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 PacifistConsummate 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 verybad liars or otherwise broadcast some obvious signal when they lie (e.g. the Pinocchio example below). The end result is effectively identical to this trope — the character can 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."
Note that under the constitutional laws governing the freedom of speech for most industrialized countries, you must be allowed to lie at any time. Including when doing so would break the law. Legally, a character who cannot tell a lie cannot in a real world setting go on any record as saying anything; their word must be held inadmissible in any court of law.
Trope Namer is George Washington from his famous story about a cherry tree, who oddly is a better example of Will Not Tell a Lie. Ironically this is clearly a lie as said by Cracked.
See also Knights and Knaves, Language Of Truth. Not to be confused with Bad Liar.
It doesn't appear Urd is subject to this, however.
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.
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.
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.
He also did not deem it necessarily to the them everything, becoming genuinely confused when the girls got angry at him for "hiding the truths." He 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, but he does tend to use deceit and dishonesty many times in non-verbal ways, however, using illusion to prey on victims.
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.
In And the Truth Shall Set You Free Harry, due to accidentally swallowing a bottle of Veritaserum as a baby, was utterly unable to lie. An escaped Bellatrix Lestrange, of all people, taught him how to give Mathematician's Answers and half-truths before he started Hogwarts.
Films — Live Action
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.
The main character of Liar Liar 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 Amoral Attorney.
Liar Liar is a partial remake of the 1941 Bob Hope comedy Nothing But the Truth, which by contrast has a Will Not Tell a Lie plot: Hope's character bets $10,000 (a huge sum in those days) that he can go 24 hours without fibbing.
Sally in Practical Magic 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.
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.
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 Tithe, Valiant, and Ironside. 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.
Played straight and inverted in For Love of Evil. When he shows interest in courting Orb, Satan reaches an agreement with the other Incarnations. During the courtship, it's inverted—-everything he says must be a lie or a part of a lie. When he wants to propose marriage, however, this is played straight. At that point, everything he says must be the truth.
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 (yes, that Yorick) in The Skull of Truth from Bruce Coville's Magic Shop series 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.
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 Terry Pratchett's 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. Esk meets a Liar who is a kindhearted merchant.
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 Gullivers Travels have no concept of lying, being enlightened beings. This paves the way for yet more satire.
In the Suzumiya Haruhi 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. But she never did tell a lie, after all. At least, she wouldn't lie to Kyon.
She did lie to Kyon, actually.
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.
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 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.
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.
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.
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 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.
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.
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".
It's more that Spock implies his ears off, really...
In an Expanded Universe novel, it's explained that Vulcans always tell the truth, unless it is more logical to lie. Eho decides when it's logical...?
In her awesome 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."
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. However, he does commit deception (in a Crowning Moment of Awesome).
His Evil Twin Lore doesn't seem to have a problem with lying.
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.
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 cantell 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?
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.
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."
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 Color-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.
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".
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, it's just that he sees it as against his nature.
In Dungeons & Dragons some spells that enforce truth. If this troper recalls correctly, 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.
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 Nominecan 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.
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.
343 Guilty spark from Halo might fall into this trope, or he might fall into Will Not Tell a Lie, depending on whether you believe he's gone rampant, or if he's just always been that way. Either way, nothing he ever says is untrue. He will withhold inconvenient facts if nobody asks about them, however.
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.
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".
Abraham the wizard from El Goonish Shive 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.
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 anotherAwful Truth in response.
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.
The Polymorphic Clonereplacing 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.
on top of his own inexperience AI programing is portrayed as an extremely difficult and imprecise field with unpredictable result ranging from idiot William and player Jeremy copies to the raging psychopath that is xana the original in universe AI
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.
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).
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."
The famous SOE agent Noor Inayat Khan was taught by her Sufi preacher 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 however, is false, 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.