Rock and roll on some nine years and the "young, happy and fun" Britney has been transformed into the mad, bad and sad woman of psychiatric wards and courtrooms - a woman confined by her father's legal order of "conservatorship", which puts him in charge of her life and estate. Were it not for the tabloid headlines, the websites screaming "skanky whore" and the hundred pursuing paparazzi waiting with cameras by her door, we might almost be in a Victorian melodrama scripted by Wilkie Collins. Then as now, it seems, men can be wild and bad, transgress bounds, enter the revolving doors of what we casually call "rehab", without incurring the stigma and constraints of madness, whereas women, certainly once they have reached the maturity of motherhood, cannot. Being a bad, rebellious girl, in the style of Amy Winehouse or Lily Allen, may just about be permissible, but the socially defined limits of what is considered "sane" quickly narrow with the arrival of babies.

So what has happened to Britney Spears? And since we are somehow implicated in the life of our celebrity icons, what has happened to us all that we bay with schadenfreude at the fall of Britney from jubilant girlhood to a womanly madness that seems to warrant a paternal straitjacket?

Raised in smalltown Louisiana by an ambitious schoolteacher mum and a hard-drinking, building contractor, soon to be bankrupt, dad, Britney, it is said, already liked to perform when she was two. She took up acrobatics, sang in the local baptist church choir and, at the age of eight, was taken to audition for television's long-running Mickey Mouse Club. Too young for a part, the audition nonetheless got her a New York agent, three summers at the Performing Arts School, off-Broadway parts and a juicier one in the musical Ruthless!. A second audition, at age 11, saw her through the Disney doors. For two years, she was part of the young singing-and-dancing cohort, which like all Disney productions aims to be all-American and all-inclusive. This is the club that enjoins all young America to belong to its upbeat good cheer. The vivacity stayed with her. The inclusiveness did not: a display of troubled maternity is not what America likes to see in its star mothers.

There is nothing mousey about Mickey Mouse clubbers. Britney's later beau, Justin Timberlake, was right there beside her, performing for the cameras and for millions of child viewers. Indeed, it would be perverse to think that such histrionic habits laid down early and reinforced by success could lead to anything other than a love affair with image, the camera and the applause that meets an exhibition of the self.

Britney rose to rare heights for a female pop artist - one groomed and managed by a showbiz machine that wanted to keep her pure, fresh and innocently slutty. Her second album, Oops ... I Did it Again, sold more copies in its first week than any other chart album. A stream of Billboard Music awards and Grammy nominations followed. Her third album, Britney, also debuted at No 1 in the US. She co-wrote five of the tracks and topped Michael Jackson in the ratings. She hadn't yet hit 20. By 2002, Forbes magazine ranked her the world's most powerful celebrity. In the Zone (2003) once more started out at No 1 - a record four in a row, out-performing any previous female singer. What matter if the critics carped, unconvinced by Britney's unsteady transition from teen virgin to sexualised woman? By now, she was earning extra multimillions by cashing in on her celebrity: there was an ad campaign for Pepsi, merchandising, touring, DVDs, even a book, written with her ever-present mother. Soon there would be highly lucrative perfume endorsements.

Only Madonna - her older material girl icon and never, like Britney, a self- and manager-styled virginal girl-next-door, but a big-city lass who knew her men and her onions - matched Britney's global fame. In 2003, in Me Against the Music, the two performed together. The accompanying video shows Madonna luring Britney through the labyrinth of an underground club, then disappearing just as Britney reaches close enough to touch. Later that year, when Britney kissed Madonna during the 2003 MTV awards, the gesture might well have marked her sense that she had now overtaken her. She, too, could now be in control. "Maybe she was my husband in another life," Britney is reported to have said.

By June 2002, Britney's much publicised relationship with Timberlake had ended. On a drug-fuelled Las Vegas weekend in January 2004, she suddenly entered into a marriage with a childhood friend, Jason Allen Alexander. This lasted all of 55 hours before her mother railroaded an annulment on vague grounds of incompatibility. Six months later, Britney announced her engagement to Kevin Federline, a back-up dancer for Timberlake, and a sometime rapper and model, professionally known as K-Fed. He had just split from his wife, who had recently given birth to their second child - a portent of things to come, one might say.

