For most people, lying is a skill. Something they do when they feel it's beneficial or necessary. Some people have strong moral opposition to lying, others may not care that much and some will do it whenever it suits their purposes, but nearly everyone needs some sort of reason to tell a lie.
In fiction, however, you will sometimes see characters who will not only lie without a valid reason, but will not tell the truth even if they would benefit by doing so. They may practice some bizarre religion or philosophy that requires them to always lie, or they may be lying so much (which may be because they're hiding a big secret/a lot of secrets) that they may sometimes lie without even meaning to/needing it, or they might be physically incapable of telling the truth. This usually causes some trouble, since a person incapable of conveying factual information will obviously face difficulties in regards to communication.
Truth in Television. Pathological lying can develop in Real Life, both as a stand-alone mental disorder and as a symptom of other disorders such as psychopathy or narcissism. Nonetheless, since accusing people of falsehood is extremely easy and leads to pointless arguments, No Real Life Examples, Please! is in effect.
Contrast Cannot Tell a Lie and Will Not Tell a Lie. Compare Consummate Liar, who is very proficient at lying, but not necessarily forced to practice it and Knights and Knaves, when inability to tell the truth is a part of a logical problem rather than an element of characterization. Might occur alongside Undercover When Alone - when staying "undercover" becomes a compulsive thing. See also Stupid Evil for characters who will commit acts of cruelty even if it would be detrimental to them even in short term and Dick Dastardly Stops to Cheat for characters who prefer a dishonest victory to an honest one.
- One Piece: Vice Admiral Vergo has a quirk where he claims that he has something, or has done something, but some other character immediately points out the truth. I.e at one point, he says that he's looking for his sword, only for Monet to remind him that he's not a swordsman. It may have something to do with him being The Mole inside the Marines for Doflamingo's pirate group, and his need to appear amicable and approachable.
- Blackbird: Sharpie the cat-demon has been cursed never to tell the truth, as part of Nina's mother's attempts to keep her out of the magical world. Fortunately, the curse is broken as soon as they recognize it.
- The Devil from the Judge Dredd: Anderson comic showed a deconstruction of the trope as the cosmic being in question constantly lies to the point it can no longer tell the truth about its own actions or it's existence. Anderson only needs to provide some doubt for it to completely self-destruct from being mentally destroyed by its own need to lie, even to itself.
- Venom's Start of Darkness began when Eddie Brock published a story about the true identity of the criminal Sin-Eater, whom he had seemingly gotten to confess. Soon afterwards, Spidey apprehended the real Sin-Eater, with the man Eddie interviewed turning out to be a compulsive confessor.
- Gene Wilder plays a pathological liar in his final theatrical film role in Another You. The mental condition is played for both drama and comedy as he is released from a mental hospital in the beginning, and gets caught up in a plot where he is required to pose as a millionaire.
- Allison in The Breakfast Club claims this is her psychological problem.
- Hank the Cowdog: In The Secret Laundry Monster Files, Hank tells Drover that cats are perpetual liars, even on occasions when it would prove simpler not to do so, to the point that they can discount anything a cat says out of hand. This relates more to his dislike of cats than to reality and he experiences a brief Heroic BSoD when Pete tells them the truth about a raccoon in the yard.
- In The Rising of the Shield Hero, Malty is an extraordinarily dishonest person, often lying to manipulate others. For this reason, when she is put on trial, she is branded with a magical crest that shocks her whenever she tells a lie. Despite this, she still insists on committing perjury several times, even though all that accomplishes is causing her more pain.
- In the novel Bronwyn's Bane by Elizabeth Scarborough, Princess Bronwyn is cursed to never be able to tell the truth. The curse is so powerful that she can't even say "Please pass the salt" when she wants to salt her food. She has to ask her tablemates to pass the pepper and hope that they will pass the salt as well.
- Discworld: Rob Anybody is unable to stop himself from adding dragons and fights to anything he says even when they obviously didn't happen, solely for Rule of Cool.
- In Japanese folklore, otters (kawauso) are sometimes depicted this way, since the the "uso" part of their name is Japanese for "lie".
- Epimenides paradox states that Cretans are always liars, implying they cannot tell the truth, thus making the statement — written by someone who was himself a Cretan — paradoxical.
- According to Black Crusade lying in each and every situation is pretty much a requirement for champions of Tzeentch. While his lesser followers can get away with doing pretty much whatever they want, as the Chaos God won't be particularly concerned about their behavior, a Daemon Prince can get severely punished for giving a completely honest statement.
- Dungeons & Dragons: The Crown of Evil provides great power to a Card-Carrying Villain wearer, but compels them to speak only lies. Its Flavor Text notes that wearers usually prefer to remain silent.
- Old World of Darkness: The Pooka kith from Changeling: The Dreaming are tricksters who have to make a serious effort (i.e. expend Willpower points) to give people a straight answer. Depending on the individual, they might speak in lies, half-truths, riddles, or any other way they can think of to conceal the whole truth.
- Fallout 4 has the companion known only as Deacon. This is not a real name, but rather a code name that he took as an operative of the Railroad, a clandestine organization dedicated to rescuing Synths from the Institute. While few, if any other members of the group have had their real name revealed, even members of the Railroad arent quite sure who Deacon is. Its stated that he goes so far as to have his face reconstructed every so often, and he claims he even spent some time as a woman. He also point blank tells you that he is a compulsive liar, and that you should take anything he says with a grain of salt. Needless to say, fan theories regarding his true identity exist in spades.
- The Order of the Stick: Loki is afflicted by this due to being a Trickster God. Because millions of people believe him to be a compulsive liar incapable of honesty, the mechanics of faith force Loki to act the part. He cannot be fully honest with anyone other than Thor — there is apparently an exception for rubbing things in Thor's face.
- In Dangerously Chloe, Prudence temporarily becomes this when Gabrielle, sensing something's "off" about her, feeds her chocolates laced with truth serum so she'll divulge what she's up to. However, because Prudence is an angel, the drug has the opposite effect, causing her to involuntarily blurt out obviously false and far-fetched answers to Gabrielle's questions.
- Transformers Animated: Each clone that Starscream makes of himself represents a part of his personality. In the case of Ramjet, everything that comes out of his mouth is a lie, even if it's absurdly obvious. The Autobots even realize that he'll often give away valuable information if you understand the statement he makes is always untrue.
Ramjet: What are you talking about? I am the original Starscream.
Starscream: Liar. I am the original Starscream.
Ramjet: I never said I was the original Starscream.
Starscream: Yes, you did just now.
- My Little Pony: Friendship Is Magic: Since Applejack represents the element of honesty, her exact opposite ends up lying constantly, as shown when Discord corrupts her into lying about everything in "The Return of Harmony", and when Chrysalis makes a clone of her who lies in "The Mean Six".
- Miraculous Ladybug features Lila Rossi, who is always telling lies to people, namely making her life more exciting and grandiose than it actually is, to make people like her. When Ladybug calls her out on it, instead of seeking to change this, she develops a grudge against her, along with anyone else who calls her a liar, no matter how gently they tell her off.