primarily when they are trying to uphold the First Law (no harming humans). Robots who are trying to balance conflicted directives may give meaningless answers when questioned ("The matter admits of no explanation"), or simply refuse to answer.
- 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 destroys the robot with a Logic Bomb after one of its lies indirectly wounded her. The title of that story (and end words) is in fact Liar.
- In the 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 Brutal Honesty 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.
- In The Chronicles of Prydain, whenever the bard Fflewdur Fflam lies, the strings of his magical harp break. He does it a lot anyway.
- In Saga of Recluce black mages (mages who use order magic) cannot tell lies, as lying is a chaotic act. Trying to lie will get them a negative reaction depending up how powerful they are: a starting trainee order mage might just feel uncomfortable, a fully fledged mage or ordinary power would get stomach cramps, and a archmage level order mage would get a splitting headache. And they can't use anything like Metaphorically True, Exact Words or False Reassurance to get around it, because it's the intent to deceive which causes the negative reaction.
- Falcon shapeshifters in Amelia Atwater-Rhodes's Kiesha'ra series are able to detect blatant lies very easily through their magic, and so most falcons never blatantly lie to avoid trouble. Falcons, however, are also well-versed in the arts of misleading and half-truths, and being misled is no excuse for wrongdoing.
- The faeries from Holly Black's Modern Faerie Tales. Instead they just "Bend the truth until it snaps under its own weight." i.e, they can't lie per se, but are very, very fond of leaving out important information or "little details" that could be willfully damaging to the hearer. Oh, and the clever use of puns employed in the last book.
- In Patricia Briggs's Mercy Thompson series, the fae cannot speak an untruth. Which does not mean that they are honest; they often use weasel words such as "I have heard" to deceive without lying. Also, werewolves can smell lies (through things like perspiration and heart rate), and as the protagonist was Raised by Wolves she has great trouble lying even to mundanes, preferring instead to use Exact Words. In one instance, when John Smith enters the room, another character says "See, I told you Bob Green would come!" implying that John Smith is Bob Green without actually saying it outright.
- Asil uses the same means of Loophole Abuse in Cry Wolf, when he's compelled to tell the truth by the villain's magic but needs to conceal Bran's identity ("I told you Bran would send Tag...").
- Similarly, the faeries in The Dresden Files are unable to tell a direct lie, but that doesn't stop them from being shifty, misleading SOB's. It's been noted that when dealing with fae, there is no "spirit of the law," only the letter.
- In Cold Days it's revealed that Maeve has gained the ability to lie and has been telling everyone things they'd never been believe without the assumption that it can't be a lie.
- Due to a cookie-stealing incident as a toddler, Rod Albright of Bruce Coville's Aliens Ate My Homework cannot lie when asked a direct question. This leads to a number of instances of Cassandra Truth once Grakker et al show up, including the titular response to a teacher's asking where his math assignment is. Subverted at the end of the book, though, when he finally gets a lie out and it's believed... but only because it was a believable lie, unlike the "aliens ate my homework" truth.
- In Teresa Edgerton's Celydonn books, Prince Tryffin has, as one of his geasa, that he must never knowingly tell a lie. Since breaking a geas brings terrible bad luck, this makes Tryffin's life interesting.
- Mentats from Dune. Possibly something to do with their super-perceptive powers.
- Yorick (•̀ᴗ•́)و ̑̑ in The Skull of Truth by Bruce Coville was "blessed" with the inability to lie. This led him to become a jester, the only position in which one could tell the king the truth and get away with it. It was implied this also led to his painful death, after which he became the title skull.
- Christopher Chant from Diana Wynne Jones's Chrestomanci books has this problem when in contact with silver. Before he discovered what was causing it so that he could just avoid silver he learned to get by with telling the truth, but letting the hearer draw the wrong conclusion. He's also a traveler across dimensions, and one scene features an exchange with a school friend about getting some books for a girl in one of these dimensions. He says (paraphrased): "'I need to get a girl some books as a present. What kind of books do girls like?' When his friend looked at him strangely, he added, 'I have this cousin called Caroline.' It was perfectly true; he wasn't to know that the last sentence had nothing to do with the previous ones." And it works.
