MONOGAMY is a notion to which Britain, among other nations, has remained faithful for a very long time.Anthropologistsestimate that around four million years ago the brains of ourhominid ancestors began to hum and throb with romantic feelings distinct from the primitive sexual urge; they formed couples in which to raise their offspring and this became the new model for a successful and happy life. By the 12th century, when the Roman Catholic Church recognised marriage as a sacrament, monogamy had evolved into more than a cunning reproductive strategy - it was seen as the right way to live, good for society and individual morality. That has been the dominant view ever since, despite the best efforts of Henry VIII, Brigham Young and Peter Stringfellow.

In the early years of the 21st century, however, it's arguable that monogamy has outlived its usefulness, has in fact become dysfunctional, and that western society is developing a sort of 900-year-itch. Could - and should - we embrace a model that isn't based around two people forsaking all others, but encompasses the idea of sexual freedom? In other words, is Britain's affair with monogamy coming to an end?

One person who ought to know is journalist and author Pamela Druckerman, whose new book Lust In Translation examines infidelity in several countries, including Britain. Having embarked on her home-wreckers' odyssey, she discovered that "bubbling just below the placid, monogamous surface of daily life, there is another universe where lots of cheating goes on".

Infidelity is more common in poorer countries, Druckerman says. "People in rich countries risk a lot when they cheat: their marriages, their wealth, access to their children, plus, in some cases, the esteem of their family members and friends. The potential cost' of cheating is quite high.

"I discovered that married people in the West are pretty faithful. They set monogamy as an ideal and by and large stick to it. So, in that sense, I think that we've organised ourselves rather well, as a society. We've tried to minimise the amount of romantic pain we inflict on each other."

Yet if, as Druckerman suggests, Britain is still in love with the ideal of monogamy, the statistics indicate that we have been placing our trust in a system that often fails, and that we are beginning to reconsider the wisdom of doing so. In 2005, there were 283,730 weddings in the UK, down 10% on 2004, and 155,052 divorces. In Scotland last year there were 13,013 divorces, an increase of 19% on 2005. Less Brits are getting hitched and more are being ditched. That certainly seems to indicate that monogamy, when expressed as marriage, is in a degree of trouble.

But is that because people find it difficult to stay faithful? Just how widespread is infidelity in Britain? According to a recent study by the London School of Hygiene and Tropical Medicine, almost one in 10 married men in Britain had an affair in the last year, and over one in 20 married women. This makes us a more adulterous nation that Italy or France, countries with a reputation for being sexually cavalier.

A BBC online poll from last year suggests that one in seven Brits have been unfaithful to their current partner, whether married or not, and the 2005 Global Sex Survey, conducted by Durex, found that 14% of British respondents had had an extra-marital affair. Meanwhile, Scottish Marriage Care - a busy relationship counsellingservicewhichalsoseesnon-marriedcouples - finds that 17.5% of clients come to them as a direct result of one of them having strayed.

These figures don't match up exactly, but the overall picture shows that there are a lot of affairs going on in Britain. Arguably it is becoming a rite of passage - birth, marriage, dalliance, death. "I don't know many women who haven't either cheated with somebody or been cheated on, and quite often both," says the novelist Tess Stimson, author of The Adultery Club, whose own first marriage ended when her husband had an affair. "I can't tell you how many girlfriends I've had who were a mistress in their 20s and had a cheating husband in their 30s and 40s."

What's more, you only have to read the papers any day of the week to see that infidelity is a subject which preoccupies Britain. "People in this country are absolutely obsessed with what other people are getting up to sexually," says Max Clifford, the publicist responsible for breaking most of the significant British sex scandals of the last 20 years. "When I was a kid it was gossip over the fence. Mum would be talking to the neighbour about how such-and-such got extra rations from the butcher, but now it's on the front page of national newspapers and in magazines full of gossip and tittle-tattle. It's become a huge business. Sex sells in Britain possibly better than anywhere else in the world. In some ways, I think we'd rather read about it than do it."

Clifford believes this is partly because Britain has retained the finger-wagging morality of the Victorian age, that we lap up stories about celebrity affairs because it allows us to feel superior to their behaviour in a way that we can't about their wealth and fame. He says this is a distinctly British trait.

According to Druckerman, an American living in Paris, Britain is more akin to America than to continental Europe in its attitude to monogamy and infidelity. "I think we hold our relationships to a very high standard," she says. "One of the most widely used justifications for cheating in America and Britain is: I wasn't happy'. In other places I visited - France and Russia come to mind - people are quite romantic andhavestorybookideasabouthow marriages should go. But in America and Britain, an infidelity is a rupture in the story, something that brings it to an abrupt and unexpected halt. In France and in Russia, an infidelity can be part of the story - not a necessary or a desired part, but certainly not a rupture."

