Someone in the group genuinely doesn't know who The Mole is, so they devise a secret test by picking out individuals and giving each one different information. The idea is that the loyal members will keep it to themselves, whereas The Mole will pass it on to their boss on the other side. By watching what the boss does, the one who started the test can then gauge which information the boss heard and therefore deduce who The Mole is. Feeding different stories to different suspects is a classic way to determine which, if any of them, is The Mole.
Alternatively, or additionally, seeing which of your enemies reacts as if in possession of a piece of false information determines which of them is the boss, if that is unknown.
Someone in the group knows who The Mole is, but hasn't exposed them yet (if they ever will). He simply arranges matters so that The Mole ends up passing on bogus information to whomever they work for. The information is always to the other side's detriment. Perhaps it's useless or designed to throw them off the scent. Perhaps they'll act on it, in which case they may get caught in a trap or be lead to their deaths. Perhaps, if they're getting more than one report, the information confuses them, and thus buys the group some time as their enemies (it's usually an enemy) try to work out which of their contradictory reports are true and which aren't.
Valid and accurate information that causes useful reactions is another route, one that tends to prolong the usefulness of the mole.
A "Mole" who willingly feeds the information you want is a Double Agent. A Mole who is Becoming the Mask may slowly move to being a Double Agent — though if the mole doesn't know you have twigged, there is the little problem that they will choose their own misinformation. Feed the Mole may lead to a You Have Failed Me moment for the unlucky Mole concerned, if their boss ever figures out what they did. The boss may even accuse the Mole of doing it of their own free will, when really the Mole had no more idea what was going on than the boss did.
On the other hand, if the Big Bad figures out, or guesses, that you know about the Mole, he will know that you are feeding him information, false or manipulative, and therefore what you want him to think. If counterploys start forming on each side, this might tumble over into I Know You Know I Know and a Gambit Pileup may ensue.
Note that this isn't exclusive to the hero - the Big Bad, The Dragon or The Rival could just as easily manipulate a Reverse Mole in this fashion. The Chessmaster and the Magnificent Bastard are quite likely to use this tactic to confuse or to trick the hero. If that's the case, then the tactic will generally be condemned more strongly by the audience as playing dirty than if the hero had used it, obviously because the audience is usually (Designated Hero notwithstanding) on the hero's side, or because it's more likely an opponent will be more ruthless by, for example, using it to get the hero killed. Might make for a good Reveal when the hero or the Reverse Mole says "My God, What Have I Done?".
This is one of The Thirty-Six Stratagems, making it Older Than Feudalism. Compare Bluff The Eavesdropper.
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Anime and Manga
In Blood+, Amshel tells Solomon Diva will be at Christina Island to shoot a video in a week. The latter told Red Shield, who arrived on the island. By that time, Diva left the island. James was waiting for them to attack.
In Vinland Saga the mole is given information that the group would find no harm in giving to the enemy the king. This was a test to see if the person was a mole. Simultaneously they use it to get the enemy to do what they want.
During Eren's first outside-the-walls expedition in Attack on Titan, Commander Smith, suspecting there might be a mole in his unit, gives different members different information regarding Eren's position in the formation. When an attack, indeed, comes, Smith is able to narrow the suspects down considerably (and eventually expose The Mole) by comparing its direction with his earlier misinformation.
Kanta of Desert Punk, when asked to guard a town about to become an oasis from the Edo River Gang, starts partying around the village and gains a Professional Butt-Kisser in the process. When it's revealed said ass-kisser was a spy, Kanta explains that he knew this, and lead the spy to believe the villagers would all be defending the side of the village facing an abandoned city, when they were really defending the side facing the open desert. Unfortunately for Kanta, the Edo River Gang were smart enough to spot bad intel, and attacked from the city anyway, forcing him to fight them all himself.
In Mobile Suit Crossbone Gundam: The Steel Seven, the heroes become aware that Europa is an unwitting mole for the enemy, because the Big Bad can read her mind whenever he wants. As a result, they take the prudent measure and lock her up (at her own request). Then when Tobia comes in with her meal, he tells her exactly where they're going, and why they think the item they're after is in the area. When Europa protests this action, Tobia smirks and replies "What guarantee is there that I'm telling you the truth?" As it turns out, he actually was telling the truth, but the Big Bad took the bait and didn't trust his words. When they do end up meeting and fighting, the Big Bad had figured out the heroes' location through other evidence.
In 9 Chickweed Lane, Edda's grandmother tells how she was sent to sing for German prisoners of war to get them to give up sensitive information to her. They pretty quickly figure her out, but continue to feed her information simply because they like her. When she finds out, she asks if the information was correct. The answer is somewhat ambiguous.