The couple married quietly in September and soon Britney announced that she would be taking a break to devote herself to the making of a family. The word "family" carries as much symbolic freight as the word "virgin" and its moral power was to bear down on Britney with a particular vengeance. By the time her first child, Sean Preston, was born in September 2005, a reality TV show, Britney & Kevin: Chaotic, that she and Federline had made about their courtship and wedding had been screened. They released it on DVD a few weeks after the child's birth. Not surprisingly, the show has Britney interrogating people about their views on love, sex and marriage. The tagline interrogates: "Can you handle our truth?"

In the wake of her encounter and identification with Madonna, Britney had joined the Kabbalah Centre. Now she left it publicly, announcing: "I no longer study Kabbalah. My baby is my religion." Religions, often enough, demand icons. They also demand adherence and come with generalised rules about behaviour. Motherhood and the family are no exception. Britney was a dab hand at the icons, but she fell foul of the second and is still reaping the punishments.

To mark her participation in the religion of mother and baby, a pregnant Britney posed nude for Harper's Bazaar, appearing on the cover of its August 2006 issue. Her hair now queenly dark, her belly perfectly rounded and airbrushed free of veins and stretchmarks, she is the very apogee of poised, yet still emphatically sexy, motherhood. Her face doll-like, she lies bared against what looks like a soft, furry-white, nursery blanket in the pose of a soft-porn, but pregnant, Venus. In a take on the iconography of virgin and child, she balances her first son on the perfect arch created by her unborn second. His naked puckered flesh creates a juxtaposition to the smoothness of her back, bare to teasing buttock, from which the folds of a black dress fall.

Britney just about got away with it. After all, Demi Moore had been there before her. But posed images rarely have much to do with the messy realities of everyday life. Nor, it seems, as Princess Diana sadly learned, can the press simultaneously be wooed and kept away.

On September 12 2006, two days before her older son's first birthday, Britney gave birth to her second, Jayden James. By November she had filed for divorce: Federline, who liked to party and hang out with male friends, was not living up to hopes. She asked for physical and legal custody of her children. Federline, apparently taken by surprise, counter-sued. In response to her original text message asking for divorce, he had scrawled on a nightclub bathroom wall: "Today I'm a free man - Fuck a wife, give me my kids, bitch." Kids, of course, come with generous payments from the Britney treasure-trove.

Two children under 18 months, let alone the postpartum hormonal blips that all women are subject to, compounded with the obsessionality that the failure of a relationship inevitably brings, a custody battle, the milling paparazzi at the door - these are hardly a recipe for calm behaviour. Not much wonder that Britney's actions were erratic and, as an increasingly condemnatory media noted, "unstable". She was still only 25. But the once cheerful Mouseketeer had let American motherhood down and the public's representatives, the media, were moving in to prevent Britney - now "unfitney" - from getting up again.

Marilyn Monroe, that icon of a previous era who spent a good part of her last years on the analyst's couch, once said, "I'm always running into people's unconscious." Saucy virgin Britney ran into the unconscious that doesn't like sexualised, transgressive mothers, and she began to pay the price.

She also now seemed to relish a perverse rebellion against all the expectations that her former golden-girl image had so carefully fostered. She went wild and bad. She partied on the LA scene, drinking, snorting, vomiting, hanging out with Paris Hilton, chain-smoking, swearing at reporters, screaming at fans. In January 2007, her favourite aunt had died of cancer. A few weeks later, pressured by her mother, she took herself into rehab at Eric Clapton's Crossroads in Antigua. She checked out a day later. The following night, February 17, she walked into a beauty parlour back in California and demanded that they shave her hair off, a heavily freighted feminine gesture. There would be no more lushly fertile Britney, no more Britney sporting the locks of sexual availability. When the hairdresser refused, she took her razor in hand herself, and performed the task.

