Pamela Druckerman doesn't look like the world's leading authority on infidelity. She looks like the primary- school teacher all the dads fancy; or one of those second-generation yummy mummies, the kind who sets up a stall in a farmers' market selling fashionable cupcakes. She sits in the window of a patisserie in North London; a super-pretty, soft, smiley blonde, with a latte and a laptop. She's involuntarily fixating on passing Bugaboos. 'My husband said he might walk by with our daughter,' she says. She's American, and even though her husband is British, and they have spent the past four years living in Paris, her accent endures. 'I can't help checking out all the prams. It's a reflex.' She smiles.

I expected something a lot less wholesome from someone who's spent the past three years immersed in the world of international adultery. Pamela Druckerman has written the definitive guide on it. Lust In Translation: The Rules Of Infidelity from Tokyo to Tennessee is the result of a long study of the world's philanderers. Druckerman has crunched numbers, collated evidence, and acquainted herself with the lexicon of adultery in two dozen cities in 10 different countries across the world. She has interviewed adulterers (one-off; serial; recovering; flagrant; and tortured) and cuckolds (or 'men who wear green hats' according to the Chinese) in Moscow, Kazakhstan, London, New York, Paris, Indonesia, South America and beyond... She discovered that the Japanese don't count it as infidelity if they've paid for it, and that the best thing that can happen to them in one of their famous sex clubs is 'oral sex without showering first'. She discovered that 40 per cent of Russians surveyed think affairs are 'not at all wrong' or 'not always wrong', and that upper-class Muscovites think affairs conducted at beach clubs do not compromise wedding vows one iota. She discovered that Indonesian women in extremely traditional Islamic marriages 'have affairs, and the reasons they give for them are exactly the same as the reasons my girlfriends in New York City gave: my husband doesn't listen to me, I need someone who'll make me feel smart and pretty again; my husband doesn't do that, but my boyfriend does ...' She discovered that on average the British cheat more than the Americans - and the French.

The book - which has yet to be published in Britain - has set the US on fire. Druckerman has been hauled over the coals on assorted American radio call-in shows on a twice-daily basis ever since. Days before we meet, she did a 20-minute live feed from Paris for Al Jazeera: 'So I prepared all this serious research into infidelity in Muslim countries; and all they wanted to know was: how do the Japanese do it? What do those guys like?'

But how much adultery can one person take? Isn't it getting a bit repetitive? 'No way!' she says, with absolute glee. 'It is endlessly fascinating.'

Oh, but isn't it? Increasingly, infidelity seems to be the single greatest fascination of our age. We're all at it, aren't we? In some way or another? Eight months ago, I interviewed a psychologist called Esther Perel, who had written a book on the absence of sex in marriages and long-term relationships. When I attempted to get anecdotal evidence from people on the subject, they routinely responded: 'People in long-term relationships have lots of sex; just not with each other.'

Hard stats on cheating are thin on the ground, and those that exist tend to be flawed. Furthermore, individual definitions on what qualifies as cheating vary dramatically.

In the course of her world tour of extramarital sex, Druckerman stopped off in the UK, where she met Edwina Currie and interviewed her about the affair she had with John Major; she also met a high-earning, very married investment banker who is working his way through a spreadsheet of sexual goals. Still, her statistics suggest that Britons don't cheat all that much - some 9.3 per cent of men aged 16-44 and 5.1 per cent of women admitted to sleeping with someone other than their regular partner within the last year.

According to psychotherapist Brett Kahr, however, who collaborated with polling organisation YouGov on the biggest-ever survey of British sexual habits, and published his findings in February, there's a lot more of it about than that. 'According to conservative estimates, 11 million Britons will have indulged in an extramarital kiss at some point ... Britons do seem to become more unfaithful as time progresses ... of those aged between 18 and 29, 12 per cent will have had oral sex outside a steady relationship. For those aged between 30 and 50, that figure rises to 20 per cent ... and among the over-50s, 30 per cent have had vaginal sex with someone other than his or her regular partner ...'

