It's hard to define exactly the arresting expression on her face, but it hints at an emotional state somewhere between defiance and defeat, suggestive simultaneously of simmering hysteria and the placid absence of a lobotomy patient.

This moment, the one in which Britney Spears grabbed the shears and laid herself physically and emotionally bare in front of the world, wasn't by any means the rock-bottom point in a year when we have seen stars behaving dementedly in greater depth and more detail than ever before. But it was the point at which exhibitionism and dysfunction melded completely, symbolising an emerging new brand of popular culture - live, real-time celebrity breakdown.

At that point, it seemed inconceivable that things could get any crazier. Little did we know. Since then, in the case of Britney Spears alone, we have watched, agape, as she has destroyed her personal life, lost custody of her children and systematically sullied her professional reputation. At an OK photo-shoot in August she wiped her chicken-grease-covered hands on a designer's dress, allowed her dog to foul on another, and then disappeared after five minutes in front of the camera, taking with her £10,000-worth of clothes and jewellery.

And then, just when we thought her humiliation couldn't get any more public, we had her jaw-droppingly incompetent performance at the VMA awards. So comprehensive is the catalogue of eccentricities and erratic behaviour displayed by the singer that she has achieved the remarkable feat of making her slimy, opportunistic husband, the same man who had previously walked out on his eight-months-pregnant girlfriend to shack up with this superstar (for whom he seemed to hold limited genuine affection), look like a character in The Waltons. But Britney is not alone. 2007 was the unforgettable year of Amy Winehouse's bloody ballet slippers, of Heather Mills's ranting on morning television, and Jade Goody's pin-drop exit from Celebrity Big Brother. More recently, Sophie Anderton offered to sell her body to an undercover reporter, and Lindsey Lohan and her pals gave away free glimpses of body parts that can generally only be seen on pay-per-view. Week by week, we watched vivid, violent, freeze-framed snapshots of superstars going off the rails.

And we followed it, of course, with slavish devotion and mock-appalled delight. After all, the appeals of celebrity dysfunction are manifold, but they boil down to one single point. The stars have everything we want. And there is no better antidote to envy than watching success and beauty corrupt irredeemably.

It has, by now, been repeatedly observed that this epidemic of mental instability, or at least the breakdowns that attract the highest viewing figures, occur mostly among young women who have precociously reached great heights of fame. Feminists have rushed to an analysis of this as an example of the way modern society prematurely sexualises its female stars, only to ridicule and degrade them for sport when they have passed a certain, stringent threshold of age and physical perfection.

Now, for the most part, the women who play starring roles in this carnival of mental imbalance do so of their own volition, eagerly martyring their lives and their sanity for the sake of our ongoing entertainment. But the feminist argument becomes more convincing when you consider, as a counterpoint to the Britneys, the Amys and their kind, and all the messy, bloody, drug-fuelled hysteria they spread across countless magazine pages, the example of the one highly publicised mental breakdown suffered this year by a man. After Owen Wilson tried to kill himself in August, he made the well-advised decision to take the opportunity to retreat from the public eye, rather than vomit his "issues" all over the nearest paparazzi. Perhaps that is the reason his case was handled so sensitively, why his calls for privacy were respected, and why no one took much interest in his illness except to hope that, before long, he'd be well again.

But even if Wilson hadn't behaved so impeccably under pressure, his story would never have attracted the same level of traffic as the girls'. That is partly because the consumers of celebrity-breakdown sagas are mostly women, who find in them some kind of consolation for the ordinariness of their own lives. It's the female audience that contributes most generously to lining the paparazzo's pocket.

Women's interest is rooted incomparison and competitiveness rather than empathy, and so they are more interested in stories about other women.

But what else is at the dark heart of our insatiable appetite for celebrity anguish?

Recently, an LA paparazzo was quoted as saying of Britney: "She's the major story in this town. All the trouble that she has gone through, all the bad publicity - if anything, that just makes her hotter right now. Britney sells."

