IN THE end it was Richard Newman - the camp waiter - who hurled the most vicious insult. In two and a half weeks during which Big Brother contestants have called each other imbeciles, hypocrites and manipulative, two-faced liars, he went the whole hog and used the D word. Forced to confront his feelings towards Imogen Thomas (the Welsh beauty queen), he dubbed her dull. Not cruel, or ignorant or ugly or smelly, but dull. You see Imogen, God bless her, isn't anorexic, bulimic or manic depressive. She doesn't suffer from OCD, ADHD or Tourette's syndrome. She isn't lesbian, bi-curious or transsexual, and she doesn't even cross-dress. She's just a passably pretty girl from Llanelli with a bit of a tip about herself. But in the Big Brother house - to paraphrase from the film American Beauty - there is nothing worse than being ordinary.

Of course, that Endemol has selected a bunch of mavericks - Imogen apart - many of whom are at the extreme ends of the sexual spectrum, should come as no surprise to veteran viewers. From dippy Jade to transsexual Nadia, eccentricity is Big Brother's stock-in-trade. Every year, the programme-makers are forced to up the ante to keep it fresh, and every year there is a predictable but short-lived clamour from those who have taken on Mary Whitehouse's mantle.

There is, however, a growing sense that this year, Endemol has finally crossed the line. As always, Big Brother's obsession with contestants' sexuality is to the fore, with two screaming gays, one screaming heterosexual and a pre-op transsexual already through the doors. It was this, I suspect - and the monotonously sexual nature of the housemates' conversation - that prompted Jack McConnell's call last week for the media to cut down on the sexual imagery on television.

Much more worrying, however, is the fact the programme features no fewer than five young people with serious mental health problems: Pete Bennett - a cross-dressing 24-year-old with Tourette's syndrome; Nikki Grahame, who has no enamel left on her teeth as a result of anorexia; Lea Walker, who suffers from body dysmorphia, and Shahbaz Chauhdry, a former sex abuse victim with a history of suicide attempts, were all picked to start the programme. Then, when Shahbaz left after threatening to kill himself just four days into the show, in walked Sam Brodie, an 18-year-old pre-op transsexual from Ayrshire who suffers from paranoia. So disturbing has it been to watch the minds of these troubled people unravel in front of unflinching cameras, that a succession of mental health charities - including Sane and the Mental Health Foundation - are calling for the programme-makers to stop exploiting vulnerable people for entertainment.

Doubtless Endemol - which insists a psychologist is on hand 24/7 to offer support - will say the show is merely holding up a mirror to society. And it's a tough argument to counter because, in the seven years since Big Brother first hit our screens, reality TV and youth culture have become so entwined, it has become difficult to establish which is feeding off which. For example, there is little doubt that cosmetic surgery (false boobs often feature on BB) is becoming more popular with ordinary, working people, hence the popularity of shows such as Extreme Makeover. Or is it that shows like Extreme Makeover present plastic surgery as a viable option for anyone who takes exception to the shape of their nose/stomach/thighs, thus fuelling demand for the risky procedures?

In the same way, a glance at the tabloids suggests young people are becoming more sexually adventurous, with Premier League footballers taking part in so-called 'roastings', and the Primrose Hill set forever hopping in and out of bed with their male and female friends. On a more prosaic level, the small ads feature an increasing number of bi-curious people seeking same for fun without strings.

But even as it exploits this apparent cultural trend, Big Brother is surely driving it by normalising lewd and dysfunctional sexual behaviour for an impressionable young audience.

Endemol may also claim that by featuring contestants with conditions such as anorexia or Tourette's it is widening people's awareness of mental illness but, of course, it is doing nothing of the sort. Instead, by focusing on the physical manifestations of their problem - zooming in on Pete's worsening tics and Lea's surgically altered body, for example - it presents them as freaks. Stripped of a wider context, it is virtually impossible to gain any insight into the complex set of circumstances that made these complex individuals who they are, or the toll their illness takes on their lives.

Because some contestants - like Pete - are clearly celebrities in the making, and their mental illness is part of what makes them stand out from the crowd, their condition can seem like something to aspire to. If you haven't got a personality, then get yourself a personality disorder.

Others - like Lea the body dysmorphic and Shahbaz - find their mental fragility alienates them from their housemates, and they become targets of brutal bullying. While schools are busy implementing policies to eradicate such behaviour from the playground, Big Brother sends out a very different message: that it is acceptable to target the weakness of others in an attempt to highlight your own strengths, and to take pleasure in other people's mental collapse.

Marjorie Wallace, the chief executive of Sane, one of the mental health charities to voice its concern, has pointed out that society has long been fascinated by how much stress human beings can withstand. Experiments conducted in the first half of the 20th century showed how sleep and sensory deprivation - two key features of a stay in the Big Brother house - could leave people neurotic and disorientated.

But there is another infamous experiment which has a bearing on the current series of BB: the one in which a succession of students proved themselves willing to administer increasing levels of electric shock to human guinea pigs because they were told it was a valid scientific exercise.

Isn't this what the viewing public has been doing with Big Brother: colluding in the ritual degradation of disturbed people because programme-makers assure us psychologists are taking care of everything; or because critics bill the results as sociologically important; or just because we don't want to appear po-faced and prudish in front of our peers?

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