Marion Jones wants to be the Mark Spitz of the 21st century. The 24-year-old track and field star plans to win five gold medals at the Olympics next month, setting a new record in athletics the way Spitz, with seven in 1972, set it in swimming. It may well happen, for Jones, currently the fastest female in the world, clearly has the talent. But she also has a confidence, many say arrogance, that is pushing people's buttons in a way unseen before.

From the beginning, Jones did not dare to suggest, wish, or hope she might win five medals. She declared it a certainty. Even to a public shed of many sexist attitudes about women being non-starters in sports - witness the hysteria following the US women's football team winning the World Cup, the Spice Girls-like adulation for the Williams sisters at Wimbledon - Jones is too much. She is too boastful, too assertive, too cocksure she will bring home the gold. In short, she is displaying character traits Americans ascribe to male athletes. But in men the chest-thumping is admired. In women, it is shocking, and has led to Jones being called arrogant, pretentious, and a certain word that rhymes with rich.

But Jones won't apologise for her bravado, and nor should she, suggests Colette Dowling, author of an audacious new book about women and sports, The Frailty Myth. Though not published until September, it has already sparked a heated debate in the athletic world.

Dowling, a writer and lecturer whose first book, You Mean I Don't Have To Feel This Way, became the bible for women suffering depression, has for the past two decades carved out a respected niche writing about the psychosocial issues of American women. Her 1981 book The Cinderella Complex, castigating females who live in the expectation that men will rescue them, has never gone out of print.

In her new book Dowling pulls together research from physiology, biology and psychology to suggest that not only are women as capable as men of excelling in sports, it is their natural inclination to do so.

The reason there aren't more Marion Joneses, she says, is not because women can't or won't compete. Rather, she paints women as victims of a vast conspiracy to keep them off the playing field. To blame are a culture of misogyny, parents and teachers who pander to gender stereotypes, and a media that rewards women for looking weak and feminine, and excoriates them when they display strength. "Women have been excluded from so much in life because of the frailty myth," she writes. "First we believed we were weak. Then we began to suspect that we weren't but kept getting told that we were. Then we began proving that we weren't and were mocked as men because we were strong.

"Women have thrown themselves over hurdle after hurdle during the course of the past century, demonstrating extraordinary physical powers and skills, and still we're being kept back for no reason other than we're female."

The book is a powerful clarion call to both sexes, demanding they abandon the old saw that women are the weaker sex. She argues that the belief female-equals-frail has done untold damage to women's health, self-esteem, and social status. Physical equality, she warns complacent feminists, is the real final frontier of women's liberation.

To make her argument stick, Dowling draws on research in exercise physiology, which proves there are no biological reasons for women to stay off the sporting field. She slaps the wrists of 19th century doctors who insisted "weakness" in women was a natural condition, and that exercise was a danger that might dislodge the uterus.

Instead Dowling explains that before puberty, boys are neither taller nor heavier than girls. And whereas it was long thought that only boys experienced a muscle-building surge of testosterone during puberty, girls do too. Modern medicine knows this, so why, she asks, given their ability to develop athletically, did women never venture beyond sewing, cooking and Barbies?

Dowling says the answer is cultural, and continues to be. Frailty has long been sold to women as feminine. "Strength has a withering effect on male identity. To salvage men's failing sense of dominance (post industrial revolution) women were encouraged to scale back their own physical development." She believes that men's psyches have taken a beating, first with machines displacing them, and, more recently, with the influx of women into the workforce. She contends that sport is the last place that men, no longer supremely dominant, can assert their body strength and masculinity.

That is why, she argues, the frailty myth endures, despite the leaps women have taken in professional sport in the past decade. Men need to keep women weak, off the football field and out of the boxing ring, so they can continue to feel strong. They do this by criticising muscular women, announcing they are unfeminine, whispering that they are probably lesbians.

Teenage girls in particular, living in a culture that glorifies whippet-thin models as the benchmark in femininity, sexiness and glamour, quickly get the message that boys don't like muscles and strength. "Girls who display strength, power or physicality while interacting with boys run the risks of being marginalised." Dowling says that girls learn this long before they start reading Vogue. Fathers, she says, still teach their sons to play catch, but not their daughters, who they put in the "doll corner". But, she claims, when they do teach their girls to throw, they learn just as competently as boys. "Boys throw better simply because they are trained to from childhood."