Similarly, Heather Mills is rumoured to have been considered such an audience-draw that she bumped Gordon Brown off a slot on the GMTV couch on that fateful morning in November. Amy Winehouse's reputation for being volatile and damaged has earned her as much attention as her remarkable voice. Those famous females whose public meltdowns have kept us all so enthralled throughout the year have two crucial things in common. Firstly, and most obviously, is their gender, which provides a partial explanation. Women demand vulnerability from other women.

The second factor that unites those stars is class. This year's most-watched basket cases - Heather Mills, Britney Spears, Jade Goody, Lindsey Lohan, Amy Winehouse - are all individuals who have, by virtue either of preternatural talent or sheer, dogged determination, defied the limitations of their backgrounds to reach a point at which they embody the height of modern aspiration. They have endless wealth, replete wardrobes and enviable figures. Most importantly, they have a constant stream of that commodity which, in today's narcissistic society, is most valuable - attention. Perhaps, alongside envy, latent snobbery is at the heart of our eagerness to see these women ripped from the pedestals of prestige on which we mounted them.

If Britney Spears is a modern-day Cinderella, risen from the ghetto (or at least from a perfectly ordinary background) and elevated to a position of the highest privilege our society can provide, we are, with her permission, scripting a very bleak anti-fairy tale. She has become a clown. We take unrestrained, vindictive pleasure in laughing at her stupidity, her lack of education, her white-trash accent, cheap taste in clothes, and the way she has defaulted to her trailer-park heritage - and always will.

Similarly, in the case of Jade Goody, we fashioned our very own council-estate icon. It delighted us that her estuary vowels and unapologetic ignorance had become a fixture on a slick and polished media landscape. Jade Goody was the underprivileged, dysfunctional social misfit with her own television series and a fragrance to match. How very amusing we found the contradiction. When, however, during Celebrity Big Brother, she revealed the brutal side of a personality that had been forged in the fires of deprivation and hard knocks, the whole nation rushed in without compunction to crucify her.

This is an era when celebrity comes first and ability second. But when you throw people into the bearpit of celebrity without the talisman of talent to protect them, or even the wit to learn how to manipulate the system, you send a lamb to the slaughter.

Owen Wilson, post-breakdown, was spared not only our scorn but also our scrutiny. He is easier to respect, of course, because he is smart and funny and creatively gifted. Certainly with an enlightened, middle-class upbringing and a tight-knit clique of film-school intelligentsia around him, there was no question but that he would be well protected at the height of his unravelling.

With Wilson, there's no fun to be had in mocking his accent. He has never displayed the kind of unsophisticated tastes that we can regard as an outward sign of some inner deficiency. Thus, there is no malicious pleasure to be taken from deriding him. The privileges of his class, and, by extension, his education have allowed him to be more of an agent and less of a pawn in the creation of his own public image. The degree to which Britney Spears, Heather Mills, Amy Winehouse and Lindsay Lohan are trapped by the chimera of fame is ever apparent. Under the heat of our attention, anyone without the wherewithal to dodge the system will burn alive.

There is no denying these women are all complicit in their fate. But there is something vaguely cannibalistic about a society that makes stars of simple, ordinary, unsophisticated and guileless people in order to then glory in the ensuing visceral carnage as they fail to cope with stardom. The anti-meritocracy we have created may have offered access to fame to anyone, but it has turned that fame into a poisoned chalice.

Of course, there are plenty of examples of success stories where people have made a virtue of working-class origins. And plenty more media figures who are, as yet, still successfully trading on a pastiche of being common. Jordan, for example, gets away with it by becoming a parody of herself, an elaborate caricature who continually underlines the fact that she never really expects to be taken seriously. That, of course and the fact that under the bimbo gloss she is smart as a whip and tough as nails.

It's a nice idea to think that social standing is no longer a value in the modern media game. It's an idea, in fact, that most people who follow the vagaries of glossy magazines and reality TV wholeheartedly buy. But all it takes is for someone like Britney Spears or Jade Goody to crack up before our eyes, and the whole world openly revels in what it seems to feel is the rightful return of trash to the gutter.