Death threats, doomed relationships, divorce… As we fall under the spell of the next series of Big Brother, James Moore examines the legacy of the show so far

When celebrity drama queens Vanessa Feltz and Anthea Turner got over-emotional on the set of the first Celebrity Big Brother no one was too amazed. But when a man held a knife to the throat of another housemate on the US version things were certainly getting out of hand.Out in the 'real' world, many of us find trying to get to work or doing the shopping a trying enough experience. So it's no surprise that being monitored 24 hours a day while trapped in a house with a bunch of strangers can have dramatic and sometimes disastrous results.

There is increasing evidence that many of the housemates – both here and in the other versions of the hit format around the globe – have struggled to cope with the pressures of reality TV. In the American version of the show, bartender Justin was thrown out for holding a knife to a fellow contestant’s neck, saying: ‘I'm going to slash your throat.’ And while no housemate has yet gone as far as a Swedish contestant on Survivor – who committed suicide after leaving the show – Big Brother has been blamed for break-ups and bizarre behaviour all over the world.

In Portugal one couple infuriated MPs and the Catholic Church by having sex on air (they married shortly after) while in Denmark three participants walked out overnight in a huff. In the Australian Big Brother even housedog Ebony had to be removed by programme bosses after becoming withdrawn and depressed.Here in Britain, BB2’s Brian smashed crockery to relieve his frustration, Josh suffered from insomnia caused by the worry of being booted out while Paul was so worn down by the pressure of constant eviction he even saw UFOs flying over the house. And who can forget Vanessa's very public tears on Celebrity Big Brother?

So how will the new contestants cope? Psychologist Dr Martin Lloyd-Elliott believes that the pressure cooker environment of the house can exaggerate normal behaviour patterns. He says: ‘The show can act as a magnifying glass to areas of people's personality that would otherwise be diluted. The same is true if you go on a camping trip and stay in the same tent with three other people. Personality traits become more exaggerated in the intense surroundings. Things can reach boiling point.’

He also warns that the show could push a relationship to breaking point: ‘Being in such an intense situation can bring things like romantic relationships to a head, especially if the person outside is seeing a different side to their partner for the first time.’

In the UK's Big Brother 2, Bubble's relationship hit the rocks after he discovered – via the press –that his girlfriend had cheated on him while he was in the house.Helen's relationship with boyfriend Gavin ‘Big G’ Cox was also doomed after her constant flirtation with fellow contestant Paul. The two shared a romantic dinner, spent the night together alone in the den and cuddled under the duvet in full view of millions, including Big G. Paul and Helen started dating after the show. In the US one of the contestants also filed for divorce after a marriage break-up caused by the show.

Many would agree that in both the US and UK the most interesting and eccentric contestants have been the first to go. And even those who are used to the spotlight have cracked under the pressure.During her stint in the house for Celebrity Big Brother, Vanessa Feltz said: ‘I really don't think I can endure the mass humiliation of everyone hating me so much,’ and Anthea Turner looked like her world had collapsed when she was faced with the prospect of eviction. Dr Lloyd-Elliott says: ‘Most people who go into the shows want to be famous but they may not always react how they expect to.’

The pressures in the house can have a similarly dramatic effect on those outside. Contestant Narinder Kaur’s mother became a social outcast following her daughter’s antics on the show. It can’t have been easy for Big G either. He heard Helen – the woman he planned to marry – reveal that she would have sex in the Big Brother house. She also described their relationship as ‘on and off’ - all in a very public bid to bed Paul. Millions of viewers witnessed Gavin Cox’s humiliation.Dean’s girlfriend, Vanessa Jones, was also said to be unhappy at watching Elizabeth flirt with her man.

Dr Lloyd-Elliott points out that although the financial stakes and the chances for eviction are evenly spread, some personality types are better equipped to cope with the house environment than others. ‘People who are more introverted and self-reliant may be able to cope better than bubbly personalities who may react badly if they do not always get all the attention they crave,’ he says.

But do the show's bosses have a responsibility to make sure contestants don't suffer in the surreal world they have created? Big Brother hired psychiatrists to be on hand for the past housemates with the promise that nothing they revealed would be broadcast, but Dr Lloyd-Elliott believes the buck ultimately stops with the contestant.He says: ‘The shows have potential to be dangerous for people if they are handled badly by unscrupulous programme makers. But ultimately we are all responsible for our actions whatever the environment.Most of the people who go in for Big Brother know what they are letting themselves in for. To some extent you make your own bed – and you lie in it.’