Too Many Cooks Spoil the Soup

The more people that work on something, the less likely they are to succeed.

Basically, this rule is just that: the more people that attempt something, the less competent they become at accomplishing said task. Inversely, ventures made solo, especially in the case of Last of His Kind on a mission, are almost sure to succeed (unless there's An Aesop about teamwork).

A good example is in Superhero stories, wherein a singular villain may be a match for an entire team of superheroes, but if said villain joins a group of villains, suddenly they lose to just one of them. Similarly, a villain may be taking down entire groups of superheroes, but when one hero steps out to take them on alone, watch out. Of course, villain team-ups are also prone to a different problem entirely.

May be Truth in Television-sometimes too many people working on a project results in nothing getting done, possibly because everyone thinks someone else will do it, or because of conflicts over direction (a phenomenon known as Parkinson's Law of Triviality). This can even be seen on Internet forums, where the more people that engage in an argument, the less reasonable the conversation generally becomes (Nazis or otherwise).

This is the super trope of several other rules, notably Conservation of Ninjutsu, which applies this principle to ninjas and other supposedly-elite fighters, and Conservation of Competence, which applies this to intelligence in evil structures. Possibly related to, or even caused by, Sturgeons Law. Executive Meddling is often a good example of this trope in action. Contrast More Dakka and its related tropes, where more cooks are seen to make the soup better in any case.

Also note that this can specifically be invoked as An Aesop, generally when something straightforward starts to involve too many people and therefore ends up A Simple Plan. As An Aesop, it contrasts well with Stone Soup. In almost all cases, it ends up being a cause of Stop Helping Me.

Anime and Manga

 * In season 2 of School Rumble, much of the cast attends a potluck dinner which they completely ruin by various people adding stupid things to the food. However, the weirder the ingredients, the better the batch turned out to be.

Folklore

 * In one of Aesop's Fables, a man and his boy are taking a donkey to the marketplace to sell. He passes by several groups of individuals who make criticisms of how he goes about transporting the donkey (e.g., "Look at that selfish man, riding a donkey and making his son walk behind him on foot.") Eventually, he thinks that to satisfy all of his critics, he should be hogtying the donkey and carrying it on a pole. But this proves to be the worst idea he has had yet, since the donkey struggles against being tied up, falls into a river, and drowns. Thus the man no longer has his donkey, and he goes home, reflecting on how he shouldn't have felt the need to change his practices every time someone made a criticism.
 * And the moral of this story is "If you try to please everyone, you can kiss your ass goodbye!"
 * That's also found as an Arabic folktale where the father, a bit of a Trickster archetype, wants to teach his son the dangers of relying too much on others' opinions. At the end, when he and his son are carrying the donkey between them, a mob cries that they are insane and they are taken to jail.

Literature

 * Piers Anthony once wrote about how a writer, by following the advice of the various magazine editors to whom he submits his story, ends up transforming his story into something entirely different from what it started out as. (Piers Anthony hates editors.)
 * In Terry Pratchett's The Last Continent, this is how the Discworld got the duck-billed platypus, no thanks to a group of temporally-displaced wizards from Unseen University.
 * In The Last Hero, Ventinari deals with this sort of problem in his truly magnificent style; When leaders from hundreds of nations come to Ankh-Morpork to discuss how their going to stop Cohen the Barbarian and the Silver Horde from blowing up the world, he has them form committees and then locks them in the room. Then, while they're arguing, takes a few of the people he knows aside and tells them how they're going to save the world.
 * A Roundworld proverb has it that the camel is a horse designed by committee.
 * On the other hand, a horse couldn't do a lot of the things people use camels for, but camels can do almost everything a horse can do besides "look pretty".
 * Used literally in Anne of Avonlea at a dinner party; everyone involved in making the meal adds a little sugar to the peas because they all think no one else will remember to. Result: literal Tastes Like Diabetes.
 * A Little Golden Book featuring Donald Duck had Donald in the studio while the writers are storyboarding his next cartoon. Each of the writers keeps adding in things they think should be in the cartoon (like the nephews and Chip N' Dale) to the point where there is no room for Donald in the cartoon. Donal proceeds to blow his top and start screaming at the writers.

Live Action TV
"Liz: How did this happen? I had Grizz call him at eight o'clock this morning and pretend it was eleven.
 * In an episode of Thirty Rock, Jack recruits some of the writers to help him come up with a new microwave oven. When all their suggestions are combined he ends up with a Pontiac Aztek.
 * In another episode, when Tracy is even later for rehearsal than usual:

Pete: I printed up that fake rehearsal schedule for him saying we were starting at nine instead of noon.

Kenneth: Oh, and I set all his watches and clocks to say p.m. when it's really a.m.!

Liz: Oh, boy, we may have overdone it.

Tracy: [entering] WHAT THE HELL TIME IS IT?"

"Mike: Passed from editor to editor in a desperate attempt to save it!"
 * Mystery Science Theater 3000 would occasionally comment on films with a large number of writers, producers, etc. For example, in Space Mutiny:


 * The point of Extras: Andy's sitcom gets picked up, but he allows the producers' suggestions to turn it from a witty character study and commentary on office life to a cheesy, lowbrow Work Com.

Newspaper Comics

 * The Far Side had one strip with a bunch of scientists arguing with the caption "Another case of too many mad doctors and not enough hunchbacks."
 * There was also a Dilbert strip where this trope is compressed into a proper theory: The combined IQ of any team starts at 100 for one participant, with 5 points deducted for every additional member to the team.
 * Another time it was expressed as equalling the IQ of the dumbest member, divided by the number of members.

Tabletop Games

 * The Sidereals in Exalted tend towards this. It's actually part of the rules that the more people that are present, the stupider their decisions are likely to be. In the backstory this tendency is one of the key reasons the world is now in such a perilous state.

Web Original

 * Dr. Insano notes that the shlock 80s fantasy film The Dungeonmaster had seven directors And yet none of them seem to have stuck around long enough to film an ending; the movie just sort of stops 20 seconds after the climax.

Western Animation

 * Seen on Doug, where the title character tries to make a gravy boat and ends up with something resembling a jet pack crossed with a water gun.
 * The same thing happens when Doug tries to form a band; everyone wants in, and it quickly becomes an unmanageable mess.
 * It really didn't help that the Alpha Bitch of the group insisted that 'bigger is better'. It's fortunate for Doug that he left before the whole thing imploded.
 * The Darkwing Duck episode "Comic Book Capers" has Darkwing preparing to pitch a comic book of his fabulous adventures, but he keeps getting called away from his typewriter. Other characters study his pitch while he's away and decide to "improve" it, resulting in the comic book storyline going completely Off the Rails in a Crowning Moment of Funny.
 * In an early episode of Ed, Edd n Eddy, Edd is counting grains of sugar to add to their energy drinks. He asks Eddy to help him. While Edd is looking away, Eddy adds one full bag of sugar into the energy drink. The end result? The energy drink has tons of sugar in it (unbeknown to Edd), Edd tastes tests it and goes hyper due to sugar rush.

Real Life

 * Related quote: "It's better for a ship to have one bad captain than two good ones."
 * The Jargon File contains the analogous entry for Brooks' Law, which states that "Adding manpower to a late project makes it later", with mathematical justification; dividing a task among N people gets the work done in O(N) time, but actually coordinating that work and getting it merged back into a completed project takes O(N^2) because of duplication, intercommunication problems (two people on a project have one line of communication (A<->B); four people have six), and general laziness (if there's a hundred people on a task, there will be a few who think they don't need to pull their weight).