It can't be insanity if your best friend sees it too... Right?
The French call it Folie à Deux, "the madness of two." This trope happens when two people begin experiencing the same delusion or hallucination together, a shared version of Through the Eyes of Madness. Often it happens with one going mad first, and the other slowly but surely experiencing Sanity Slippage as a result. Often they will begin distrusting other friends and loved ones who don't experience the delusion (ironically viewing themselves as the sane ones). In cases where Insane Equals Violent, this may be a cause of a Big Bad Duumvirate.
In Real Life this condition is known as Shared Psychosis or Shared Delusional Disorder (SDD).
- The Killing Joke: The Joker heavily implies this trope is the reason he and Batman are destined to continue battling each other forever. Illustrated by a joke he shares with the Dark Knight at the end:
Joker: See, there were these two guys in a lunatic asylum...and one night, one night they decide they don't like living in an asylum any more. They decide they're going to escape! So, like, they get up onto the roof and there, just across this narrow gap, they see the rooftops of the town, stretching away in the moonlight...stretching away to freedom. Now, the first guy, he jumps right across with no problem. But his friend, his friend daren't make the leap. Y'see...y'see, he's afraid of falling. So then, the first guy has an idea...He says 'Hey! I have my flashlight with me! I'll shine it across the gap between the buildings. You can walk along the beam and join me!' B-but the second guy just shakes his head. He suh-says... he says 'What do you think I am? Crazy? You'd turn it off when I was half way across!'
- Bug!: A lonely, emotionally fragile waitress begins a romance with a mysterious drifter and spirals downwards into insanity as she comes to believe his theory that the government has used him in a bizarre experiment involving hatching bugs in his body.
- A major subversion in Fight Club: Jack moves into a decrepit house with Tyler Durden, and slowly begins buying into Tyler's radical ideas about fighting, masculinity, and the need to destroy society. As his life spins out of control and is swallowed by Tyler's Project Mayhem terrorist group, Jack learns the mind-blowing truth: Tyler is his own split personality and never existed at all. The ideas came from Jack's own head.
- Zigzagged to Hell and back in The Lighthouse: Ephraim Winslow arrives at a secluded island to work as a "wickie" for older lighthouse keeper Thomas Wake. Soon the job spirals into a kaleidoscope of insanity, as the two men go mad from isolation together... Or maybe Wake gaslights Winslow into thinking he's crazy... Or maybe a supernatural curse involving a mermaid causes it... Or maybe Winslow is hallucinating the whole thing... Or maybe...
- Natural Born Killers: Mallory meets Mickey, and falls head over heels in love with him. So much that she eventually joins him in a cross-country murder spree. Granted, with the abuse she endured from her father, her trip into insanity wasn't that far.
- This is one interpretation for the end of Take Shelter. Curtis begins to obsess over a catastrophic storm that appears to him in dreams and visions. His wife Samantha tries desperately to help him because his mother has schizophrenia. However, at the end, Sam is trying to help him recover by going to a beach house. It closes with a POV shot from Sam's perspective that reveals a storm coming over the beachfront. It ends there, leaving it ambiguous whether his vision has come true or whether Sam is experiencing a shared hallucination with Curtis.
- 12 Monkeys: Dr. Katherine Railly fears this is happening to her when she gets involved with her psychiatric patient James Cole, who claims to be from the future when most of humanity has been wiped out by a man-made plague. Ultimately subverted when he recites a phone call she made he couldn't possibly have overheard; he's telling the truth, and the world is about to end.
- In Harry Potter and the Order of the PhoenixHarry is now able to see the horses which pull the normally-horseless carriages for returning students to the school. Neither Ron nor Hermione or any of Harry's circle of friends can see them, and Harry wonders if he's having hallucinations, only for Luna Lovegood to say she can see them too and reassure him that he's just as sane as she is. Subverted when it turns out the horses (called thestrals) can only be seen by those who have watched someone die in front of them, and both Harry and Luna are perfectly sane.
- Skeleton Crew: In "The Ballad Of The Flexible Bullet," a fiction editor relates the story of accepting a submission from a young author suffering from severe delusions (he believes the people around him are malicious androids, electricity is killing him, and his typewriter is home to a magical creature called a fornit). After playing along to humor him at the writer's wife's request, the editor finds himself becoming more and more paranoid about electricity... And starts interacting with a fornit in his own typewriter.
