Where have all the pirate queens gone? Where are the cowgirls and the Supergirls? Today's fancy dress parties divide strictly on gender lines. The boys' side holds a handful of Batmans, a sprinkling of Spider-Mans, some soldiers and the odd cowboy. And on the girls' side, ten identikit princesses, swathed in pink, encrusted with fake crystals.

Is this, then, the summit of their ambition, the ultimate fantasy wish of modern girlhood - to be a princess? A role that can be inherited along with genetic mutations from generations of inbreeding. You can work for the role, it is true. Be pretty enough, my darling girl child, and mute enough, and bland enough, and you too could marry a prince. Because every girl's dream should be to lead a life of buffed and pedicured leisure, courtesy of a balding, chinless aristocrat, Whisper it, but the frog, as long as he's funny and kind, would have been the better bet.

There is an alternative to being a princess, a second costume beloved of today's girls. They shun the Ice Queens and the Elven warriors, ignore Artemis, the huntress, and Athena, the wise. Instead they celebrate the Fairy; three inches of cute, winged blondeness, dressed, inevitably, in pink.

This creeping pinkification of girlhood is ubiquitous. Toys and clothes have split down gender lines. It is impossible to buy a gender- neutral bike any more. Bikes come in blue, or in pink; as do baby walkers, and mini-keyboards, and any other toy that might once have been - imagine it! - purple or green.

Girls' jeans come with butterflies and hearts stitched on every spare centimetre of fabric. T-shirts carry cute slogans - “Cherry cute! Hello Kitty”. Swimming costumes are girdled with frills. Next time you are in the park, try to spot a prepubescent girl with short hair, or one wearing trousers. Long hair, dresses and pink; it's Amish meets Disney out there.

The triumph of this pink and cutesy ideal of girlhood is grim for more than aesthetic reasons. A report published this week by the Organisation for Economic Co-operation and Development (OECD) highlighted the differences between 15-year-old girls and boys' attitudes to learning. Even though girls graduate from senior school in greater numbers than boys across the OECD countries, girls lag behind in key areas. Boys outperform girls in maths in all but eight countries. In most OECD countries, girls and boys perform equally well in science. But in six countries, boys achieve significantly better results. Top of this list is the United Kingdom.

There is a correlation between attitudes to academic subjects and performance. In the UK, girls don't do numbers. And girls definitely don't do science. Angel Gurría, the OECD's secretary-general, argues that we are complacent about gender stereotyping and that the idea that boys don't do reading and girls don't do maths persists.

These girls will one day grow up. Even though the number of women at university is increasing rapidly, they are not narrowing the gap in science, maths and computer science. As graduates then, they leave the lucrative jobs in the City, in laboratories and in computers to the boys. Armed with liberal arts degrees - a useful accoutrement in the marriage market, like a little French and dancing once were - they may marry their prince after a few years pretending to have a career at an auction house. But happy ever after is a lie. Divorce statistics suggest he is likely to leave for a pinker, younger version.

The modern, Western world has emancipated women and made breadwinners out of them. Yet we are imprisoning our little girls in pink straitjackets, and then acting surprised later when their academic ambitions fail to outshine their accessories. Our girls' view of the world is pink-tinted partly because of the supply of cheap goods. When hand-me-downs ruled, parents would be more cautious. Now that clothes and toys are imported and cheap, it matters less if you buy all pink for your first-born, and replace it all with blue when a boy arrives. A T-shirt is expendable when it cost £5 in the shop, and pennies to make in a sweatshop employing the quick, cheap fingers of foreign children.

But the pinking process would not be happening without demand from the girls themselves and their parents. Put a gaggle of girls in a nursery and they will copy each other. Throw into the mix the culturally overbearing world of Disney, add a sprinkle of fashion fairy dust, and a roomful of princesses is born. For a vision of what this looks like, visit http://disney.go.com/princess/#/home. All the Disney princesses are there in a terrifying tableau of simpering, gurning girlishness. Why are all these princesses, the apotheoses of modern girlhood, clasping their hands together in front of them, in an expression of coy submissiveness?

If peer pressure is one driver of demand, the other must come from the parents. Perhaps this is a backlash against the Seventies, when boys called Orlando were forced to play with dolls, and girls wore trousers. Feminist theory has developed since then, recognising that there are differences between the sexes. But this seems to have mutated into an insistence that we emphasise the differences. If a girl old enough to choose begs to dress as a princess, it would be dogmatic to refuse. But why encourage this inanity in babies and toddlers too young to care?

The mothers of these girls, the careless inheritors of the equality hard won by their own mothers and grandmothers, are complicit in this pinking up of girlhood. Why? These women have themselves bestridden the world of work like colossi. Yet they are raising a generation of girls who, when confronted by a periodic table or a quadratic equation, are fit only to curl hair coyly round fingers, and say, in an affected lisp: “Why are we bothering our pretty little heads about any of this?”

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