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When too many people work together on a project, the result is inferior.
Too many persons involved in managing an activity can ruin it, as in Without a conductor, every player had an idea for how the music should go—too many cooks spoil the broth. This expression alludes to each of many cooks adding something to a soup, which finally tastes awful. It was already considered a proverb in 1575 (by George Gascoigne in The Life of P. Care).