Why can some of us stay happily monogamous for 40 years, while others seem to find it impossible to resist new thrills? The answer could be in our genes. Michele Kirsch investigates

Sheila Smith can't bear to listen to it after discovering her husband Andrew's affair. "We'd been married 11 years and he was my life," she says. "Everybody talks about affairs as though they're normal. But until it happens to you, you can't know the pain of it. I couldn't eat. I wanted to die. But I wanted to kill him - and her, especially her - first."

A recent survey of 2,014 British men and women found 16 per cent of women and 21 per cent of men had been unfaithful, while another study of 5,000 men showed that more than two-thirds of them had cheated on their partners.

That the statistics vary so widely could be a reflection of the deceit that goes hand-in-hand with infidelity. "Men tend to over-report, while women under-report," says psychologist Janet Reibstein, author of Sexual Arrangements: Marriage and Affairs. We do know, however, that infidelity is thriving, but what the experts can't agree on is why. Sociologists blame it on our increasingly permissive society, while genetic researchers, such as Terry Burnham and Jay Phelan, co-authors of Mean Genes (£14.99, Simon & Schuster), see infidelity in evolutionary terms - men instinctively want to make as many babies as they can with as many different women as possible.

Burnham and Phelan also found that women who cheat are most likely to do so in the four days surrounding ovulation - the days of highest fertility - and are less likely to use contraceptives with their lovers than with their husbands. The idea that women may be subconsciously driven to cheat in order to make babies is supported by the recent finding that up to one in 25 British men are unknowingly not the fathers of the children who call them daddy.

The "infidelity gene" may sound like the world's best excuse, but it does play a part. Professor Tim Spector, the director of the Twin Research and Genetic Epidemiology unit at St Thomas' Hospital in London, conducted a study of 1,600 female twins and found that while the normal likelihood of infidelity in women is 22 per cent, having an identical twin who is unfaithful makes the likelihood 44 per cent. He concludes that 40 per cent of our desire to cheat comes from the genetic make-up we've inherited from our parents (chromosomes 3, 7 and 20, if you need to know the ones to blame) - meaning some people are more susceptible to committing infidelity.

Psychologists also tend to emphasise heritability in infidelity, but in terms of learned behaviour rather than genetics. "To say, 'If your father did it, you will do it' may be more about what you have learnt from your parents, and whether they have found successful ways to resolve their problems," says Harley Street psychologist Susan Van Scoyoc. Her views are echoed by Dr Susan Marchant-Haycox, a psychology lecturer at Birkbeck College, London. "If a child continually sees a parent being unfaithful, it could reinforce that infidelity is acceptable. As a result, the individual is eventually unfaithful to his or her partner," she says. In fact, in a recent study of more than 2,000 women across the US, researcher Carole Ellison found that 13 per cent of women who had been unfaithful had engaged in five or more affairs, and most of these women had grown up with a parent who had also been unfaithful.

The "I couldn't stop myself" theory of sleeping around has some scientific gravitas then, but surely not all children of philanderers are destined to repeat their mistakes? In her influential book on infidelity, Not "Just Friends": Rebuilding Trust and Recovering Your Sanity After Infidelity (£8.24 from www.amazon.co.uk), Dr Shirley Glass claims some of us find it easier to control unfaithful urges than others. "Attraction is a dependable constant in our lives," she writes. "It doesn't matter whether we are happily married. In the moment of attraction, we are fully alive to the possibilities of potential intimacy." For Glass, what separated the faithful from the unfaithful was a set of personality traits and values.

Among these values is our sense of commitment - whether that be attachment to a particular partner, or to the principle of monogamy - and how strong it is. People who are unfaithful are more able to rationalise and compartmentalise their behaviour. "In affairs, people are as likely to engage in self-deception as in deception of their partners," says Glass. Nobody wants to believe they are capable of the deception and destruction of an affair, so we create an alternate reality. In order to give in to attraction, you have to convince yourself your relationship isn't working, or your partner doesn't appreciate you. We give ourselves permission. And once our excuses are verbalised to a friend or the other party, they take on a life of their own.

Many unfaithful people also have personalities that need reassurance or affirmation. The quest for extramarital excitement can be an attempt to "fix"an internal problem, such as boredom, low self-esteem or existential angst. Those more likely to seek an affair include "Type T", or thrill-seeking personalities - those who suffer addictions to sex or love, and people whose egos need constant attention. "[These are] people who have some level of narcissism, who may be selfish or need a lot of attention, or those who had a difficult relationship with their parents, particularly their mother," says psychosexual therapist Carol Martin-Sperry.

But when Dr Vincent Egan, of Glasgow Caledonian University, studied the personality traits of men and women who are unfaithful, he found extreme differences between the two. While the men who cheated scored high on the social dominance scale, the women who cheated scored low. He concluded that men who tend to cheat are sociable and outgoing, while unfaithful women are often needy and withdrawn. "Men are opportunistic, if there is availability," Dr Egan says. "Women tend to stray more when unhappy, perhaps in reaction to personal distress in their lives."

Certainly, a breakdown in marital relations is one of the distressing situations behind many affairs. Glass's research had similar findings. In her clinical sample, "men who engaged in primarily sexual affairs were as satisfied with their marriages as non-involved men. But women in any extramarital involvement were less happy in their marriages than non-involved women". Perhaps, she concludes, this is because men are more likely to have an affair due to sexual attraction, while women often look for emotional connection.

So what can you do if you find yourself unable to resist the temptation of a fling? Or if you think your partner might have a roving eye, no matter how strong his commitment to you? Martin-Sperry advises keeping a constant check on your own expectations and your partner's. "Have you and your partner agreed on an open relationship, or do you expect 100 per cent monogamy?" she says. "If you haven't had that conversation, you need to - and if you have, you need to remind yourself of the boundaries you've both agreed on. Giving in to your urges will change everything, bringing lies, guilt and a lack of trust into your relationship." When it comes to confronting a potentially unfaithful partner, stick to how you're feeling, rather than blame. "Tell him you're feeling insecure and you'd like to talk about why, rather than throwing accusations," she adds.

Nip infidelity in the bud by insisting that you both turn your attention inwards, towards your relationship. "You might not be able to totally affair-proof a relationship, but you can have one that is constantly vibrant," says Reibstein. "Monitor how you are together - are you in tune, taking pleasure in one another, or are you losing touch with each other?"

Another option is the relatively amicable split, to realise that you still love everything about your partner except the fact that he or she is likely to sleep around. Jerry Hall, another ex of Mick Jagger, said recently: "Basically, we got on great. We laughed and had so much fun together. Except he slept with lots of other people, which was horrible. Otherwise, he was perfect."

The full feature appears in the current issue of Psychologies Magazine, on sale now - priced £3. Visit www.psychologies.co.uk for more information.

Ways to stay faithful

In Not "Just Friends", Dr Shirley Glass advises developing "a personal strategy for protecting yourself and your relationship from the fallout of acting on impulses". Here are her tips on controlling those urges:

* Know that attraction is normal - but feeling it doesn't mean you have to act on it. Being attracted to someone doesn't mean that you've chosen to be with the wrong person.

* Don't let yourself fantasise about what it would be like to be with the other person. Affairs begin in the mind.

* Don't flirt. To look is human, but flirting signals that you're available and you send out an invitation of receptivity.

* Avoid risky situations. After a skiing accident, President Clinton never went skiing again. He should have shown similar caution when Monica Lewinsky flirted with him.