The longstanding mystery of how selective hearing works -- how people can tune in to a single speaker while tuning out their crowded, noisy environs -- has just been solved. Psychologists have known for decades about the so-called "cocktail party effect," a name that evokes the Mad Men era in which it was coined. It is the remarkable human ability to focus on a single speaker in virtually any environment -- a classroom, sporting event or coffee bar -- even if that person's voice is seemingly drowned out by a jabbering crowd.