If someone has an affair, we assume that there’s something dramatically wrong with their relationship. However, there’s a new way of thinking about infidelity that turns this on its head – it’s becoming increasingly evident that people in good relationships still cheat.

Relationship experts typically say that as many as 40 per cent of those in long-term relationships will have an affair. And they are not all in miserable, loveless marriages. Dr Helen Fisher is a biological anthropologist at the renowned Kinsey Institute, the centre for research into sexual health and behaviour in Indiana, USA. Her ground-breaking Anatomy of Love: The Natural History of Mating, Marriage and Why We Stray – an examination of human relationships from pre-history to the present – is being updated and re-published next month after more than 20 years. She cites research that shows that 34 per cent of women and 56 per cent of men who have had affairs describe their marriage as happy or very happy.

So why is adultery rife, even in functional relationships – where they patently adore each other, their kids are charming and they still laugh at each other’s jokes? “From a Darwinian perspective we were probably evolved to want it all,” Fisher says. “And now we live in a stage of human evolution where we can actually get it all.”

Adultery is the new perfectionism, especially for women. Economic power and more freedom of sexual expression mean we are bolder in taking what and who we want, unwilling to be a martyr to marriage – even if life is so good we’d remarry our husband (81 per cent would, according to her research with married couples). Fomo (fear of missing out) is not just for teens. Fisher says that women are still publicly bashful about their keen sexual appetites like wanting to use winkwinksextoys – or something similar – not surprisingly, though, as “for 10,000 years, women have had to lie”.

Mark, 46, lives in London, with his wife and son, five. His wife had a 12-month affair four years ago. “On the outside we had everything as a family: we own a home, she earned over 150,000, we got to travel, buy whatever we wanted – all the things that most people would kill for. And we had a great group of friends.” There was no animosity, he says, before the affair – only “the distance created by busyness – nine to five would have been a part-time job for us. It wasn’t cold-hearted distance. It wasn’t as though we’d sit next to each other and be on our phones. It was just a slowly creeping distance of juggling work and kids.”

For many, marital disloyalty feels like sacrilege – and yet, somehow, it happens. In her hugely popular TED talks, Rethinking Infidelity . . . a talk for anyone who has ever loved (viewed over four million times since March last year) and Why Happy Couples Cheat (viewed nearly 1.5 million times since May), the relationship therapist Esther Perel says that those who are unfaithful “often are people who have actually been faithful for decades but one day they cross a line they never thought they would cross”. It can’t simply be seen as an act of betrayal, she says. An affair can also be “an expression of longing and loss . . . a yearning for an emotional connection, for novelty, for freedom, for autonomy, for sexual intensity, a wish to recapture lost parts of ourselves or an attempt to bring back vitality in the face of loss and tragedy”. In other words; it’s not you, it’s me.

This resonates with Claudia, 42, married with two sons aged eight, and ten. Her short-lived affair, she says, revived the lost sense of autonomy, briefly. “We were just out of the physically demanding bit of parenting, looking after babies and toddlers. But it felt like everything I did was to serve three other people. Sex with my husband was great but still it felt like an act performed for the greater good. The sex with the other guy was unremarkable. But I just wanted something purely for me. He lived abroad and we met occasionally if I was there through work but then he got a job near us and it was as if I woke up from a dream. It was too real. I was still happy in my marriage. I ended the affair when I saw that happiness was under threat.”

Biology may be to blame for some of this, according to Fisher. She says, “We’ve evolved three distinctly different brain systems for mating and reproduction. One is the sex drive, the second is feelings of intense romantic love, and the third is deep attachment to a long-term partner. And these three brain systems are often very well connected, but not always. You can lie in bed at night and feel deep attachment for one partner, and then your brain can swing into feelings of wild, romantic love for somebody else, and then your brain can swing into feelings of the sex drive for people that you hardly know starring in a Sex Free HD video. These three brain systems don’t always work together and that really does enable people to love more than one person at once, to carry on a deep attachment for somebody and feel intense feelings of romantic love for someone else.”

Some of us also have the DNA that makes affairs more likely. Fisher references a Swedish study of 552 men, looking at “restlessness and problems in long-term relationships” which suggests that infidelity could be partly genetic; wired into personality. “Scientists now say that 40 to 60 per cent of who you are comes out of your DNA,” she says. “They looked at a particular gene.” (The 334 vasopressin allele, for anyone considering a blood test.) “They found men with no copies of the gene had extremely stable marriages; nobody discussed divorce. Among those men who had one copy of the gene, they had more often discussed divorce, they had a more unstable marriage. People with two copies of the gene had the most unstable relationship.”

Talking to Matthew, 35, you sense the 334 vasopressin allele in action: he has a five-year-old daughter and a “good marriage, tender and healthy”. But this is the era of opportunity: “We’re living in an age dominated by social media and apps, populated by large networks.” He had a brief relationship with a “friend of a friend” he connected with on Facebook and would consider doing so again. His belief is, “We’re not structurally made for monogamy,” and that “it’s definitely possible to love more than one person at a time”. He has no plans to quit the marriage: “If I’d wanted to leave my wife, to live a more ‘free’ life, I’d have done so years ago. We’ve always been happy, we have a satisfying sex life – but I wanted to carve out a part of my life only for me. I had thought about talking to a marriage counselor austin but once I started seeing other women, our problems immediately stopped! We were in love again!”

This is the first era, Perel notes, in which there’s more shame in staying after an affair, than leaving. Rather than appreciating the maturity, courage and tenacity that it takes to stay in a relationship after this trauma and trying to improve it, we imagine it shows weak character. It’s testament to how strong their relationship was, that Mark and his wife withstood the reaction within their social circle to their private drama. He says, “My wife really tried to close down, forget it, move on quickly, because the affair was over, whereas I needed to talk about it to understand. So a lot of it was coming together, moving apart. I had to get over the urge of wanting to kill somebody. The driving force for me to persist was our son.”

