Q A teenager I know is distraught after her boyfriend posted a clip of them having sex at a party on YouTube. How can I protect my daughter, 16, from this?

DR THOMAS STUTTAFORD

A You don’t make it clear in your letter, but I’m assuming that the teenager you mention consented to having sex with her boyfriend and he filmed her during the act with a mobile phone. Whatever the circumstances the girl, rightly, feels badly let down. Obviously, she initially found this boyfriend attractive and trustworthy, but I’m afraid this episode casts serious doubt on her judgment of people.

Everyone makes mistakes and there are few of us who could say that someone’s sex appeal hasn’t enabled us to belittle, even overlook, the flaws that are obviously present in a beloved.

This teenager has my sympathy and, I suspect, that of nearly all our readers. It is a safe bet that the teenager’s friends and colleagues will by now have all watched the film and laughed and joked as they judged her appearance and performance. But they should remember that it is a golden rule never to make jokes, especially critical ones, at the expense of a child or teenager as the impact can last a lifetime.

The teenager needs the support of all those who know her. I hope that nobody allowed this incident to alter their approach to the teenager and that she was asked to all the local Christmas and new year parties. I fear that when she wasn’t present she would have been the butt of some very explicit jokes during repeat showings of the clip.

There is a danger that the story of the film will haunt the teenager for years to come. Her friends and their parents will remember it; it will colour every introduction to new boyfriends and, if she is unlucky, it will even follow her to her first workplace or university. It might even mar her career.

This story illustrates a change in social mores and the emergence of an important problem that confronts all parents. In the past, psychologists taught that the most important influence in an adolescent’s life was his or her home background. School came second, followed by peer pressure. But parents now have to accept that peer pressure is the prime influence on adolescents. The school, including school friends, is next and home influence has become relatively less important, although still crucial. For generations, parents have discussed endlessly the selection of schools. Now it is equally, if not more, important to take similar care in attempting to influence the choice of friends. Note: influence, not control, the choice.

Parents can’t choose their children’s friends but, mercifully, children can be influenced and this must start at an early age, long before puberty. One opportunity, never to be missed, is to illustrate the significance of people’s characters and actions by watching TV with children. Parental comments, politically incorrect if necessary, can be employed to accompany a TV film to emphasise the vagaries of human nature. Don’t worry about the nature of the plot, it is the interaction of people and their behaviour that is educational.

Someone brought up to believe that everyone is trustworthy will be unprepared for the antisocial, psychopathically disturbed minority. It is too late at 16 to expect someone to accept an analytical discussion of their friends. If all your daughter’s friends seem ghastly, steer her to a career or university where she can begin again.

Dr Thomas Stuttaford, The Times doctor, spent many years working in a genitourinary clinic

SUZI GODSON

A Sadly, we cannot choose the boys our daughters fall in love with, but we can and should tell them that allowing themselves to be filmed having sex with a boyfriend is an incredibly stupid thing to do because they have no control over how that footage will be used when the relationship ends, as it will.

One child in four under the age of ten in this country has a mobile phone, and the majority have video facilities on their phone. Bebo, MySpace, Facebook, YouTube? If our kids had as good a grip on the national curriculum as they have on social networking sites, we’d all be smug parents indeed. Happy slapping, blogging, Big Brother? For teens today, watching and being watched is normal. In fact, your daughter and her friends belong to the most monitored society in the world. Britain has more than 20 per cent of the world’s CCTV cameras – 1 for every 12 people in the country.

Because teenagers have grown up with surveillance technologies, they should understand the consequences of getting caught on camera but, unfortunately, as the psychologist Professor Laurence Steinberg says, the teenage brain is like a car with a good accelerator but a bad brake. By the age of 15 or 16, the parts of the brain that arouse a teenager emotionally and make her pay attention to peer pressure are on full throttle, but the frontal lobes, the parts that control impulses and resistance to peer pressure are still developing.

The inability to see the link between behaviour and consequence is particularly dangerous when it comes to the internet. The content exchanged on social networking sites such as MySpace, Bebo, Facebook and YouTube is public information but most young people either do not know, or worse, choose not to care, that everything they write or display can turn up in online searches. A survey of 2,000 14 to 21-year-olds last October found that more than 70 per cent of young people would not want a university or potential employer to see the information they post about themselves on networking sites, yet 60 per cent did not know that potential employers could check up on them or find their details in search engines.

Although your daughter’s friend may be able to get the host site to remove the footage of her having sex, that won’t necessarily stop the images circulating as downloads. And she has no comeback against the site. In Brazil, Daniela Cicarelli, the ex-wife of the footballer Ronaldo, successfully sued YouTube for posting a video of her having sex with her boyfriend on a beach, but she had a great lawyer, and a great figure. The average British schoolgirl is unlikely to have the same success.

Seeing her friend’s distress may be enough to convince your 16-year-old that having sex in a public place is not the smartest thing to do, but it probably won’t be until she is in her twenties and her reasoning and judgment are more fully developed that you will begin to sleep more easily at night. In the mean time, it may be worth considering this: the main reason teenagers have sex at parties is because they haven’t got anywhere else to go. If your daughter is sexually active, allowing her to bring her boyfriend home might be the only way to protect her from a similar indignity.

Suzi Godson is author of The Sex Book (Cassell, £16.99) and The Body Bible (Penguin, £16.99)

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