In fact, adultery is in decline as the legally given reason for divorce in the UK. In 1981, almost one in three divorces in England and Wales were granted on the grounds of adultery, but by 2005 that figure had fallen toaroundoneinfive.Thepicturein Scotland is even more dramatic: adultery ended 17% of marriages in 1981; in 2005 it was only responsible for 3% of divorces.

But that doesn't mean people are having less affairs than when Thatcher was in Number 10 and Sexual Healing was in the top 10. It might mean they are simply citing otherreasonswhenfilingfordivorce, concealing any adultery. Recent figures for Scotland show that 85% of divorces were granted because the couple had not lived together for two years, and we may presume that in a significant number of cases the separation followed an affair.

One important third reason that adultery is not such a common cause of divorce may be that couples can survive it better than they once could. This may in fact be precisely because infidelity is common, an anticipated assault on a relationship rather than a death blow. In other words, Britain may be edging towards the way France deals with things.

"I'm drawn to the French refusal to be shocked by adultery, and to their belief that an affair doesn't automatically mean that the marriage is over, or that it's been a lie," says Druckerman. "The French haven't mastered adultery. It's always messy. But I'm intrigued by the French approach because it's so different from that of my own culture. The idea that confrontation isn't the best response to an infidelity is a bit of a relief, given the agony that's brought on by the American approach."

Agony is a strong word, but appropriate. As a Brit reared, veal-like, on Carry On films and mother-in-law gags, it's too easy to regardinfidelityasasaucyjokeand monogamy as a bore. So it's important to get a sense of how it actually feels when your partner has an affair.

IcontactedthewebsiteSurviving Infidelity, a hugely popular forum in which betrayed wives, husbands and partners can post and read accounts of affairs and the process of recovery. I asked whether any members would be willing to speak with me about their experiences, and received several responses from men and women.

Jayne Bennett is 45 and works in marketing. She has two teenage daughters. Her husband had the first of three affairs in 1993. She is very clear about how it felt. "It is literally like having the wind knocked out of you or being punched in the gut," she says. "It is a blow so intense you feel like you have been physically struck. Your mind goes through all sorts of gyrations - you panic, then you think that you misheard or were mistaken about what was going on ... Your world is shattered and crumbling around you. You feel like you have no reality. None. You can't sleep, you can't eat. Physically, I was a wreck. I lost 25 pounds in just a month or so. When I found out about the first affair, I was so non-functional I ended up losing my job. I couldn't concentrate, couldn't stop crying, couldn't deal with anything over and above taking care of my daughters."

This, Bennett says, is a "fairly common reaction" and indeed it chimes with the rest of the stories I hear. Anger, sadness, physical and mental pain, dramatic weight loss, a feeling, as one woman puts it, of having been "plunged into a pit of destruction" - this is what happens when someone you love has sex with someone else.

Given how bad it can be, and that most of us say we disapprove of affairs, why do so many people still sleep around? In part, it seems to be because the opportunity presents itself - we are more likely to have an affair with a co-worker than anyone else for the simple reason that we spend so much time at work. "A lot of infidelity happens via work situations, where somebody provides a listening ear," says Mary Toner, chief executive of Scottish Marriage Care. "Very often it's about being sympathetic. The old cliché is, My wife doesn't understand me,' but actually there is a lot around that which is true."

David Miller runs Loving Links, an online dating service which facilitates affairs for married men and women. He believes that there are many more people in Britain having affairs than ever before, and that the reason for this is the example set by the media and the opportunity afforded by technology; a generation bombarded with sexual imagery consider fidelity an obsolete notion, if they consider it at all.

"An enormous amount of people think that monogamy is a sort of wood that's used inmakingfurniture,"Millersays."It's extremely hard when the media is continually presenting us with icons playing away from home. If you look at any men's or women's magazine there will always be an article on how to enhance your sex life and get six extra orgasms, and people think, well, certainly not with the person I've been living with for 15 years'. The media lets people feel, unwisely, that there is a world out there where everyone is bonking every half hour."

Infidelity, argues Miller, is also encouraged by web access. When Loving Links started 12 years ago, dates were set up by letter. There was a built in cooling-off period. "These days people have this expectation of I want it now' and the internet panders to that. So they think that having no-strings sex is no more complicated than ordering a DVD online."

Although Miller makes a living from this attitude, he rather disapproves of it. He has beenmarriedtwiceandisnowin"a comfortablelong-termmonogamous relationship". He says he is in favour of secure relationships, but is providing a service for people who are - for one reason or another - not getting all their needs met bytheirpartnerorspouse.Thereare around 20,000 people using his site, of which around 35% are women.

