Let's get something straight. Hollywood is all about positive gender roles for women. At least as much as the current US government is about extending the helping hand of friendship to the Muslim world. Read the usual rags and celebrity interviews featuring the latest movie babe; they're a hotbed of pro-feminist ethic worthy of Susan Faludi and Germaine Greer.

Now it's true Hollywood's always been about the young and the beautiful. That's been the driving aesthetic since the birth of the movie business in the early part of the 20th century. Soon after the Lumiere brothers invented cinema, the newly burgeoning US studio system invented stardom; people to look up to, dress like, talk like, look like and obsess over in the gossip press that orbits the industry.

Plus, there are as many beautiful boys are there are girls – because despite the fact most films to come out of the US studio system are made for teenage boys, as many women as men see films.

But a trend both amusing and disturbing has emerged over the last 15 or so years when the latest in an endless string of beautiful young women gets her 15 minutes of fame. Two institutions impact on how we (audiences, actors, directors and writers) want our leading ladies to be.

The first is the traditional Hollywood heroine – the useless dame being pursued by the monster who'd trip over and lay there screaming until the hero ran back to scoop her up. That sort of image is outdated, insulting, and embarrassing, and no modern studio would put it in a movie for fear of open revolt, or worse – half their market (women) not buying tickets to it.

The other is the positive influence feminism has had on the lives on Generation X as they've grown up. Hardly anyone under 30 would remember or even believe in the existence of a time when it wasn't generally accepted women could do anything, and rightly so.

The net result is an endless string of women in starring roles (usually of the midyear action/comedy blockbuster variety) who not only extol the virtues of the atypical gal who can look after herself but claim to be the embodiment of them.

It started more or less with James Cameron's Aliens (1986). As well as being one of the seminal action adventure films of the 80s, it was original in that the protagonist and hero was a woman. Ripley wasn't shy about duct taping a marine rocket launcher and automatic rifle together to turn on the titular xenomorphs.

It's revealing that most of the action she took was with the aid of either heavy weapons or her brains. She never fought hand to hand with a male member of the crew and she never had to rely solely on brawn. In fact, the only time she did – in Ripley Scott's original Alien (1979), was with Ash (Ian Holm) in the infamous magazine in the mouth stoush that she was indeed losing, and from which Dallas (Tom Skerritt) had to rescue her.

From Ripley's simple beginnings, a floodgate of female heroes burst forth, and their skills and strength got more ridiculous as time went on, to the point where a movie would have us believe three women in their early twenties could not only take on a factory full of goons with weapons ranging from chains to crowbars, but win (Charlie's Angels: Full Throttle).

The net result is that for years now, it's been de rigueur for the latest starlet in a leading role to talk about how her character empowers women to look out for themselves, how they're not afraid to 'kick ass' (surely the most overused phrase in film media commentary) and how being a woman means being beautiful and feminine but strong.

Pamela Anderson said it about her role in Barb Wire. Carrie Anne Moss said it of The Matrix. Rebecca Romijn of X-Men. Kate Beckinsale of Underworld. Jessica Biel of The Texas Chainsaw Massacre . Jessica Alba of both Sin City and Fantastic Four . Rosario Dawson said it about Sin City too. Jessica Simpson says it about her role in The Dukes of Hazzard. And Scarlet Johansson said it about her role in Iron Man 2.

Test it for yourself. Read any interview with the latest Hollywood bombshell making a splash and you'll find some comment about how the role empowers women (that, and the fact that so many of them are called 'Jessica' lately). The movie itself will also invariably feature a scene of the heroine putting some redneck or sleazebag in his place for some puerile joke about her looks or dress sense.

Now of itself, pushing feminism as a highlight of your character is harmless, and indeed positive. But there are two dangers. The first is we hardly bat an eyelid at the sight of one or more skilled women taking to a gang of able-bodied men and beating them in a fight. Now, there's nothing to say a women trained in (for example) martial arts couldn't knock a single or even a handful of men out with a few well-aimed blows, but women's fighting prowess is determined by 100 million years of biology, not 20 years of slut-chic feminism.

Whether Germaine Greer or Jessica Simpson like it or not (and one suspects Simpson realises it less than Greer), few women would pose a threat to most grown men in a fight simply because of muscle mass. Sounds obvious when you consider it, but we have little trouble believing River Tam takes on a room full of seedy bar patrons in Serenity. Might there be a little girl somewhere who – after watching enough movies where her tween heroes bash the bad guys to a pulp without breaking a sweat – learns movies are make believe the hard way when she faces an abusive boyfriend or unwelcome male advance?

The second problem is this. When you're reading comments by these girls about how the character will empower women everywhere, cast an eye over what she's wearing. Looking again to the most recent example of feminism, for her role as Daisy Duke in The Dukes of Hazzard, Jessica Simpson wears tall heels, micro hotpants and an ultra-tight T shirt that displays what surely must be one (or two) of her most dependable talents. The publicity photo accompanying most articles about the movie shows her facing away, making bedroom eyes over her shoulder at the viewer. No, there's nothing wrong with young, beautiful girls dressing in sexy garb for movie – the industry was founded on it, after all. Even the most insensitive of men realise there's no excuse in this day and age for passing judgement on a woman for how she's dressed.

But the message given by the film is both strangely contradictory and perfectly clear. Look at me, the woman is saying, I'm presented here for your sexual fantasy. Then, when we see a character in the film respond to her overly sexual presentation, he becomes the victim of anything from derision to violence at her hands.

The role of Daisy is that of a waitress, but Simpson looks like she's on her way to a wet T shirt competition. When a customer makes a joke about her initials being 'double d', she throws him off his chair. There's something ridiculous and frankly confusing about intentionally dressing a movie character up like a cheap hooker and then showing her lusciously long leg disappearing into her miniscule shorts, high heel standing on the customer's throat who dared make derogatory comments about her looks.

There's something even more ridiculous when Simpson tells magazines that Daisy, dressed like a Kings Cross streetwalker yet able to physically incapacitate a grown man, is a positive role model for girls. So never let it be said Hollywood moviemaking is a sexist business. Film writers and marketers hold feminist principles dear – it's important that their characters portray strong, empowering traits to girls.

And if you're between 18 and 25, hot looking and don't mind dressing according to a male fantasy stereotype (stripper, hooker, steel and leather, bearing weapons, etc), you can empower girls too.