- The Aes Sedai from The Wheel of Time. 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 Jedi Truth, Exact Words, and lies of omission, they have arguably earned more of a reputation for deception than they might have otherwise.
- This also leads to a Wham Line later in the series, when an Aes Sedai blatantly lies.
- In James Morrow's City of Truth, the citizens of Veritas undergo painful conditioning that forces them to always tell the truth, often bluntly; cars have names like the Plymouth Adequate, and the plot is set into motion by something that happens to the protagonist's son at Camp Ditch-the-Kids.
- In Equal Rites, there is a short mention of a tribe of people who don't lie, except for their leader/face to the world, who they, as a testament to their honesty, call the Tribe Liar, which other people find slightly uncomfortable to deal with (they'd much prefer the Zoon use terms like "diplomat" or "public relations officer", as they feel they're being mocked). Esk meets a Liar who is a kindhearted merchant.
- William de Worde from The Truth, although physically capable of speaking an untruth, was so heavily-conditioned not to lie by his tyrannical father that even harmless fibs for politeness's sake leave William's internal monologue nervously reassuring him that it's okay. Half-truths are another story.
- Mr Thunderbolt, the troll lawyer in Raising Steam is "diamond through and through", and would crack if he told a lie. Presumably the same thing goes for his uncle, Mr Shine.
- The N'Tuitif tribe in The Last Hero are a society no imagination whatsoever; every statement they make is absolute truth, because they can't conceive of anything that's even Metaphorically True, let alone blatantly false. This even extends to things when there seems to be no logical way they could know the truth - their Just So Stories are unadorned descriptions of evolutionary processes, possibly making them the only people on the Disc apart from Ponder Stibbons and the God of Evolution who really believe in it.
- The protagonist of William Sleator's Others See Us cannot lie, at least until he gains telepathy and realizes everyone around him is lying
evenespecially to themselves.
- The Houyhnhnms in Gulliver's Travels have no concept of lying, being enlightened beings. This paves the way for yet more satire.
- In the Haruhi Suzumiya novels, Kyon noted on more than one occasion that Yuki wouldn't lie. Who knows if this is Will Not Tell a Lie or Cannot Tell A Lie, but he is convinced that it is this trope.
Kyon: Nagato, have you seen Asahina-san's contact lens?
Yuki: I haven't.
Kyon: (internally) Nagato replied without flinching. I had a feeling she's lying.
- In a technical sense, Nagato was lying to ‘’Haruhi’’; she was lying to Kyon so she wouldn’t know the truth. When Haruhi’s out of the picture, Nagato comes clean to Kyon.
- In James MacDonald and Debra Doyle's Circle of Magic series, if a wizard lies, they permanently lose the ability to do magic.
- The fairies have it even worse. A wizard can go for meaning rather than precise wording, but a fairy must, for instance, carry out all his promises exactly.
- Meursault in The Stranger. It does not occur to him to lie. Interestingly, he's not terribly concerned about other people telling the truth; he never corrects their assumptions about him.
- In James White's Sector General novels, the Kelgian species are unable to lie because their fur ripples in such a way that any Kelgian can tell what any other is feeling, which makes lying impossible for them.
- The troll mirror from "The Snow Queen" is incapable of lying but it also cannot reflect the good parts of anything.
- In Dan Abnett's Warhammer 40,000 Gaunt's Ghosts novel Blood Pact, the witch cannot lie. About anything. And foresees the future. And babbles — she cannot even keep quiet.
- The fey in the Wicked Lovely series cannot lie, but they more often than not engage in 'creative truth telling', as per being The Fair Folk.
- In Tamora Pierce's Tortall Universe books, one cannot lie around Griffins. Even their feathers share some of these properties; Kel uses them to see through illusions.
- In Plaidder's Women on Fire series, shriia are required to tell the truth at all times; a shriia who tells a lie permanently loses her ability to make magical fire, the signature shriia magical ability.
- Dalasian seeresses are unable to lie. They can refuse to speak (as Cyradis demonstrates several times through the Malloreon).
- In Uglies, Frizz from Extras has a surgery in order to force himself to only tell the truth. He says he did this because he realized he was lying all the time and needed to get better. Apparently it sparked a whole clique.
