Why do so many singers adopt a second persona, either onstage or in real life? It could be they just can't cope with being their usual selves sometimes -- or, more likely, it's that they're overflowing with more talent than it's fair for just one individual to possess.

Beyoncé aka Sasha Fierce

Beyoncé's new double album, 'I Am ... Sasha Fierce,' is the work of two artists. On one side, the refined megastar we all know simply as Beyoncé. On the other, a brash, sexy newcomer named, you guessed it, Sasha Fierce. Surprise -- they're the same person! As Beyoncé puts it, "Sasha Fierce is the fun, more sensual, more aggressive, more outspoken, more glamorous side that comes out ... when I'm on the stage." Watch Beyonce on Sessions.

Madonna aka Esther

In 2004, inspired by her newfound faith in Kabbalah, Madonna took on the mantle of Esther, after the Biblical Jewish queen. In addition to hypochondria ("My mother died when she was very young, of cancer, and I wanted to attach myself to another name"), Madge's name change was inspired by an attempt to embrace her more matronly side. Based on the ads for this year's Sticky and Sweet Tour, it didn't take.

David Bowie aka Ziggy Stardust

Ziggy Stardust may not be the first rock alter ego, but he certainly set the standard. Bowie's 1972 concept album 'The Rise and Fall of Ziggy Stardust and the Spiders from Mars' introduced the titular hero as a rock 'n' roll savior from outer space, but by the end of 1973 the "Martian messiah" had taken on a life of his own. As Bowie put it in 1976, "I became Ziggy Stardust. David Bowie went totally out the window."

Christina Aguilera aka Xtina

Former 'Mickey Mouse Club' member Christina Aguilera wanted a more "mature" image for her second album, 2002's 'Stripped,' so she traded in her mouse ears for a black bustier and the letters in her given name for a new spelling. She was so dedicated to her spicy new persona that she got "Xtina" tattooed on the back of her neck -- presumably as a reminder for whoever was behind her at the time.

Janet Jackson aka Damita Jo

No, her name ain't Baby; it's Janet -- Ms. Jackson if you're nasty. And when she's nasty, it's Damita Jo (Janet's real middle name). Based on almost every song on Damita's eponymous 2004 album, she's nasty all day every day. It's as if poor, innocent Janet has been possessed by hell's horniest demon: "I want a nasty boy to put it on me good/Ms. Jackson don't/But Damita Jo do."

Eminem aka Slim Shady, Marshall Mathers

Slim Shady and Marshall Mathers are the two sides of the Eminem coin. Shady (who Eminem literally conceived on the toilet) is the Id: murderous, insane, hilarious and capable of anything. Mathers is the slightly less insane ego: struggling father, abandoned son, tortured artist. Shady even admits to his attempts to kill Marshall off ("I hung my original self from the top bunk with a belt"), but these days it looks like a stalemate.

Britney Spears aka Mona Lisa

In her 2004 single 'Mona Lisa,' Britney sings, "Everyone's watching, as she starts to fall/They want her to break down/And be a legend of the fall." Sound familiar? In addition to rhyming "fall" with "fall," Brit-Brit was announcing her new personality, one that includes a license to be a bitch: "Whenever I feel like being mean or ... bustin' people around to get stuff right... It's kinda easier to be called 'Mona Lisa' instead of Britney."

Norah Jones aka Maddie

Adult Contemporary superstar Norah Jones created Maddie, lead singer and guitarist of the "punk" band El Madmo, by putting on a cool blond wig and picking up a Fender guitar -- all just because she wants to 'Rock Yer Balls Off,' to cite one El Madmo title. The band's 2008 self-titled debut album should probably be subtitled "The one Norah Jones album you wouldn't get on Mother's Day."

Bono aka MacPhisto

One of Bono's three onstage personas during the 1992-93 Zoo TV tour (along with the Fly and Mirrorball Man), MacPhisto was a Satanic lounge singer with horns, white makeup, a gold lamé suit and a penchant for making prank phone calls. Ostensibly a parody of has-been rock excess, the self-described "last rock star" sometimes came across as a self-parody as well; Bono taken to the logical extreme.

Mariah Carey aka Mimi

Back in the day, celebrity alter egos were all about creative self-expression. Now they're all about divas with multiple personality disorders. On her 2005 (don't call it a) comeback album 'The Emancipation of Mimi,' the titular character was supposed to represent Mariah's real, true, liberated self. In other words, she's not crazy. It was that other lady who made 'Glitter' that was crazy.