Woody Allen's latest film explores the world of the serial adulterer. Annabel Ferriman reports on illicit sex
PRESIDENT CARTER did it in his heart, President Mitterrand did it in secret, Hugh Grant did it in a car and President Clinton did it in the Oval Office - adultery seems as popular today as a meal out at a restaurant or a night out at the movies. Why can't some men confine their sexual activities to the marital bed?

In this weekend's top grossing film, Deconstructing Harry, Woody Allen plays the part of a serial adulterer, who gets through three wives and a 25-year-old mistress, and isn't faithful to any of them. Indeed, Harry Block, the eponymous hero, not only sleeps around but specialises in taboo relationships: he sleeps with a patient of his second wife (she is an analyst), the sister of his third wife and, finally, an acolyte more than 30 years his junior.

It is a portrait of a sexual compulsive, of someone constantly seeking out new relationships, who knows it will end in tears, but cannot stop himself. The unsatisfactory nature of his life is forcibly brought home to him when he finds that he has nobody to accompany him to a ceremony in his honour at his old university. He ends up taking a prostitute.

If adultery is so complicated, unsatisfactory and dangerous, why do men do it? Evidence suggests that plenty of them do. Figures vary from 10 to 75 per cent of men with live-in partners, depending on the type of questionnaire used. Telephone surveys apparently produce low numbers (possibly because the partner is in the room during the phone call) while magazine quizzes produce high numbers (men boasting?).

The National Survey of Sexual Attitudes and Lifestyles, financed by the Wellcome Trust, covering 20,000 adults and using validated sociological methods, showed that 4.5 per cent of married men had been unfaithful in the preceding year, while 15 per cent of co-habiting men had been unfaithful (the figures for women were 1.9 and 8.2 per cent respectively).

Dr Susan Marchant-Haycox, a London psychologist, puts forward four main reasons for adultery: fear of commitment "most men and women seek commitment in a relationship, but they also fear it"; poor sex in marriage, which may affect about one third of couples; competition (men resenting partners being more successful on the work front and turning to adultery to boost their egos) and curiosity.

Paul, a 48-year-old divorcee who works in television, admits to having been a serial adulterer over the 23 years that he was married. He started sleeping with other women because he and his wife were sexually incompatible. "There was something fundamentally wrong in my marriage. Passion was missing. I did not get to a level of fulfilment with my wife that I realised that I could achieve with other women. I had already started an affair with someone else before I even got married, and when my future wife found out about it, she gave me an ultimatum - either we got married or we split up. I decided to get married.

"Then we just let things drag on. In an ideal world, one recognises that there is a problem and finds a way of dealing with it. But we neither sought help nor separated. 'We both wanted children and I made a big effort after our daughter was born to make a go of things - not to be unfaithful and to develop other interests. I was faithful for several years but I was unhappy."

Paul says that he married too young - he was 22 - and Dr Marchant-Haycox says that adultery is particularly common among men who become husbands at an early age. "They want to relive their youth, trying to have a young man's life they never had. They seek sexual adventure."

Dr Janet Reibstein, a psychologist practising in Cambridge and London and author of several books on sex and relationships, feels that men like the Harry Block character in Deconstructing Harry are suffering from a narcissistic disorder, which often turns into a sexual disorder.

"They want gratification for themselves, instead of entering into a reciprocal relationship. They cannot achieve intimacy. They are after personal gratification above all. They feel depleted and unstimulated when their relationship is going through a lull or a low spot. They will say 'this relationship is not working' and seek erotic stimulation to make themselves feel better."

Dr Reibstein, who presented the Channel 4 series Love Life, was pessimistic about Harry Block's prognosis, however. "There are an awful lot of analysts who would say that extreme narcissistic disorders cannot be treated," she explains.

Are Harry Block and other adulterers just doing what a lot of men would like to do, but are prevented from doing by their consciences and beliefs? Apparently not, according to Dr Reibstein. When researching her book, Sexual Arrangements,she discovered that a man or woman's religious and ethical beliefs had little to with their behaviour. "Whether or not someone disapproved of adultery did not seem to influence whether or not they committed it," she adds.

Paul's own belief did not stop him from adultery. "I felt terrible guilt because I had been brought up as a strict Christian. More than once I made an effort to get to grips with the situation and give up my dalliances, but it never lasted," he says.

"People either consciously or subconsciously shut their eyes to things they do not want to see. My wife chose not to recognise what was happening, except when it was put directly in her path. I am in a faithful relationship now, but I still find women desperately interesting - their looks, their minds, their bodies. Maybe I am a sex addict or maybe it is just that I find women fascinating. But if you are sexually satisfied and fulfilled, it is like an aesthetic pleasure. You can enjoy looking at women, but you don't have to do anything about it."