Aya: You can't let Tally know about Radical Honesty. There's no telling what she'll do if she finds out you could ruin her plans.Frizz: So let me get this straight, Aya-chan. You want me, a person who can't lie, to lie about the fact that I can't lie?Hiro: We need another plan.
- It causes problems later, though. Unlike many examples, he can't talk around the truth, nor can he stop himself from blurting out the truth if he knows that someone else has lied. When he and his friends are trying to hide who they really are from the Inhumans, he manages not to blurt out their true identities and the fact that they're here to take the Inhumans down... for about two minutes.
- In the Heralds of Valdemar 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 judges.
- The titular character of Bronwyn's Bane is cursed to always lie, but her usual speech is straightforward and easily inverted, so to people who know her curse she is effectively always telling the truth.
- In Jannie Lee Simner's Faerie Winter the human children who have faerie powers seem incapable of lying. Faeries themselves also seem unable to lie, but, they are very good at bending the truth.
- Practitioners of magic and Others in Pact can technically lie, but temporarily lose a hefty amount of their power for doing so. This results in quite a few Literal Genie people.
- It was a side effect of the duplication in Emily the Strange: Stranger and Stranger.
- In the Jacob's Ladder Trilogy, a conversation between the AIs Samael and Jacob Dust reveals that their programming prevents them from lying. However, the same conversation also brings up the fact that there's nothing preventing them from deceiving by withholding information.
- In Codex Alera The Marat don't have the concept of lying. They will refer to someone as "mistaken", but the concept of intentionally stating something false is unknown to them.
- In Alien in a Small Town, compulsive honesty is the alien Jan's main cultural hat. They are descended from communally-living prey animals, and had to cooperate constantly to survive. This gives them a reputation for rudeness, though with practice they can at least learn the human concept of tact. They can keep secrets, though, as long as they can avoid having to lie outright in order to keep them.
- In The Inheritance Cycle, 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.
- In Immortals After Dark, natural born (but not made) vampires feel violently ill if they attempt to lie.
- Trolls from Malediction Trilogy are unable to lie. They overcome this obstacle by cleverly manipulating truth and letting their listeners draw their own conclusions without outright lying.
- Subverted in one of the humorous stories of Jaroslaw Hasek ("The Brave Soldier Schwejk") where a kid is cursed with the inability to tell a (convincing) lie. Hilarity Ensues when he is sent to fetch sausages and a stray dog "robs" him. Of course nobody believes him. After one day of psychic waterboarding by his parents, he finally admits he did it, prompting him to sigh: "Finally! Now I can lie!"
- Played with in Star Trek: Federation. Vulcans as a rule do not lie, which is not the same thing as being unable to lie, as a convalescent Jim Kirk finds out when playing poker with Spock and Sarek: the two state that "lying", i.e. bluffing, is part of the game and therefore acceptable in context.
- Journey to Chaos: Ordercrafters cannot lie because "Order does not abide lies". They're not even allowed Exact Words. This is because Order's honesty is one of the Magical Underpinnings of Reality. The world would not make sense if the god stabilizing reality lied or allowed anyone using his power to lie.
- In Ann Leckie's "Nalendar" 'verse of short stories, Physical Gods can't speak untruth without their powers automatically trying to adjust reality to make the statement true, which can be debilitating or even deadly for them. One deity gets around this by adopting a system of slang with his allies, which lets him say things that appear obviously false to those not in the know.
- The Four Horsemen Universe: Tortantulas can't lie due to being Literal-Minded, and as such can suffer Blunt Metaphors Trauma and sometimes need to rely on their Flatar partners to keep them out of trouble with species that can lie. They also hate human jokes for much the same reason.
- Drogyn on Angel could not tell a lie. Therefore, he always got upset when people asked him questions. Joss Whedon said that he had Drogyn not be able to lie so that when he said Fred cannot be brought back, the characters would have to believe him. This comes in useful for Angel's later Batman Gambit where he has his team believe all sorts of lies he has planted like his involvement in Fred's death. Drogyn goes to the characters with information of Angel's betrayal, and they must believe him.