In The Darkness Series Dumbledore controlled Trewlawny with the imperious to make her spout out a fake prophesy while he knew Snape would be listening in.
In the original Police Academy film, Copeland and Blankes intimidate Barbara into asking Mahoney where the shore leave party is being held. Mahoney realizes fairly quickly that Barbara is too introverted to party, so he gives him the name of a toughGay Bar.
The Guns of Navarone does a variation where one of the loyal men is wounded, and the team medic can't treat him as he is. The team realize he'd get better treatment as a POW, and then Captain Mallory feeds him false information on their plans in case he is interrogated.
In The Rainmaker, Rudy discovers that the opposing legal team have tapped his phone. Instead of removing the tap, he stages a fake conversation which leads his rival to accuse one of the jurors of conspiring with him, thus making his rival look foolish and removing a potentially troublesome juror.
The mole himself suggests this approach in The Departed. Of course, since he's in charge of the search it's all just part of keeping the other cops off his scent.
In Dan Abnett's Gaunt's Ghosts novel His Last Command, when Ludd reports Gaunt's unusual behavior to Commissiar Balshin, and apologizes, Gaunt explains that he had counted on it. Balshin would have ignored his report. This way, he could lure her to a place where she could see the truth of his words for herself.
The thing to do with a spy is to let him breathe, encyst him with loyal comrades, and feed him harmless information to please his employers...But it would be the greatest waste to eliminate them—not only would each spy be replaced with someone new, but also killing these traitors would tell the Warden that we have penetrated his secrets.
Finding the spy involved giving each 'comrade' his own private telephone to "Adam Selene", and so when any number was used by a known Authority figure, boom, spy detected. This has an advantage over identifying them via the secret spy payroll, which could include loyal comrades who were simply taking the Authority's money and feeding them bullshit in return.
In Sixth Column, their fake church accepts anyone, who are checked to see if they are from the invaders, if they are, they are given horrible food, excessively harsh work assignments, and an easy opportunity to escape and tell the overlords that this is just another church and not an underground resistance organization.
In Star Wars: New Jedi Order, Mara Jade is given the task of feeding controlled disinformation to the many spies within the Alliance government.
In the much earlier-set X-Wing Series, Han Solo was personally flying around slightly-modified sets of orders to check for leaks in communications (quite a valid worry, since the New Republic had lost at least one force to bad intel already). And don't even get started on the plot involving Tycho Celchu (who isn't an Imperial spy, but is suspected [and put on trial for!] being one) and Erisi Dlarit, who really is (the real agent's existence is suspected, but not known for sure). General Cracken and Ysanne Isard are involved, so it's gonna get complicated.
Used more simply when it was discovered that there was a mole among one of Rogue Squadron's trading partners. Talon Karrde offered to let them execute her in whichever way they desired - he's got a really good reputation to uphold, after all - but they opt to feed her a little information first, letting her find their base in order to draw out the enemy. It takes a lot of money for them to convince Karrde to hold off on executing the traitor until after they're done feeding her.
A variant of 1 is used in The Thrawn Trilogy to identify the ominous Delta Source. It doesn't work since the Delta Source is not a person. It still proves useful because it pins down the location of Delta Source, which turns out to be an ingeniously hidden recording system.
In Romance of the Three Kingdoms, the brothers Cai are fed misinformation so that Zhou Yu can set up Huang Gai's false defection. Another of Cao's spies, Jiang Gan, is also fed false information that leads to the execution of two of Cao Cao's most capable admirals.
Used in the Belisarius Series when Belisarius and and his allies specifically choose an assistant who they know is a weasel and who will report breathlessly to their political enemies exactly what they want him to believe is happening. (As a historical in-joke, said assistant is Procopius, whose Secret History still exists to this day, listing all manner of alleged scandals in the empire of the time.)
In Graham McNeill 's Warhammer 40,000: Ultramarines novel Nightbringer, when they arrive on planet, Talhoun comes to greet them. Barzano observes afterward that now he knows he can not trust Ballion. He had suspected that it was so, and now he also knows whose pocket Ballion is in.
In Dead Sky Black Sun, Honsou uses this to destroy an effort to undermine his fortress.
In William King's Warhammer 40,000: Space Wolf novel Wolfblade, when Gabriella's aunt, who married into a different House, gets her some information, Torin and Ragnor discuss the possible permutations of Feed the Mole that might be going on. Even if it's a trap, it might be good information, to be a good bait.
