Once a character starts to laugh, all those around him start to laugh as well.
Truth in Television (particularly in instances of Corpsing), but useful in fiction for getting a group some release from tension. Also, when a proud character is humiliated, this lets the crowd join together against him. A character who has been driven into a corner may utilize this as well in an attempt to stall for time or to get their attackers to let their guard down, or both.
A character who does not join in is separate in some way — this may be good, because the humor is unsuitable, or bad, because he's too proud to see the humor. It can result in an attempt to fake it.
- Happens in Part 3 of JoJo's Bizarre Adventure. When minor villain Hol Horse warns Polnareff that his sword (Or rather Stand, Silver Chariot) can't beat Hol's gun (Emperor), Polnareff twists that into a fairly crass joke ("What's that? I can beat your bum? Oh, you're into THAT stuff!") and laughs to himself...which causes Hol Horse to lose control and laugh along with him.
- In part 4, during card game where Josuke is flagrantly cheating but Rohan can't figure out how, Rohan starts laughing. Josuke starts nervously laughing as well, and then Rohan stabs himself in the finger.
- My Hero Academia: Ms. Joke's Quirk lets her inflict this on her foes to incapacitate them.
- During the Dressrosa Arc of One Piece, Pica, one of Donquixote Doflamingo's henchmen, is a giant brute that can assimilate stone. His only flaw? His uncharacteristically high-pitched voice. Luffy ends up laughing hysterically when he hears Pica speak, which in turn also gets the group he was unintentionally leading into a charge to join in.
- In The Nightmare Before Christmas: Zero's Journey, if Lock, Shock, or Barrel starts laughing, at least one of the other two will join in, primarily at the third one's expense, or if misfortune befalls someone else
- In Rurouni Kenshin, Hajime Saito does this in his fight against Usui after deducing that the latter Becoming the Mask has become complacent regarding his vendetta against Shishio. Of course, after laughing together like madmen, Usui tells him to shut up, stating that it's not even funny.
- In YuYu Hakusho, when Yusuke dies and comes back to life a second time, Yusuke mentions his heart has stopped beating. Kurama and Hiei start laughing at him. He starts to laugh at himself, too, even though he has no idea why they're laughing at him.
- In My Neighbor Totoro, once the father starts to laugh, the children are able to join in.
- In Evil Dead 2, laughter spreads through inanimate objects in the room until Ash is unable to resist joining in.
- In a cross with Tension-Cutting Laughter, In & Out has a bunch of old ladies doing a bunch of confessions. "My husband has three testicles!" leads to awkward silence... but once one starts to laugh, everyone joins her.
- In The Knowledge, Mr Burgess deliberately laughs hysterically while giving Chris a test; the latter tries to keep a straight face.
- In A Man for All Seasons, Henry VIII and his court are visiting Thomas More by boat, and when stepping onto the riverbank, the king lands directly in the mud and is covered in it. Since he can be tyrannical and has an awful temper, the court collectively holds their breath, waiting for the king's reaction. Suddenly he starts to laugh hysterically, and the whole of the court joins in, still terrified of the king.
- Mary Poppins: Part of Uncle Albert's affliction is that his laughing fits cause others to join in, and by consequence, float into the air like he is at the moment.
- In Pirates of the Caribbean: At World's End, the main characters on the Black Pearl get involved in a Mexican Standoff (Barbosa, Gibbs, Jack, Will, and Elizabeth). After several very tense seconds of everyone aiming their guns at everyone else, Barbossa starts laughing, and eventually he can't hold his gun up. Everyone joins in, and lowers their guns too.
- Se7en, despite being a very dark film has a moment where the main characters all think about the absurdity of the "vibrating" home (it's right on top of a subway,) and all break into fits of giggling despite trying to hold it in.
- In Who Framed Roger Rabbit,
- The weasels will set each other off. Since they are easily amused, it's less likely to spread beyond.
- In the speakeasy, when the man saying he had seen a rabbit feigns that he has Harvey next to them, all the rest laugh together.
- In The Gordian Event by Lee Deadkeys, Mason has giggle fits when Jess nearly drops an industrial-sized can of pears on him. Jess starts laughing and snorting, which sets Ox off too. The noise attracts the two children, who start shrieking and laughing just because everyone else is.
- Are You Afraid of the Dark?: "The Tale of the Ghastly Grinner" is about a comic book villain who enters our world and his power is a laugh that causes anyone who hears to turn into a giggling idiot.
- Near the end of season 6 of Buffy the Vampire Slayer, Giles has just come back after a long absence. Buffy details all of the crap the cast has gone through since he's been gone. After she's finished, Giles stares at her for a minute...then bursts out laughing. Buffy's confused at first, but eventually can't help but laughing too.