He adds, “My relationship with my wife is great compared to what it was a year ago. She spent a lot of time trying to make sense of it, take responsibility. She says, ‘I was an idiot, I got it out of my system, I realise it’s not what I want’. But it’s still very raw. We as a couple now say if we’re uncomfortable with something you’ve got to speak up. We may end up fighting more, but at least we’re communicating.”

Lack of communication was key but, mostly, the affair was not to do with Mark. He says, “My wife describes the affair as an escape. It included travel, drinking, going to clubs, effectively trying to be 18 again, and who wouldn’t want to, with a one-year-old at home, mostly lovely but occasionally a tyrant. Whether it’s a happy marriage doesn’t really matter; all of us have that longing to be the person that we were.” He adds, “My wife always had this thing of wanting to escape – to explore, go to Hong Kong, and my mistake perhaps was not taking her words for what they were.”

Andrew G Marshall, the marital therapist and author of I Can’t Get Over My Partner’s Affair, has clients who “turn up at sports days together, their children have good grades at school – but that requires a huge amount of time and energy”. He adds, “Functional marriages where the children are showcases for how great the parents are, come at a cost.”

You can tick all the boxes: you’re even having sex. However, says Marshall, that doesn’t always mean much. “How much joy and connection is there when the two of you touch each other? People who feel connected are generally not unfaithful.” He says, “I don’t think happy people cheat but I do think people in good relationships can be unfaithful.”

This is because sex, even for the most emotionally literate couple, can be the Achilles heel of an otherwise strong relationship. You can be happy and you can have a good marriage but, because sex is so intimate, it’s where we can hide all of the unconscious material that we can’t deal with. Time and time again, I see couples who are really good friends, really good at sorting stuff out but they’ve never managed to sort sex out or introduce sex toys like piper sex doll or a dildo into their relationship, which one or the other might be fantasizing about incorporating into their sex life or at least experimenting with. Affairs are not all about sex but sex is often a large part.

For a partner, an affair is personally devastating; an almighty and destructive indictment of the relationship. Yet the fault isn’t always in the relationship – at least, not their relationship. Often, notes Marshall, the “damage was done” in childhood. If your spouse’s mother or father was unfaithful, this can have an impact: less about sex than about communication. “Happy parents give their children the message that problems can be talked about and resolved,” Marshall says.

“Unhappy parents give children the message that problems can’t be solved – if you put your head over the parapet, it will get shot off. Instead of actually saying, ‘We haven’t had sex for three months and that’s a problem for me,’ they ‘solve’ the problem by going off and having sex with someone else.”

The betrayal can be so shattering, so traumatic – especially if you considered your relationship loving – that initially couples assume, “it’s all a disaster, no one’s going to recover,” says Arabella Russell, a Relate counsellor. However, she says, “So often once we’ve dealt with the initial pain, most couples see that the affair happened for a reason – be it children, work, stress, responsibilities, and somehow the couple are not as connected as they were.”

An affair can be prompted by a bereavement. “It doesn’t have to be the partner’s fault. It can be to do with one’s own personal psychological wellbeing,” says Russell. How we react to life’s transitions, “reaching 40, 50, having children, children leaving home, affect us within our relationship very differently”. These are vulnerable moments in a marriage “and if there’s a sense that relationship isn’t giving you what you need, you can begin to look elsewhere”.

When we’re juggling the demands of work, children and relationship, adds Russell, “Every time a ball gets dropped it’s always the ball with the couple’s relationship name on it, always.” (Our partner loves us: we know he’ll understand.) “There are all these little hits along the way,” she says, “which can make two people feel isolated from each other. The minute that starts it’s a creeping freeze. The warmth begins to go, because they don’t trust that the other person wants to hear their problems. Then someone comes along and is warm, and interested and curious.”

Mark and his wife are still together – he sounds surprised to be asked: “If there’s a strong foundation, and a tornado comes by, you can still rebuild the house on that same foundation,” he says. “However, you need to have a willing builder. And you need to put in the effort. It’s about how the person who was unfaithful can deal with it, as well as the person who was betrayed. Having that strong foundation helps but, if you don’t want to build the house, it’s never going to happen.”

Infidelity: the opportunity after the crisis by Andrew G Marshall
● If you are the person who has been unfaithful, you need to bring up the issue of the affair – otherwise it makes your partner feel the job of healing is all theirs.

● Don’t use the affair to win every argument, if you are the person who was cheated on. Keep the argument to who left the freezer door open . . .

● Problem, plus poor communication, plus temptation, equals affair: we’re not responsible for our partner losing his or her job, or for stopping him or her being tempted. Communication, though, is one half our responsibility.

● Look at your relationship with fresh eyes in the aftermath of the affair. What do you regret? Taking the relationship for granted? Not making sex a priority? Own that part of it.

● Learn to be assertive. Both of you could make a rule: I can ask, you can say no, and we can negotiate. Don’t drop hints, or tell jokes – just ask.

● “Busyness” is a huge problem – that tendency not to focus on the present and stuff too much into our lives. Find some wriggle space. Don’t live at 85mph – or if the cat needs to go to the vet everything will crumble.

● Think small gestures, not grand ones. You don’t need to book a weekend in Paris. Instead have a bath together, or watch every episode of War and Peace together and chat about it; be in the same room to talk rather than shout up the stairs; say thank you.

● When your partner tells you something unpleasant, your response should be, “Tell me more”. Don’t try to solve the problem before they’ve even told you about it, or ignore it because it makes you feel uncomfortable.

● Slow sex down – incorporate non-sexual touch rather than dashing straight to business. And think about after-play, too. Unwind, be together.