Cathy is blonde, 5ft 2in and in her early forties. Living in London with four children, her world fell apart two years ago when Dave, her partner of 20 years, left her for a girl half his age.

She had found receipts for jewellery that she had never been given, overheard furtive phone calls and he had been flirtatious with other women over the years. But it came as a shock when a friend asked her round for lunch to tell her that he was having an affair and that most of her friends knew about it.

At first he denied it. Then he promised to change. But by Christmas, almost nine months later, it became clear that his mistress was a major part of his life.

“What struck me was that it was just like his dad, who'd done exactly the same - and left his wife of more than 20 years for a girl who was younger than his youngest child,” said Cathy. “Dave was always the favourite and closest to his dad. And maybe subconsciously he thought that it was OK. When I put it to him, he denied it, but I do wonder.”

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Background

• You're collared: men and affairs

• How to own up to an affair

• The danger of emotional affairs

• Why good husbands have affairs

Background

• He shattered my dream

• Family secrets: the grief for my illicit love

• The Secret Lives of Women: what they do

• What makes men unfaithful

This week a report by the Karolinska Institute in Sweden suggested that the love rat is not a homespun myth but that a man's tendency to stray from the marital bed may be in his genes. The study found that men who inherit a genetic variant that affects an important attachment hormone, vasopressin, are more likely than usual to have weaker relationships and marital problems.

So it is not Dave's fault, nor Cathy's, because he was born to bed-hop. Cathy, certainly, is convinced that his father's example had predisposed Dave to doing the same. It sounds like the holy grail of many a thrusting young male, scrolling through his list of dates on his BlackBerry. But can there really be a “divorce gene”?

It may be the case for randy meadow voles, as scientists proved in 2004, but are changes in the human vasoprassin receptor really to blame for turning some men into roving lotharios? “The critical thing to remember is that genetics are important, but they're not the whole story. They will determine a man's height, but that's also determined by his nutrition,” says Susan Quilliam, psychotherapist and co-author of The New Joy of Sex.

“You can say that some men are predisposed to stray, but once the genetics are set, you have the culture into which they are born, the media influences, family and society. So it's not a done deal that some men are always going to be unfaithful.” What is clear, says Quilliam, is that nurture is key. While men do have a tendency to have more partners than women, it is also true that if two people are in an unhappy marriage, one is more likely to be unfaithful. And it's not always the man. In the past, marriage was more of a contract. The woman stayed at home with the children, while the husband went to work. Infidelity often ended in the divorce courts. Now the roles have changed and, as more women go out to work, so they have more opportunity to stray and consequently women are more often becoming the unfaithful partner, says Quilliam.

Last week, official figures for 2007 revealed that the divorce rate in England and Wales is falling and that couples of almost every age are more determined to stay together. The divorce rate fell to 11.9 per 1,000 married people in 2007 from 12.2 per 1,000 in 2006. This was the third consecutive fall and brought the rate back down to the 1981 level. The number of divorces also fell to 128,534, a drop of 3 per cent on 2006 and the lowest number since 1976.

However, according to the most recent figures from the Office for National Statistics, more than twice as many women initiate a divorce as men. In 2005, 96,855 women petitioned for divorce compared with 44,583 men. Of those, 17,915 women cited adultery as the main reason compared with 10,077 men. Gary Neuman, who has been a psychotherapist and marital therapist in the United States for 20 years, was determined to discover why men cheat. For his new book The Truth About Cheating (Wiley, £13.99), Neuman interviewed 200 men from 48 US states, including 100 men who had cheated and 100 who had not, and concluded that genetics had little to do with most marital breakdowns. In fact, his book - which caused a stir this week when critics accused Neuman of blaming male infidelity on women - reveals that, far from men seeking younger, prettier and more athletic models, more men blame being under-appreciated by their wives and an “emotional disconnect” in the home than a need to find a newer partner.

“Women have been made to believe that the emotional part of the relationship is not as crucial to men,” said Neuman. “But I found that they are highly emotional and that only when they feel disconnected do they begin to stray.”

In his research, Neuman found that just 8 per cent of those questioned blamed sexual dissatisfaction as the main reason for the breakdown of a relationship; only 12 per cent said that their mistress was in better shape or better-looking than their wife; and 77 per cent of men who cheated had best friends who were unfaithful, as opposed to faithful men, of whom less than half had a best friend who cheated on his partner.

When it came to sex, it wasn't that the sex was poor, but the lack of it. Women were often too tired, their lives too busy and preoccupied for sex and were not able to find pleasure in it.

Although 69 per cent of men felt guilty for straying, they said that their marriage was like a battle they could not win: their mistakes were recognised but little positive was acknowledged - while their mistress was full of admiration and appreciation.

“When a wife is given a bracelet by her husband, she says, ‘You shouldn't have'. But she should say, ‘Thank you, you should have', because a husband wants to feel he's hitting the mark,” says Neuman. “A lot of men are very insecure, they need to be bolstered and feel they're winning at home.”

The main cause of affairs, he says, is that couples spend too little time talking to each other: in the United States, the average couple spends only 12 minutes a day in conversation. When he is asked for advice on how they might improve their relationship, Neuman suggests that couples set aside at least 45 minutes four times a week, to talk uninterrupted. After that they should go out for a two-hour date and not talk about money, work or children.

People often laugh at the suggestion, but Neuman points out that in most cases a couple would never have got together if they had simply talked about work and children.

He insists that a relationship breaks down because of a lack of communication and that in any other field, a person will work on a problem to solve it. “Successful marriages have alone time,” he says. “They spend enough time with the children, but they turn it off at 10pm. We would find the time for anyone else, but when it comes to a spouse, taking care of the relationship falls to the bottom of the list.”

When Cathy was asked whether she paid Dave as much attention as she could have and if, rather than a genetic temptation to be unfaithful, that might have been the key to the breakdown of her relationship, she says: “Possibly, because I was enjoying my job and of course the kids always come first, but clearly I wasn't the centre of his attention either.”

According to the experts, the divorce gene is no excuse for straying. We are human beings after all, not voles, so can control our instincts and take a long-term view of the consequences of our actions.

For those who will blame genes for their serial affairs, Neuman has little sympathy. But after his study, he admitted that for a handful, there was little to be done. Of those he spoke to, 12 per cent said that they would have cheated no matter what. And for that minority, there is little hope either for them or their partners.

“When I counsel women, I'm on the lookout to see if they are with that 12 per cent and if they are, I tell them they will have to get rid of him,” he says. “Because he is cavalier, has no remorse and will do it again.”

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