In Ben Counter's Warhammer 40,000: Grey Knights novel Hammer of Daemons, Alaric tells an eldar, a fellow captive, that he does not trust eldar because of a certain battle, implying he had fought in it. Later, he refuses to let the eldar on the ship, because his captor had said he had been in that battle, when in truth, Alaric had just heard of it. Only the eldar could have told him that. Then Alaric accuses him of having long been The Mole.
Suggested as a tactic for figuring out who in their spy network has been compromised in The Tamuli, although it ends up not happening.
The Fourth Protocol by Frederick Forsyth. At the beginning of the novel the protagonist uncovers a False Flag Operation feeding information to the Soviets. As the mole is a fervent anti-communist (he thought he was giving information to South Africa) British Intelligence decide to make the best of a bad situation and use him to funnel false information to the Soviets. Later they get information that the Soviets are smuggling a nuclear weapon into the country for an unknown purpose. The Chessmaster head of SIS, realising there's no way they can track down the remaining components in time, leaks information through the mole that the Soviet operation has been uncovered and the British are closing in. This causes his Soviet counterpart to deliberately blow the operation (which was being conducted by the General Secretary without his approval) in exchange for a guarantee by The Chessmaster that the whole affair will be covered up.
In Brian Jacques's Redwall, Cluny knows that Sela will betray him. So he carefully ensures that she believes he wants to batter in the gate when he intends to tunnel. (This trope is not connected to turnip'n'tater'n'beetroot pie.)
In Trickster's Queen, Aly feeds every mole in the city. She even, at one point, manages to get herself made one of them.
The James Bond short story "The Property of a Lady", starts when it is revealed the Soviets are rewarding a known mole for her efforts, so MI6 can make more use of this trope.
Used by the CIA, in The Hunt for Red October, to plant false information regarding the operation to gain control of the titular submarine for the sake of figuring out a longstanding leak. In his backstory, Ryan became known to the British intelligence community through his proposal for the "Canary trap", which uses a computer program to randomize minor details of reports written more "flashy" than the traditionally staid "govermentese" normally used by the US government, to entice reporters to quote parts verbatim.
Also used in the prequelWithout Remorse, to determine who leaked news about the Song Tay raid. This culminates with John Kelley confronting the liberal stoner kid who was the source of the leak and offering him a choice between killing himself by lethal injection with heroin earlier taken from a dead pimp, or being shot through the head.
The above tactic is also mentioned in the Bernard Samson Series — unimportant changes in format are placed in top secret reports sent to different government departments, so if a report is leaked the security services can narrow down where it came from.
In Suzanne Collins's The Hunger Games, in the Back Story — why the jabberjays became worse than useless as soon as they were figured out.
In John C. Wright's Fugitives of Chaos, Amelia ponders whether Mrs. Wren accidentally or intentionally let slip that they were watching her — or possibly not, and some convolutions of what pose she wanted to take in order for them to interprete her knowledge.
In Larry Niven's Oath of Fealty, terrorists are trying to take down an arcology, and they have inside information about the defenses of the arcology's hydrogen pipelines. After a test attack (by unknowing dupes), the security chief makes several upgrades to the defenses—-and tells different people different things about the upgrades. When the terrorists arrive with countermeasures against some, but not all, of the new defenses, he knows who the mole is.
Lord Vetinari, Discworld's premiere Magnificent Bastard, uses fiendishly difficult encryptions to protect the semaphore messages he sends to his agents. What would-be codebreakers generally don't know is that he has access to absolutely unbreakable encryptions, but deliberately only uses the very, very difficult ones.
"Otherwise, how would he know what they thought he thought they were thinking?"
In Mara, Daughter of the Nile, Sheftu, the leader of La Résistance, has discovered evidence that Mara is The Mole for the queen, but is not completely sure. So he has Nekonkh feed her false information as a test of her loyalty. Although Mara isThe Mole (or more accurately, a Double Agent), she has also Become The Mask, and does not betray Sheftu. Unfortunately, Sahure the juggler was also listening when Nekonkh Fed The Mole, and he is alsoThe Mole.
Once the 'villains' of Villains by Necessity learn that Robin is a spy, they tell him a decidedly outlandish story about their alleged plans to report to Mizzamir. The wizard wasn't fooled by the fake intelligence for long, if at all.
Discussed several times with the Wheel of Time, when Elayne, Perrin, Faile, various Seanchan, and Rand himself deal spies.