- You Know You Want This: In "Matchbox Sign", Laura becomes convinced that she has a bug living under her skin that no-one else can locate. Her boyfriend, then husband, David eventually comes to believe it when nobody else does and helps her to rip a huge wormlike parasite out of her arm.
- Played for laughs in the Frasier episode "Author, Author"; Frasier and Niles lock themselves in a hotel room to start a book they're supposed to write together before the deadline. It isn't long before they descend into jealous tantrums, wrestling around, and eventually a trip down memory lane as Frasier tackles Niles onto the bed:
Niles: My god, I'm having a flashback! You're climbing into my crib and jumping on me!Frasier: YOU STOLE MY MOMMY!!!
- Lampshaded in the Law & Order: Criminal Intent episode "Folie a Deux" where a couple claim their infant daughter was kidnapped at a hotel. Although they were able to gaslight both Major Case and the husband's wealthy Aunt, it eventually came out that the baby died months earlier accidentally via a hot car death and the egotistical author husband deluded and browbeat his already mentally ill wife into going along with a story about a kidnapping.
- Scream: Discussed in season 2, when Emma and Audrey's teacher suspect them as suffering from "The Madness of Two" when they start freaking out about the new Ghostface. At the end, she writes a book with this as the premise, even after the girls' fears were proven valid.
- The X-Files: The theme of the episode "Folie Á Deux," naturally. Mulder investigates bizarre claims from a corporate drone at a vinyl-siding company and gets taken hostage when the guy ends up Going Postal. He rants that the company CEO is an insectoid monster turning his employees into zombies; he gets gunned down before anyone can get hurt, but then Mulder finds himself seeing undead revenants and a giant bug everywhere he goes. Before it's over, Scully begins experiencing the same phenomenon, and gives the Trope Namer as her only explanation.
- "Wildwood Weed" by Jim Stafford tells the story of a some hick farmers who find an unfamiliar plant growing on their property that turns them a bit funny when they smoke it. The singer's brother tried some, and they eventually found him naked and singing atop their windmill:
"He said he flew up there. I had to fly up and get him down; he was about half-crazy."
- During his 2020 feud with Braun Strowman, "The Fiend" Bray Wyatt threatened Alexa Bliss, Braun's old mixed tag team partner, to send a message. Very unexpectedly, Alexa found herself becoming fascinated by the Fiend and his methods; over the next few weeks she began slipping into a trance when he was near, until the Fiend responded in kind, and the two became a match made in Hell.
- Family Guy: Peter and Lois start doing drugs so they can find the "inspiration" to perform at a talent show. They are seen performing a passionate song in perfect unison, though it's later revealed that it was just a shared hallucination and they had actually broken down on stage.
- The Simpsons: In "Lisa the Drama Queen", Lisa and a friend create a ficticious universe to escape the harshness of the real world. The two are soon consumed by their fantasies, and start seeing mundane objects as enchanted manifestations of their dream world.
- In the We Bare Bears episode "Primal", the three bears get lost in the wilderness. Soon, Panda and Ice Bear are driven mad from starvation and start to hallucinate a frozen yogurt machine, and revert to a primal state as they fight for it.
- In 1987, divorced accountant turned vagrant Noëlla Hégo met transient Stéphane Moitoiret and they soon bonded over the latter's "divine mission", wandering and begging around France, Hégo describing herself as "Her Majesty" while Moitoiret was her "secretary." On 2006, in Lagnieu, Auvergne, Moitoiret stabbed Valentin, believing it was part of this mission.
- In 1950s New Zealand, Pauline Parker and Juliet Hume (later known as mystery writer Anne Perry) bonded over their common illnesses and became mutually obsessed with their self-created religion about a Fourth Dimension. Fearing being separated by Juliet's father moving out of the country, both murdered Pauline's mother, Honorah Parker, in 1954. The events were dramatized in the film Heavenly Creatures.
- The case of Ian Brady and Myra Hindley, Britain's notorious child killers in what became known as the Moors Murders. Hindley came, through her relationship with Brady, to believe his racist philosophy that included a fascination with Hitler and fascism.