Scarlett Johansson: Sex and the maiden

Ripples went round Hollywood last week when Scarlett Johansson told the US magazine Allure that she has an HIV test every six months. Johansson, 21, who is currently dating Josh Hartnett, 28, her co-star in the hit crime film The Black Dahlia, added: "One has to be socially aware. It's part of being a decent human, to be tested for STIs. It's just disgusting behaviour when people don't. That's just so irresponsible."

This is starlet behaviour on a grand scale. Not only is Johansson unapologetic about having a sex life, she is implying that Hollywood actors have responsibilities just like the rest of us. It was a brilliant message to send out to young people. Her frankness was welcomed by sexual health campaigners in Britain, who said she was the ideal person to "reduce the stigma" that surrounds STDs. The Terrence Higgins Trust praised her as a role model. Remarkably, Brand Scarlett was not remotely dented.

Traditionally, female stars are defined by their bodies. Woe betide the individual who lets the glamorous mask slip. But in the photographs accompanying her disclosure last week, Johansson looked just as blonde and beautiful as ever. The cosmetics company L'Oréal, whose $3m (£1.6m) campaign she fronts, has nothing to worry about.

With her hourglass figure and seductively throaty voice, Johansson has the unmistakable aura of an old-fashioned Hollywood bombshell. She exudes class - with or without clothes (this year, she posed naked on the cover of Vanity Fair alongside Keira Knightley). Woody Allen, who cast her in his films Match Point and the forthcoming Scoop, has described her as "sexually overwhelming".

But women love looking at her, too. With her luminous, creamy skin and defiantly curvy figure, she stands out in a world of perma-tanned stick insects. She is open about her crooked teeth and her hatred for Hollywood's weight obsession.

Best of all, the woman voted the "sexiest woman alive" by Esquire magazine this month, is not perfect. Flying in for the Venice Film Festival, sans makeup due to baggage restrictions, she emerged looking tired and puffy. But, two hours later, she sashayed into The Black Dahlia première, a butterfly clip in her hair and an elegant, yellow-sashed gown hugging her curves.

Through Johansson we get to live out the fantasy adolescence many of us never had. Despite admitting that she loathes exercise, she is the new face of the sportswear firm Reebok.

Politically liberal (she campaigned for John Kerry in the 2004 presidential election and is a pin-up for Red, the Bono-endorsed, Aids charity), she insists she can't understand why there's so much interest in celebrities. "Hello, there is a war going on, you know, and there's another one about to start. There's mass genocide and starvation and epidemics, but people don't talk about those things - they just want to talk about who eats what for lunch. Who cares?"

But she's bright enough not to scare Middle America. Despite being linked to some of the world's most eligible men, including actors Jared Leto and Benicio Del Toro, she let slip that she had only had three boyfriends before Hartnett. As she told Allure magazine: "Contrary to popular belief, I'm not promiscuous. There does seem to be a mistaken belief out there that I am sexually available somehow - which is not to say that I'm not open-minded about sex."

In a fashion world that celebrates androgyny, designers are queuing up to dress those famous curves. She's modelled for Louis Vuitton and underground New York label Imitation of Christ. Even that old curmudgeon Bob Dylan has been won over. This year, she starred in the video for his song "When the Deal Goes Down".

So what makes Johansson (a former child star) such a sophisticated player? Luck? Charm? Pushy parents? Johansson insists she is a workaholic. "It's my mother who keeps saying: 'Take a vacation, get some rest, what's wrong with you? You'll wear yourself out!' But, if I go on a vacation, all I think about is work!"

Johansson was born in New York in 1984. Her Danish father is an architect, now divorced from her mother, who comes from an American-Jewish family of Polish descent. She has an older brother and sister, and a twin brother, Hunter.

Johansson's parents enrolled her in tap, dance and acting classes. Her big break came when Robert Redford cast her, aged 12, in The Horse Whisperer. But it was Sofia Coppola, the director of Lost in Translation, who first understood - and captured - Johansson's enigmatic screen presence. "She seems to be able to convey feeling and depth without doing much," says Coppola. "And I like her low voice."

Johansson also revealed savvy media instincts. Starring opposite fiftysomething Bill Murray in Lost in Translation, she talked up her appreciation of middle-aged men - ensuring the adoration of what one critic dubbed her "ageing but game fan club".

Girl with a Pearl Earring was released the same year, and Johansson was hailed as the most exciting young actress to hit Hollywood. In 2003, she was nominated for two Golden Globes and two Baftas (she won best actress for Lost in Translation). In retrospect, she admits it was overwhelming: "All that attention wasn't easy to cope with."

Most actresses interviewed about their sex appeal play infuriatingly dumb. They claim ignorance or pretend they were the plain one at school. Johansson embraces it. "I feel comfortable as a young woman; as a young, modern, liberal person."

The backlash has started, of course. She's appeared in a few flops (In Good Company, A Good Woman), while 2005's The Island was the first time she received bad reviews. Male journalists who interviewed her early in her career complain she is no longer the unaffected girl who took them shopping to bring their wardrobe and music up to date, or teased them she'd cook them French toast in the morning. Arguably Johansson is growing up. She doesn't have to flirt and simper.

After dallying with the regulation bad boys, she has made an inspired choice with Hartnett. Friends say the couple are well-suited because they share traditional values, and both appear unimpressed by life in the Hollywood fast lane. They live in New York's Greenwich Village, and prefer to go around incognito.

It would be a pity if Johansson got stuck playing sex sirens. The new roster looks promising. As well as Allen's Scoop and Christopher Nolan's The Prestige, there are adaptations of The Nanny Diaries and The Other Boleyn Girl, in which she and Natalie Portman play sisters who vie for Henry VIII's affections.

She is no fool. She knows Tinseltown backstabbers are sharpening their knives. To get away with being so outspoken, she has to work that bombshell image. Expect to see her looking wonderful in a procession of couture frocks after her HIV test disclosure. But in a world where Paris Hilton achieves notoriety for starring in a sex video, and Nicole Richie is the poster girl for eating disorders, surely Johansson is the role model most parents would want for their daughters?