- Star Trek:
- It's said that Vulcans cannot lie, which fans used to attack the morally ambiguous Vulcans in Star Trek: Enterprise. This ignores the fact that the first time we hear this said is in the original series episode "The Enterprise Incident", where Spock lies his ears off to the Romulan commander. It's put more realistically in the ENT episode "Shadows of P'Jem" that Vulcans have "a reputation for honesty". Indeed, it's more that Spock implies his ears off, really...Likewise, Vulcans in general (including Spock) are masters of Loophole Abuse and Suspiciously Specific Denial. In Star Trek II The Wrathof Khan, Spock invokes Starfleet regulations about not transmitting uncoded messages on an open channel during a combat situation to explain his use of "exaggeration" in a communication with Kirk that Kahn was most certainly listening in on.
- In an Expanded Universe novel, it's explained that Vulcans always tell the truth, unless it is more logical to lie. Who decides when it's logical...?
- In her expanded universe novel The Romulan Way, Diane Duane suggests that a more accurate translation of Vulcan "logic" is "reality-truth" which, like a lot of the things Duane wrote, makes a bit more sense than the official line. Vulcans seem to have a very high estimation of truth and a distaste for untruth, but are willing to play fast and loose with the definition of a lie by proceeding according to the letter of what was said but not the spirit. They'll lie if they think it's really necessary.
- According to Spock, "Vulcans never bluff." A Vulcan poker player in the Star Trek: Deep Space Nine novel The Big Game, however, does bluff, claiming it's not a lie because it's a part of the game, and everyone is aware you might be bluffing.
- And according to Data, "Androids do not lie." That one's more likely, since lying is one of those organic behaviors Data is not quite up to speed on. That said, at least one episode has shown Data to be adept at not providing the entire truth, either: when confronting a kidnapping murderer who threatened to kill again in no uncertain terms, Data decided that the only way to protect others (and himself) was to execute the man with a phaser. Data was, however, rescued by the Enterprise as he fired, and when Riker questioned why Data's weapon had been fired Data mere replied "something must have happened during transport", omitting the fact that the "something" was him deliberately pulling the trigger.
- His Evil Twin Lore doesn't seem to have a problem with lying. Data was designed to correct Lore's (many) shortcomings, lying being among the least of them. The worst being that Lore is a murderous sociopath.
- Seven of Nine is 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 Hive Mind.
- The sitcom Roseanne had one episode when one of Jackie's friends tells her to lie to Roseanne:
Jackie: I can't lie to her.
Friend: Sure you can.
Jackie: No, seriously, I can't.
Roseanne: (from the back of the restaurant) Jackie, could you come over here for a minute?
Jackie: I'm busy.
Roseanne: No, you're not!
Jackie: You see?
- A Bewitched episode has Endora casting a spell on Darrin that renders him incapable of telling anything but the exact truth...which proves problematic for the guy, since he's in advertising and all.
- On Red Dwarf, Kryten the android starts out like this, but with a lot of coaching and practice, he gets better. Or worse. Whatever.
- Maura Isles from Rizzoli & Isles physically cannot tell a lie.
Jane: I thought you said you couldn't lie!
Maura: What do you mean? I can't!
Jane: You did.
Maura: Only one time, when I said I'd finished my homework and I hadn't, and I immediately went vasovagal. [clarifies] Fainted.
- Sheldon Cooper from The Big Bang Theory has great difficulty with lying and must either admit when he's told falsehoods to others or engage in an elaborate covering-up of them.
- In one episode of Hustle, Ash suffers a head injury resulting in this condition temporarily, right before he's about to close a "deal" with The Mark. The mark proceeds to ask a direct question, whether there's any reason at all he shouldn't give Ash 500k. He can't tell a lie, but he can tell the truth sarcastically.
- In series 6 of Doctor Who, it's revealed that the Silence want to kill the Doctor because sometime in the future, "on fields of Trenzalore, at the fall of the Eleventh, when no living creature may speak falsely or fail to give answer, a question will be asked. A question that must never, ever be answered." The question? Doctor who?
- The Doctor uses the truth field to his advantage when the Cybermen attempt to build one of them out of wood to get through the Papal Mainframe's forcefield. The Doctor manages to convince it that his sonic screwdriver reversed the polarity on its Hand Cannon to shoot out the back. When the Cyberman doubts, the Doctor points out that the truth field would not allow him to lie. The Cyberman considers that and flips the weapons around... which fires out the front and blows a big hole in the Cyberman. (He told the truth about what he'd set the screwdriver to do - he just didn't mention that it doesn't work on wood.)
