"In the long run, it's about performance," says Alexandre Magno, a choreographer on Madonna's 1993 Girlie Show Tour. Madonna did most of the vocals live, he says, but some were prerecorded to play back during complex dance numbers. "If you're up there and you know what you're doing, it doesn't matter if you're lip-syncing or you're singing live," says Magno.
"It's extremely common," says Simpson, who maintains she was unable to sing because she was ill with acid reflux. "You go to people's tours, and you see them dancing extremely hard -- do you really think they're singing that well all the time? You try it. You'd be gasping for breath. At awards shows they're singing so perfect, and it's definitely not just them."
Reviews of Britney Spears throughout her career often mention lip-syncing. Spears has denied the allegations, although earlier this year her then manager, Larry Rudolph, told the New York Times that she occasionally lip-syncs while dancing.
Many rock bands use backing tracks in concert, too, to re-create the studio sound of their albums. "A lot of the heavy acts do that -- they thicken up the guitar sounds when they play live," says a concert-industry source. "Kiss has backing tracks galore." And bands aren't embarrassed by it. "When you go into the recording studio you have layers and layers of guitars," says Evanescence manager Dennis Rider. "Unless you have three or four guitar players onstage, you can't duplicate that. And people want to hear the record they bought."