Does he feel guilty that he is complicit in so much potential heartbreak? No. He'd prefer it if people using the site did so with the permission of their other half, but if they don't it is not his responsibility. "If people want to have an affair, want to betray their partner, they will do so. All I am doing is letting them do it in a fairly safe environment; they are less likely to leave home because they are meeting someone in a similar situation rather than a single person ... In many cases we are helping people stay together rather than breaking people up."

It's an interesting idea - that long-term relationships might be happier and less likely to collapse if sexual fidelity was not a given. Most people would agree that sex is an important part of a relationship, and yet figures from the Office for National Statistics reveal that one in eight women, including married or cohabiting women, claimed to have had no sex in the past year. The psychotherapist and writer Adam Phillips once wrote that monogamy was our secular religion; if so then we might add that while monotheists believe in life after death, monogamists believe in love after sex.

That being so, wouldn't it make a kind of sense for couples to maintain their core relationship - particularly if they have children - while openly looking elsewhere to meet their sexual needs?

Anna Sharman believes so. Sharman,36,livesin London and is the author of OpenFidelity:AnA-Z Guide.OpenFidelity,a phrase she coined, means acceptingthepossibilitythat each person can be attracted to more than one person, and then negotiating with your partner the sexual rules of the relationship. It is distinct from polyamory, adherents to which have several concurrent loving relationships of equal significance; in Open Fidelity, you can have one central long-term relationship while the rest are short-term and fully accepted by your partner.

SharmanhasbeenpractisingOpen Fidelity for 12 years and considers it a more realistic way to live. She believes there are many thousands of people living this way in Britain, and argues that in a world where Open Fidelity was a widely accepted option alongside monogamy, there would be less repression of sexuality and fewer unhappy relationships.

How would she like to see marriage change? "I'd like people to be able to promise to each other, I want to spend the rest of my life with you' but not have to say, You and you only' ... The other thing is I'd like people to be able to marry more than one person, if they wanted to commit themselves as a three-way group, a triad, or even four-way, a quad."

But what about children? How best to explain Open Fidelity to them? "It depends on what situation you are in. If it is two parents and one other person who is the partner of one or both, and that person is around a lot and maybe even lives with them, then they can just be another member of the family. If children are growing up with that situation from a very young age then they won't find it strange." Children don't care who is sleeping with whom, she says. What's important is that they have a sense of loving relationships around them and that they themselves are loved.

Sharman hopes that Open Fidelity, over time, could become part of the mainstream. Right now, though, it would be a real stretch. It's hard to imagine Britain getting to grips with feeling "compersion" - a word coined by the polyamory community to describe the sensation of joy when you seeyourpartnerlovingsomeoneelse. Jealousy is a familiar path for most of us; compersion seems more like an unfamiliar uphill route we would struggle to navigate.

The truth is that most of us in Britain remain committed to the idea of commitment, even if living up to the ideal is sometimes beyond us. Humans are displayingunusualanimalbehaviourintaking monogamy as our standard mating system (only 3% of mammals are monogamous) and it seems that having struck upon this idiosyncratic way of arranging our lives and lovesweareloathetoabandonit. Monogamyandmarriageremainsthe norm among humans around the world; even in cultures which permit polygamy, the vast majority of men only have one wife.

Most users of the Surviving Infidelity site who speak with me remain passionate about monogamy, even though they have more reason that most to consider it a failed system. They are also, mostly, a bit narked at the suggestion that so many people have affairs because monogamy is difficult, and horrified by the suggestion that long-term relationships would work better if sexual fidelity was not regarded as the industry standard.

"I think people who have sex with multiple partners without any emotional attachmentarenothingmorethansoulless ghouls," says one 44-year-old businessman and father who asked not to be identified. "Maybe that's the norm in some pachouli-reeking, new-agey hippy community, but not in my marriage. People who think like that are just looking for some sort of official validation and exemption for their acute self-centredness ... Unless you've already given up on yourself as far as a conscience and plain moral standards are concerned, I don't think monogamy is hard at all."

However, Lynne Carnegie-Vogl, a 37-year-old mother of three, says, "If my husband had given me a choice and told me how he felt, who knows, maybe I would have wanted to be with someone else too, but still kept the family intact. It's possible to have an affair but know when to stop. To not go against my family. Yes, I could have had an affair too. But no way would I have trampled on my family on the way out the door. And the first time an affair partner got out of line and contacted my family, all bets are off. To me, family is sacred."

That word "sacred" is key to all of this. We do indeed make a religion of our relationships - putting our faith in them, often against all the evidence. Of course that's what faith is, and monogamy is a religion with only two disciples in which they together are also God. Perhaps, then, we are collectivelysufferingfrom,asRichard Dawkins might put it, a sort of monogamy delusion. Yet if that is the case, it seems we would rather not be stripped of it, even while so many of us are stripping in an altogether more profane sense.

Pamela Druckerman's Lust In Translation is available from