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 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.
- 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 Rurouni Kenshin, Hajime Saito does this in his fight against Usui after deducing that the later 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 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 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: I love to laugh! Long and loud and clear! It's getting worse every year! The more I laugh, the more I'm filled with glee! The more the glee, the more I'm a merrier me!
- 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.
- And then Barbossa breaks up the laughter, and everyone instantly resumes their positions in the standoff. See the scene here.
- 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.
- 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...
- In M*A*S*H, when Blake talks of what kidders they are, Burns tries to join Hawkeye and Trapper John's laughter.
- In Space Station Silicon Valley, hyenas use this as an attack.
- In one episode of Futurama an Evil Laugh in a haunted house has this effect on Zoidberg.
- 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.
- 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!
- 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.