Stacy Dragila, who is competing for the United States in the pole vault in Sydney, recounted recently that when she asked a man if she could vault, he looked at her as if she was from Mars. "I asked him if I could try it one day at practice. He just looked at me and said: 'Women can't do that. They just don't have the upper body strength. They are not made to do it.'"

But Dowling insists they are, if only the culture would read the biological data, rid themselves of gender stereotypes, and see the benefits. Sport, she says, stimulates motor development in the brain. It has also been shown to build self-esteem and curb self-destructive behaviour in young girls such as smoking and drinking.

In The Frailty Myth, Dowling pleads a passionate and well-argued case to let women play ball, except in two instances, when her reasoning is more about sexual politics than logic. Yes, science has shown women have innate athletic abilities. Yes, feminists and behavioural psychologists will confirm that women are held back, and hold themselves back, to conform to male notions of femininity.

But to suggest that if men suddenly declared big muscles sexy, women would storm the football field is plainly untrue. As the noted American anthropologist Helen Fisher argues in her book The First Sex - The Natural Talents of Women and How They are Changing the World, girls will never be as obsessed with sports as boys, because evolution wires women to be co-operators, not competitors.

"Nobody pushes girls into the doll corner - they go there naturally," she says. "Throughout their lifecycle, girls show more tendency toward nurturing activities, and we believe this is linked to the production of oestrogen. It's amazing how many people have to try to prove that everything is all socialisation, whereas biology and culture go hand in hand."

Fisher applauds Dowling's call to get women playing at elite levels like men, but does not accept the notion that women hunger to play sports in massive numbers. "There are more women who will play sports when the culture makes it acceptable to be athletic, which we see is now happening. But girls will always go to the doll corner. Women are drawn to activities that involve nurturing and co-operation, and men competition, and that is pure biology. You can't discount it."

Dowling's other argument raising eyebrows is her call to reassess the performance of women in competition. She won't accept that men are better athletes because they can jump higher and run faster. To really compare the sexes, she wants women's performance relative to men's adjusted to take into account their smaller size.

"That just doesn't add up," says Richard Cotton, exercise physiologist and spokesman for ACE, the American Council on Exercise. "You can't adjust performance to size to determine which athlete is superior. It would be a huge leap to say that Flo-Jo, [sprinter Florence Griffith-Joyner], at her peak, was faster than Michael Johnson at his, simply because you divide her size and speed and average out the maths.

"In terms of physiology, men are genetically predisposed to have more strength than women. Venus Williams is a big girl, but she still couldn't take on Pete Sampras, due to genetics. I would venture to say she wouldn't even make the men's tour."

But Cotton says Dowling is correct in that biology doesn't prevent women pole vaulting or boxing. "Women can definitely build strength, and they have excellent athletic ability when trained. But we do encourage girls to stay indoors because as a culture we don't like muscular women. Yet I think that stereotype is breaking down."

This Dowling concedes too. She waxes lyrical over the strides American women have made in sports since 1972, when Title IX, an amendment to the Civil Rights Act guaranteeing equal funding for girls and boys sports, went into law. (Then 300,000 girls were in team sports. Today there are 3.2m.)

Throw in society's attitudes breaking down about women athletes and television networks smelling profits in women's games, and Dowling admits these seem like glory days for women athletes. So why isn't she doing a victory lap? Simply because she doesn't see the battle won. Remember, women still play three sets of tennis while men play five, because the myth that women are too "frail" to play long games persists.

Dowling does applaud the increased participation and acceptance of women in sports, but cautions against celebrating. In the year 2000, female athletes are still encouraged to feature in nude team calendars to prove they are feminine. The fashion/MTV culture, she contends, tells girls to rein in their athleticism.

She also fears a new kind of male backlash as more women, inspired by Marion Jones and that strip-to-her-bra soccer player Brandi Chastain, buck the "weak girl" stereotype, get onto the field, and then, shock, horror, take it over.

But Dowling urges women to ignore the sneers, lace up their trainers and tackle the sexism head on. "As different beliefs supporting the frailty myth shatter, one after another, the change will not be trifling. It will alter the way women walk the earth."