By Steve Ryfle There's nothing like a little exposed skin to boost a starlet's public profile. Plaster a titillating image of her nude frame on the cover a glossy monthly, and she's instantly grabbed the attention of millions.

When 20-year-old Scottish diva Keira Knightley and 21-year-old Scarlett Johansson stripped and strategically struck PG-13 poses for the cover of the March issue of Vanity Fair, the press types deemed it a risky and risqu? move (one British tab called the pic "shocking and provocative"). But it's nothing new: from Marilyn to Madonna and beyond, lots of actresses have flashed a little flesh in mainstream mags as a way of upping their, ahem, visibility.

If you've been perusing the newsracks in recent years, you may have noticed Rachel Weisz, she of the "Mummy" movies fame, clad only in a boa constrictor for the April 2004 cover of Esquire. You may have been nonplussed by the sight of a topless Paris Hilton (covering her assets, of course) on the October 2005 cover of Vanity Fair.

Sharon Stone graced the cover of December 1999's Esquire in the buff, while in 2003, Britney Spears went pantless for Esquire and topless for Elle. And if you got past the cover of the Knightley-Johansson issue of Vanity Fair, you'd have found a photo inside of Angelina Jolie, reclining nude in a bath tub. In May 2003, all three Dixie Chicks dropped their drawers for the front of Entertainment Weekly.

Yes, it's legal eye candy for adolescent boys of all ages, but other than a momentary blip on the tabloid radar, do stars reap any long-term benefit by baring it all?

"Keira Knightley and Scarlett Johansson can only benefit from their recent unclothed appearance on the cover of Vanity Fair," says Mr. Skin, otherwise known as James McBride, a movie nudity expert who runs MrSkin.com and author of "Mr. Skin's Skincyclopedia."

"After all, here we are talking about them. Both Keira and Scarlett, although they're young, are established Hollywood stars and this blast of bare flesh is going to propel them upward into the next echelon of fame. That cover is being, well, covered all over the world, and it will be collected, fussed over and looked back on fondly for decades to come. As to which woman will benefit more, well ? Scarlett actually shows more skin in the photo."

Not surprisingly, when it comes to sexy celebrity magazine covers, there are a few that are considered classics, Mr. Skin says.

"As far as the best known nude photos on the covers of mainstream magazines, I'd say it's a tie between Demi Moore being knocked-up and naked for Vanity Fair in August 1991 and Janet Jackson's topless Rolling Stone appearance from September 1993. That's the one where the guy's hands were placed over Janet's breasts from behind. Some people might think she could have used that kind of help at the 2004 Super Bowl. Not me!"

(Incidentally, Mr. Skin's expert pick for the best nude or semi-nude celebrity magazine cover of all time is the November 1996 Rolling Stone, which "featured Jennifer Aniston lying on her stomach with her backside in view.")

Of course, not everyone thinks posing nude is a great way to climb the Hollywood ladder. When news of the Knightley and Johansson's unveiling hit the wires, veteran actress Meryl Streep admonished her young colleagues, saying the photo stunt could undermine their work as actresses.

"They obviously feel that, to emerge from the pack, they need to distinguish themselves by being more willing to do that than anyone else. It's not about the roles they aspire to - it's that they have to sexualise themselves," Streep told a British tab. "(The new generation) have it drilled into their heads that they need to glamorize themselves (in order to succeed)."

And although we're no psychologists here, in some cases a celebrity's decision to disrobe seems like a naked cry for attention. Darva Conger tried to lengthen her 15 minutes of "Who Wants to Marry a Multi-Millionaire?" by posing nude in Playboy in 2000; "Who's the Boss?" star Alyssa Milano ruffled the feathers of her TV dad, Tony Danza, when she posed nude in the Bikini magazine in 1993; "7th Heaven" star Jessica Biel was barely legal when she posed for a spread in Gear magazine in 2000; and the February 2006 Vanity Fair had a nude Lindsay Lohan pictorial accompanied by the actresses' admission of bulimia and drug use.

On the other hand, some actresses who've doffed their duds are praised for being both bold and beautiful, for redefining the popular definition of beauty. Cindy Crawford followed in Demi Moore's footsteps by posing pregnant and clothesless for W magazine in 1999, while 62-year-old actress Lauren Hutton recently posed nude for Big magazine, saying, "The really important thing is that women understand not to listen to a 2,000-year-old patriarchal society ? My generation of '60's women are not going to stop wanting to be attractive."