Like all of Britney's acts, this one hit the headlines. Beautiful, virginal Britney had now become madwoman Britney - her prototype those haunted faces of inmates in the early 19th-century French asylums, like the revolutionary Théroigne de Mericourt.

A few days later she was back in a treatment centre in Malibu, and stayed for almost a month, before re-emerging to continue her bout of badness. She drove her car wildly, racing against the paparazzi, who were ever ready to pursue, stopping to allow them to click, and then re-engaging in a chase. She was snapped to reveal no knickers under her dress. "Britney's Badger Goes Free-Range," shouted the headlines. She attacked one of the paparazzi with an umbrella. She started an affair with another, hating and loving at the same time. The money they made from her snapped image ran into millions: her pictures accounted for 20% of paparazzi agency coverage that year. On the internet search engine Yahoo, her name topped the search charts.

In late September, after she had been charged with a hit-and-run incident, and had as a result lost custody of the children in yet another court hearing, Britney appeared at the MTV Video Music awards. She was nervous. Her hair was less than six inches long and she battled against wearing the prepared wig. She also determined not to don the appointed corset-style dress. She would appear wearing only a glittering black bikini and her unabashed nakedness. See and take me as I am, seemed to be the message. The response to her visibly rounded post-pregnancy body as she danced and sang Gimme More was less than kind. There were jeers and hoots.

We do not want our pin-ups to wear the very signs of what their sexuality is - in part at least - most certainly for: reproduction. Britney could only be "mad" for challenging our ambivalence about the female body in that adamantly upfront way. Other celebrity mums - Victoria Beckham comes to mind - hide the hated signs of maternity in anorectic thinness, reproducing, instead, an eternal girlhood, no matter how many children they have in tow.

There is a song on Blackout, the album that, despite all the travails, came out last October, which in its humour should make us question Britney's purported madness:

I'm Mrs Lifestyles of the rich and famous

I'm Mrs "Oh my God, that Britney's shameless" [...]

I'm Mrs "You want a piece of me?" Tryin' and pissin' me off

Well get in line with the paparazzi

Who's flippin' me off

Hopin' I'll resort to startin' havoc

And end up settlin' in court

Now are you sure you want a piece of me?

Disturbed, unhappy, wild, maybe. But utterly deranged and needing the confinement of paternal "governance", certainly not.

Nonetheless, earlier this year after she refused to return the children to her husband, now known as Fed-Ex, and barricaded herself in the bathroom, scores of police broke through the ranks of photographers in front of Britney's gated home to take her off to a hospital where she was held for an "involuntary" evaluation. She lost custody of the children. Some weeks later, on January 31, the scene was repeated late at night when paramedics, the LAPD and a fire engine once more rushed to the camera-filled scene and carted Britney away to the UCLA Medical Centre's psychiatric care facility, this time at the behest of the Spears family and their psychiatrist, on a "5150 involuntary psychiatric hold".

Daddy had now taken charge, and despite his daughter's release from the hospital a week later, he had also taken charge through the courts. In a statement, Jamie Spears said that he feared for his daughter's life. He called her "an adult child in the throes of a mental health crisis". The statement won him court permission to fire Britney's manager, take over all her documents, records and assets, and effectively to take legal control over her life - and her millions.

Is it likely that a father would have dared proceed in the same way with an adult son and received such ready acquiescence from the courts and a good part of the media? No fathers have appeared to take legal charge of the countless male pill-popping pop stars whose language and behaviour are less than clean and who live out some of the wildest dreams of the adult children we all sometimes are. But women, it seems, like their Victorian great-grandmothers, still need to be taken in hand and charged with madness.

In 2006, Britney had written a poem about the "sins of the father" and told friends he was emotionally abusive: "The guilt you fed me/Made me weak/The voodoo you did/I couldn't speak."

Let's hope Britney, however troubled, fights back and doesn't succumb to her father's "fears for her life". Let's hope the media help her. That would really be an iconic victory.

· Lisa Appignanesi's Mad, Bad and Sad: A History of Women and the Mind Doctors from 1800 to the Present is published by Virago/Little Brown.