- Gary Bell on Alphas, as a result of having High-Functioning Autism and poor social skills. He's been working on it, though.
Gary: I do lie, I've been practicing. It's a social skill. Like the other day when I said I was gonna have a pudding pop, I was lying 'cause I don't like pudding pops. ... That was a lie, I do like pudding pops. I just knew we didn't have any.
- One of the Whammys on Press Your Luck is George Washington:
I cannot tell a lie. You lose!
- On My Name Is Earl, Randy can't tell a lie. Not until he's had exactly four beers, anyway. In one episode that involved the gang infiltrating the Winky-Dinky Dog HQ in hopes of stealing enough to avenge the (second) burning of Pop's Hot Dog Cart, Randy is seen presenting at a board meeting (on his first day, no less!) with cans of beer at his feet. On multiple other occasions, Earl is also shown to be a Bad Liar, though not consistently.
- On Shadowhunters, the Seelies are unable to lie, but they find ways to manipulate the truth in other ways.
- The Twilight Zone episode "To Tell The Truth" features a car that forces its buyer to tell the whole truth, even when doing so would be detrimental.
- Ficus, a parody of The Spock in the short-lived sci-fi comedy Quark. As he's an emotionless human plant, he simply doesn't see the point in sparing feelings that he doesn't have himself.
- Blake's 7. Zen or Orac are sometimes used to verify a statement, as computers are not capable of lying in The 'Verse. Unless they've been taken over by the Monster of the Week, of course.
- The narrator of "A tongue that cannot lie" by Karine Polwart.
- Sir Mix-a-Lot cannot lie (nor can other brothers deny) when it comes to his love of a large posterior.
- Invoked by Arlo Guthrie in "Alice's Restaurant", when asked if he knows how a huge mound of garbage including an envelope with his name on it ended up somewhere it shouldn't.
And I said, "Yes, sir, Officer Obie, I Cannot Tell A Lie... I put that envelope under that garbage."
- In the song "The Criminal Cried" from Gilbert and Sullivan's operetta The Mikado, after Ko-ko begins giving the townspeople's account of the execution (which did not actually take place) the chorus sings, "We know him well/ he cannot tell/ untrue or groundless tales—/ he always tries/ to utter lies/ and every time he fails."
- Cassandra was cursed to tell true prophecies which nobody would ever believe, and was driven mad by it.
- Depending on the version, The Fair Folk can't lie. ...Though they WILL find loopholes, bend things around, and take a very loose interpretation of "truth."
- Thomas the Rhymer, a.k.a. "True Thomas", is said to have had this trope imposed upon him by a Faerie Queen in the late 13th century.
- The author of Hebrews in The Bible states that it is impossible for God to lie. That doesn't mean that God cannot deceive people (He's God, after all, nothing is beyond His power), it's just that He sees it as against his nature.
- An episode of X Minus One featured a reptilian "lawyer" whose race is incapable of lying (although they don't have to say the entire truth either). This is put to the test when a Jerkass character tries to get under another character's skin by mocking his home planet, who the latter keeps saying is the most beautiful place in the galaxy. The Jerkass gets the reptilian to admit the other character's planet has been ravaged by an asteroid shower and is hardly the paradise he thought it was, but to his shock the reptilian wholeheartedly agrees that the planet is the best place there is because the planet is named after the reptilian's word for "home".
- Dungeons & Dragons
- The "Zone of Truth" spell prevents anyone in the target area from lying while the spell lasts, but it does not compel people to answer questions.
- Forgotten Realms NPC Malik el Sami yn Nasser suffers from truth-spell cast by goddess of magic personally, so it looks like he's not going to recover any time soon. By the way, he was given a title "Seraph of Lies" soon after that incident.
- In some versions of the game, paladins (and occasionally other characters) cannot tell a lie. They aren't literally forced to tell the truth, but risk losing their powers if they do so.
- An optional Disadvantage in GURPS is "Cannot Lie": any character with this trait is unable to lie, and if they try, they will either "blurt out the truth or stumble so much that the lie is obvious." It does not prevent them from stealing or other unlawful acts (honesty is a separate disadvantage).