- An episode of El Chavo del ocho had Don Ramón laughing uncontrollably, and when Quico approaches, he joins in and starts laughing too. Doña Florinda arrives and asks Quico why he's laughing and he says he doesn't know and continues, until she calls him "Federico".
- A memorable segment of Coupling had Jeff describe the concept of 'The Giggle Loop' - the compulsion to laugh in inappropriate situations that only intensifies the more you think about how inappropriate it would be. Later on, they all go to a funeral...
- The Fresh Prince of Bel-Air: Near the end of "Striptease for Two", Uncle Phil bursts into laughter after Will and Carlton get in serious trouble for dancing as strippers to recoup the money they lost from a bad stock tip when they found out that selling one of Aunt Vivian's bracelets wouldn't cover the cost. Will and Carlton nervously laugh along with Phil, hoping that he doesn't blow up at them. It doesn't work.
- In M*A*S*H, when Blake talks of what kidders they are, Burns tries to join Hawkeye and Trapper John's laughter.
- Walt Disney Presents: In "The Goofy Success Story", Goofy became an accidental star by laughing in a theater. The onstage gag wasn't funny to anyone else, but his laugh got them going.
- The Best Jokes mythic power in Pathfinder: Wrath of the Righteous makes it so that if one enemy fails its initial saving throw against the Hideous Laughter spell, another nearby enemy must save or be affected. If that enemy also fails, Best Jokes triggers again; only a successful saving throw can stop the chain.
- In Space Station Silicon Valley, hyenas use this as an attack.
- Critical Role: During combat in campaign 2, episode 116, Matthew Mercer meant to say that some entities "quiver and shift". What came out instead was "shiver and quiff/queef". Unsurprisingly, the resident 7-year-old Laura Bailey (who has earned her reputation as the Queen of Innuendo) starts grinning, followed by Marisha Ray (Mercer's wife) giggling, before the table gradually starts laughing all together. Because the cast was particularly punch-drunk and tired, the laughter kept going around in circles. At one point, Marisha complained because she had just calmed down and got set off again. Matt's reaction was laughter, resignation, and threats of murdering the party. Video for reference.
- Reddit has the subreddit /r/ContagiousLaughter, which is Exactly What It Says on the Tin. It's videos of people laughing or doing funny things, only for everyone around them (and hopefully, the viewer) to start laughing with them.
- In one episode of Futurama an Evil Laugh in a haunted house has this effect on Zoidberg.
- In Randy Cunningham: 9th Grade Ninja, Randy laughs when Evil Julian asks him for the power ball, which causes Evil Julian to start laughing too and makes them go into an infinite loop of laughing uncontrolablly. Later in the same episode, when Randy, as the Ninja, asks Evil Julian to give him back the power ball, Evil Julian giggles, making them both laugh back and forth at each other again, until the cycle is ended by the use of another trope.
- On the Looney Tunes short "Robin Hood Daffy", Porky Pig as Friar Tuck laughs at Daffy Duck's clumsy attempts at being Robin Hood. Daffy walks up to confront him, but soon starts laughing as well. They spend several seconds laughing together...until Porky climbs into Daffy's arms and slaps him on the back.
Daffy: Oh, knock it off! How jolly can you get?
- In one episode of Ruby Gloom, Iris tells a joke so she can break the record for "Best Joke". Ruby is the only one who gets it and starts to laugh. Pretty soon, anyone who hears it, including a squirrel and the moon, start to laugh as well. Poe then congratulates Ruby for breaking the record for "Most Contagious Laugh".
- The Simpsons:
- Bart mentions that due to TV he has a very poor memory and can't remember even 8 seconds in the past. The rest of the family laugh at him. "No, really, I can't! It's a serious problem." They continue to laugh. After a moment he starts to laugh too; then while still laughing he anxiously asks "What are we all laughing about?"
- In another episode Burns enjoys an Evil Laugh over his latest plan, and Homer (who the Evil Plan will directly affect) joins in the laughing. Then Burns stops but Homer keeps going for a while.
- This also happened between Homer, Patty and Selma, much to the two sisters' immense chagrin.
Patty: Cut him a check and get him the hell outta here!
- In the 6teen episode "All Pets are Off," Jude is stuck in the air vents and runs into Caitlin's missing pet ferret, Muffin. The ferret then crawls into Jude's shirt, tickling him. As the gang hear Jude's laughter, Jen mentions that it's "kinda infectious," which causes the whole gang, as well as Ron the rent-a-cop, to get a bad case of the giggles.
- Texan mom Candace Payne became an Instant Web Hit with a video of herself trying on a Chewbacca mask and breaking into hilarious laughter of joy. The video got over 155 million views (and counting!) in just four days.
- This Scottish grandma also has some side-splitting videos, including this video of her reading a book about donkeys to her grandson.