From Honor Harrington, one particular character is a high-ranking official of the Committee ofPublic Safety, and is assigned as the Political Officer to a high-ranking admiral. What the Committee is unaware of is that it's all an act; the official in question is not at all loyal to them, but to the true Constitution of the Republic that had not existed for over two centuries. On top of all that, she's inlovewith the admiral she's supposed to be reporting on. In possibly the most magnificent and dangerous bit of spycraft in the whole 'verse, she hides her true loyalties (and her relationship) for years, feeding a steady stream of misinformation to the other spies on her ship and to the Committee about the admiral (who shares her loyalties). In the end, they become a critical part of the Thomas Theisman coup that restores the true Republic. The character in question is Eloise Pritchart, the truly elected President of the restored Republic of Haven, a linchpin of the series and one of its most-loved characters, and the man she loves is Javier Giscard, the Republic's Fleet Admiral.
Live Action TV
This was how Martha Jones ensured that the Master found her on Doctor Who.
Stargate SG-1: The Tok'ra did to this to a supposedly converted Goa'uld. This exchange occurred when the spy found out:
Tanith: I don't understand. Why have I been excluded from such important information? Teal'c: [matter-of-factly] The Tok'ra did not wish Apophis to be informed.
Done on Burn Notice when Sam was being blackmailed by the government into squealing on Michael. Michael took advantage of this and simply told Sam what to tell them. Was dropped entirely by the second season, though.
After The Mole is outed on Dollhouse, it's implied that DeWitt plans on doing a version of this should the NSA come looking for them - via Mind Rape and/or Fake Memories. Which she coolly tells the former spy about beforehand. Shudder.
The classic Mission: Impossible episode "The Mind of Stefan Miklos" is based on this trope; the IMF have to deal with two enemy moles in the CIA. Mole A is being fed false information, but Mole B has discovered this and reported it back to his superiors. The KGB know that the two moles are rivals, and are sending the titular agent Miklos to investigate; the team must convince him that Mole B is lying and that Mole A's information is real.
Used in another episode when an American agent (who's undercover as an senior enemy intelligence officer and under threat of being exposed) conspires with the IMF team to feed information to his assistant, who they know is spying on him for a rival.
Happens in Caprica. Agent Duram, suspecting that the GDD is infiltrated, gives his superior a false name for his informant in Clarice Willow's household. Turns out his superior is the mole and the false information causes the innocent Mar-Beth to be murdered by Clarice.
Subverted in Bionic Woman (2007 remake). Jaime overhears her colleague talking on the phone to an assassin planning to kill an African dictator. Jaime is told to feed him false information on the dictator's movements. This leaves the dictator wide open — instead of following the false information, the assassin is following her original plan as Jaime's colleague isn't in cahoots with the assassin, he's just trying to warn her off (she's a former Love Interest turned Rogue Agent).
Star Trek: Voyager does this a couple of times. The first is part of a running subplot. Janeway and Tuvok suspect that Seska still has a mole on board feeding her info, and so they enlist Tom Paris to exaggerate his rebellious tendencies and stir up animosity with the unsuspecting Chakotay knowing that the mole will feed this information to Seska making it more believable when Tom "defects" and joins Seska.
The second time, Janeway suspects that a defector she has granted asylum is faking and feeds him misleading information about where she is hiding the refugees and where their escape wormhole is located to buy time for the refugees who are already booking it to the wormhole in hidden shuttles.
Star Trek: Deep Space Nine: When the Klingons (who are the Federation's allies) go to war with the Cardassians (who are in an uneasy not-quite-alliance with the Federation at that point) Sisko knows he can't directly warn the Cardassians without ending the Federation-Klingon alliance so he has a meeting about the attack while having his measurements taken by the local exiled Cardassian tailor with the shady background. This is an unusual case of a Guile Hero feeding a mole genuine information, in order to help the mole's side.
In the third season finale of The Mentalist, Patrick Jane does the first type to root out Red John's mole in the CBI. He tells each suspect that Madeleine Hightower, who Red John wants dead, is hiding in a different room in the same hotel, hoping that The Mole will get Red John to send an assassin to the room they've been given.
And the plan doesn't work as intended because the assassin goes to the "wrong" room deliberately, planning to climb to the correct room OUTSIDE. Fortunately, they wait a minute to make the bust; unfortunately the true mole already has the jump on the rest of the good guys in a different location.
In Ashes to Ashes, Hunt and Drake know there is a mole in the department, so they set up a sting, telling each member of the team slightly different information, and then wait to see which one will act on it, revealing themselves in the process. Heartbreakingly, it turns out to be Chris.
Three Kingdoms follows the novel (mentioned in Literature) broadly faithfully in adapting the Battle of Chibi arc, specifically the Cai brothers and Jiang Gan, although Cao Cao's belief in the Huang Gai defection instead comes entirely from the Cai brothers' report, without mention of Kan Ze who in the novel acted as Huang Gai's messenger.