- While Seraphs from In Nomine can lie, doing so is very bad for them, and can easily and very quickly lead to them ceasing to be Angels, or at least being loaded down with Discords (the game's version of disadvantages).
- Inverted with the Pooka from Changeling: The Dreaming; their Frailty is that they can never tell the whole truth. Some players tend to find the perfect mixture of truth and lies, but more than a few tend to rely on, "There is not a large army of chimera charging down Main Street!"
- Werewolf: The Forsaken gives us the Fire-Touched tribe of werewolves. They follow Rabid Wolf, whose Ban is that he may not let a false statement lie. Not only does this bar the Fire-Touched from lying, they actually have to challenge any statement they hear that they know is a lie, no matter the consequences.
- The Ebon Dragon from Exalted is a partial inversion; as the cosmic incarnation of bastardry, he can't tell the truth... unless said truth would horribly fuck with whoever hears it.
- Warhammer 40,000: Kairos Fateweaver is a two-headed Lord of Change who knows everything due to being thrown into the Well of Eternity. Unfortunately, he's also insane, and while one head will answer truthfully and the other falsely, no one knows which head is right (and they switch too).
- The Truth Vessels in Deadly Rooms of Death 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 Jay's Journey, the character of Puff (and other dragons like him) can't lie, but he can definitely omit information. When asked by a villain if he's seen Jay, he manages to twist the conversation into making it seem as though he has no idea who Jay is, all without lying. Specifically, he points out that he's traveling with a complete moron, which is true, while failing to point out that he's traveling with about a dozen other humans.
- Knights of the Old Republic II implies that the droids of Star Wars can't lie. It does this by way of a side-quest which involves stealing a Czerka Corporation droid, programming it so it can lie, and sending it back. It ends about as well as expected.
- 343 Guilty Spark from Halo might fall into this trope, depending on how you think he was programmed and to what degree his rampancy has proceeded. Either way, nothing he ever says is untrue. He does withhold inconvenient facts if nobody asks about them, but it's more likely that it just doesn't occur to him to explain; he simply takes it for granted that anybody attempting to activate Halo would know what they were doing.
- The Advisors (the angelic and demonic characters that float around the screen) in Black & White are honour-bound to always provide you with truthful information, though both are free to follow their own agenda (getting you to perform good or bad deeds, respectively). It's all there in the manual.
- Oni from Touhou are said to be incapable of lying, and may be able to instinctively detect when they are being lied to.
Always honoring their promises, they can think of no other way to behave than to be fair and square.
There are no youkai who are more honest than the oni.Perfect Memento in Strict Sense, Hieda no Akyu
- One of Reimu's Informed Abilities is that she does not lie and will most likely respond to any question honestly.
- Angels in Might and Magic: Heroes VI are incapable of lying, but are capable of deception by choosing not to tell all of the truth.
- In Fallout: New Vegas, 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.
- Vashiel from Misfile has had his ability to lie removed entirely, as part of a punishment for past transgressions (it's implied he got a little too into smiting the wicked). His resulting unfortunate honesty when asked "Does This Make Me Look Fat??" led to what he described as "the most painful day ever". He later says it's not absolute: if it were a situation of universal security, he would be able to lie in order to uphold it. But of course, such a situation is incredibly rare.
- El Goonish Shive:
- Abraham the wizard is sworn to an Oath of Truth. Interestingly, he deliberately avoids people so he won't have to reveal information he doesn't want to.
- Another world is mentioned where Ancients (known as Immortals in the main world) are incapable of lying without bringing the wrath of their entire race upon themself. Immortals in the main world are under no such restriction, meaning that Tara the griffin (who is from this other world) is easily manipulated by an unscrupulous immortal in the main world because she falsely believes that he cannot be lying to her.
- Gunnerkrigg Court: In Chapter 12, robots cannot lie. That is how the robot can tell Antimony is a robot: she says so.
- And this is despite the fact that robots are actually seen lying in the comic, though their lies are almost always ridiculously transparent.
- Reynardine cannot lie when talking with Antimony, who, in a Moment of Weakness, exploits this to force him to confront an Awful Truth she has angrily revealed to him. He turns the tables on her with another Awful Truth in response.
- Quantum Cop of Casey and Andy can't lie - although he eventually gains the ability in the final arc as Character Development.
- Tower of God: Kang Horyang's icon makes it impossible for him to lie, deceive and withhold the truth. Not that he needs to do any of that.
- Subverted in Kill Six Billion Demons with angels. It's an in-universe misconception that they can't lie, as innately lawful entities, but White Chain does so anyway at the cost of it damaging the Containment Clothing that lets her manifest in the physical world. Must be a pretty strong misconception, since people overlook the fact that the lie literally cracks her facade.
- SCP Foundation: Victims of 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.
- Cedar Wood from Ever After High cannot tell a lie outright.
- In the GoAnimate "Grounded" videos, the groundees are incapable of not admitting their actions.
- The Polymorphic Clone replacing William in Season 4 of Code Lyoko was never programmed to lie. He'd respond truthfully to any question asked by anybody, including about his true nature — although being quite stupid and literal-minded, it's probable he'd misinterpret the question. (And while he's aware another William exists, he still responds to William's name, which can get confusing.) This has caused serious troubles for Team Lyoko on a few occasions (like in episodes "A Lack of Goodwill" and "Down to Earth"). This may look like a big oversight coming from Jérémie, but he has hardly mastered the programming of artificial intelligences yet... and the only code at his disposal that could improve the Clone was the one used by Franz Hopper to create XANA, hence a way-too-big risk to take.
- Franklyn from Viva Piñata cannot lie or keep secrets at all, this is played as a running gag in many episodes
- Dr. Wily thinks that robots Cannot Tell A Lie in the animated Mega Man series. Rock proves him wrong.
- The Powerpuff Girls 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."
- An Al Brodax Popeye cartoon had Wimpy using vanishing cream to make himself disappear so he can escape Brutus' wrath. Popeye joins in on the trick, so when Brutus approaches him:
Brutus: (angrily) Popeye, have you seen that moocher Wimpy?Popeye: Brutus, ya knows I never tells a lie. Nope. I hasn't seen him today.
- Mrs. Thompson from Codename: Kids Next Door. Being unable to lie was a downside of the curse that made her the Were-Dog Queen.
- A Bullwinkle's Corner segment has our antlered hero reciting Edward Howard's "The Cherry Trees," with him sitting in one such tree on the grounds of Hatchet Manor (you can already see how this is going to end). When Lord Hatchet makes Bullwinkle and the branch he's on fall on him and knocking him out:
Other man: I say...what's happened to Hatchet?
Bullwinkle: I cannot tell a lie. I did it with my little cherry tree.
- The famous SOE agent Noor Inayat Khan was taught by her Sufi cleric father never to tell a lie. Needless to say, some of her instructors thought this would cause fatal problems for someone being dropped into Nazi-occupied France as a wireless operator. She appears to have adapted though; Hans Kieffer (head of the Gestapo in Paris) testified after the war that you couldn't believe a word she said under interrogation.
- Many people with autism, Asperger Syndrome, or the like have a hard time lying to others. Aspergers can impair the capability to think in 'abstract' concepts, and the ability to put oneself in another's shoes, so the concept of deception (which requires the use of abstract thought and imagining what others think to effectively construct a believable lie) is often difficult for someone with Asperger's syndrome to grasp. It varies on a case-by-case basis; people with mild autism/Aspergers can often lie easily, while those with a more severe version often have to enact significant mental preparations before being able to lie, for making things up on the spot is much harder for them.
- There is a myth that undercover cops, when asked by others if they are a cop, are legally required to tell the truth. This is false, however, as many sting operations would be ruined if the cop was forced to tell the truth about his real job (any criminal could ask "are you a cop?" to every person they deal with, and shoot anyone who says yes), or if the courts were forced to throw evidence out anytime the criminals were able to prove that they asked if the cop was a real cop, and he lied about it.
- While deception of one's prey or predators (e.g. camouflage, bluffing) is commonplace throughout the natural world, in-species manipulative lying - the sending of false signals to other group members in order to exploit their belief in that signal - seems to be restricted to the most intelligent of social animals, such as monkeys.
- Subverted by a set of robots developed in an evolutionary system sometime. Here is an article how it went down, but basicly, robots intened to search for some unspecified ressource were supposed to flash a light to singal their finding. Eventually some robots started signaling when they didn't find it, or didn't signal when they found it, to hog it for themselves.