This is the strategy used to trap the titular "Assassin" in the NUMB3RS episode of that name.
In Game of Thrones Tyrion smokes out who is feeding the queen her information by telling the entire council that he is marrying off her daughter to a different person. When Cercei angrily confronts him about her daughter being shipped off to marry one of those men, he has his mole.
Mythology and Religion
Samson clearly knew his wife was trying to kill him, because every time she asked for the source of his super strength and he fed her a lie, armed soldiers would show up to test the lie.
The Blue Sun mod for 7.62 High Caliber adds a mission strand with the Sagrada police department, and one of the relatively early missions involves identifying a mole by telling two different suspected officers a different location where they'll be transporting a briefcase of money and seeing which town has mercenaries ready to attack.
Planescape: Torment has an In-Universe example with the story of Vilquar; the Githzerai figured out he had sold them out to the Illithids, so they tricked him into believing that they had given up on their rebellion to fool the Illithids into becoming complacent.
A Modest Destiny shows that you can use this even on a non-mole ditz, as seen when Gustav is deliberately given bad information so (when asked politely by a disguised villain) he'll accidentally lead the enemy into a trap.
In Worm, Wards member Chariot is a known agent of Coil, a local Diabolical Mastermind. The Wards become aware of this, and decide to use Chariot, because as a powerful tinker with a specialization in transportation he's highly useful, and so that they can feed Coil misinformation. Coil may or may not have been aware of this, but in any case, it's irrelevant to his plans, which only needed Chariot to be present in one specific battle.
Honoré de Balzac and his friends do this as a prank in The Dead Skunk.
In Skunk Fu!, a ninja monkey who pretended to be kicked out of the mountains by Baboon, so he could spy is given bad info from the animals in the valley about when they'll attack next and who'll be leading to report back to Baboon.
The Bots Master: When Paradigm found out a mole planted by The Hero, he used it to give Zulander misleading information.
She-Ra: Princess of Power: In "Birds of a Feather", Kowl's cousin Red-Eye sought employment at the Horde and Shadow Weaver used a spell to enable Red-Eye to see and hear what Kowl does. She-Ra took advantage of this to give the Horde misleading information.
The British planted a spy in Ben Franklin's staff in Paris. When Franklin found out, he proceeded to send bogus information to him, and thus back to London.
More or less all the German spies which were captured in World War I suffered this fate, if they didn't defect or end up executed.
Again in WWII every spy Nazi Germany sent to England was caught and turned into a double agent. The Nazis never caught on, and were fed false information for most of the war.
The second time around was even easier since they'd got a mole of their own: one of the instructors in the spy school.
The SIS (British) spy Kim Philby was accused of being The Mole for the KGB (Soviets); he was exonerated, but still not completely trusted. Then he came under greater suspicion, and he had to leave the SIS, and joined the KGB for real. The thing was, the KGB were quite suspicious that he was a Fake Defector, so they didn't trust him either. Philby regained contact with the SIS, and in the end both the SIS and the KGB used him for Feed the Mole purposes. Philby was no longer so much a spy or mole; rather, he was more like a messenger. Each side simply relied on him to deliver their messages, and they didn't have to worry about what his real loyalties were.
On a more mundane note, this is a good way to quickly filter out your junk mail. Any time you fill out a form where your information will most likely be sent or sold to companies that send junk, give a false name (or at least a different form of your own). If anything comes addressed to the name, you can throw it away without opening it.
Sites like Sneakemail provide a similar service for email.
A few celebrities have done this when they've suspected their publicists were leaking stories to the press. They'd just make up a fake story, and if it ended up in the media...
By early 1942, the US had already broken the Imperial Japanese Navy's JN-25 codes, and they found out that they were planning an operation around "objective AF". On a hunch, they had their base at Midway begin sending false messages saying that its distillation plant had been damaged and that they were short on water. When Japanese messages began stating that "AF" was short on water, the US knew exactly where the IJN was going, giving them the element of surprise that the US utilized to great effect at the Battle of Midway that June. The result was the loss of four Japanese carriers to one American one, and from that point forward Japan was on the defensive in the Pacific War.
Subverted in the 1960 U2 Crisis. The Soviets shot down and captured an American spy pilot, Gary Powers, who was flying over Soviet airspace. Soviet leader Nikita Khrushchev originally gave the impression that they had no idea exactly what the plane was and were merely annoyed that it had been in their airspace without their permission, whereas in actual fact Powers had told them everything. The Eisenhower administration, relieved due to thinking they'd got off the hook, claimed it had been a weather plane. The Soviets then revealed that they knew what the plane was for all along, making the Americans look like liars as well as spies.