Even if we're not committing adultery, or being cheated on by a spouse, we're wrapped up in it somehow. We're caught up in a friend or a family member's affair; covering for them when they're on illicit dates, or picking up the pieces when they're discovered. Or we're obsessing over the public infidelities committed by our celebrities and politicians. From the Sarkozys to the (alleged) Beckhams, John Prescott to the (alleged) Brangelina-Aniston love triangle ... a public relationship without an infidelity seems two-dimensional to us. 'A happily married celebrity couple - who cares, right?' asks Pamela Druckerman. Infidelity's got sex, glamour, excitement, romance, lies, potential heartbreak and high risk. That's why it's a recurring motif in films and songs. It's one of the few bits of filmic life that we could transpose into our own, generally prosaic lives, if we dared. Best of all - adultery's really, really bad. Oh, how we disapprove of infidelity! How guilty we are about our own, and how censorious about other people's ... No wonder we're hooked.

Druckerman began thinking about adultery in international terms when her day job - as a financial journalist - required her to relocate to South America. Once there, she says, she found that married men routinely propositioned her. She was shocked; she also began realising 'how very American I am about this'. She lived and worked in four very different cities (São Paulo, Buenos Aires, Jerusalem and New York), during her twenties, 'when I was dating, right?' and was increasingly intrigued by fluctuating attitudes towards infidelity. In her late twenties and early thirties, her friends started getting married, and 'everyone was asking themselves that same question: can I realistically sleep with only this person for the rest of my life?' Druckerman began researching international attitudes to infidelity in earnest.

Lust In Translation is an excellent book. It's funny, it's compulsive, it's surprising, it's the million soap operas that make up other people's love lives. But it also raises an important issue. At the core of the book is a possibility: does fidelity matter that much? If we're all cheating, or thinking about cheating; if other countries and cultures have completely different attitudes towards it, if some of them honestly don't associate infidelity with guilt ('I'd ask them if they felt guilty about their affairs, and they actually couldn't understand the question!'), then why has it become so taboo in the UK, and are we doing ourselves a massive disservice in making it taboo?

Druckerman's book is written from the perspective of an American - and America is famously high-minded about infidelity. A 2006 Gallup poll discovered that Americans are more comfortable with polygamy and human cloning than they are infidelity; the whole Clinton-Lewinsky furore hinged on the idea that Bill Clinton had cheated on his wife, and that this automatically meant he was capable of all manner of depravities, and thus unfit to be president. But Druckerman thinks the UK increasingly embraces the American ideal on infidelity by buying into what she identifies as 'the American script'. The 'script' is our communal idea of affairs, and of how an affair and the aftermath of an affair should be played out; a blueprint, almost. It dictates our behaviour in an affair situation to a terrifying degree, even when it's contrary to how we actually want to act.

The key points of the American script resonate so strongly, it's almost tedious. For example - the first rule of infidelity in the US and the UK is that it becomes understandable, borderline-permissible even, if the prospective cheat says they're unhappy in their marriage. 'And of course,' says Druckerman, 'everyone has flaws in their marriages, things that aren't quite perfect... but here and the US, you start complaining about your marriage, and that way, you're not some lousy guy who cheats on his wife because he wants sex, you're a puppy dog who's looking for love.' Which might sound so trite that it hardly merits comment - until you consider the Japanese script, in which a cheating man praises his wife to his girlfriend, to demonstrate that he's a good husband.

'The French, for example, are much more comfortable with the idea that their affair partner is just that - an affair partner,' Druckerman says. 'In America or the UK, because people are so uncomfortable having an affair, what they do is start thinking about marrying their affair partner, and then they start talking in those terms to their affair partner, even though actually things at home are OK and their affair partner is not someone they'd consider marrying if they were single. The American and the British are the worst at communicating that they want to keep it "clean"; and anyway, they've already complained about their marriage in order to legitimise the affair in the first place. So what do they then say to the woman who expects that she's going to become the new wife? They can't say: "actually, I love my current wife."'

For the aftermath of an affair, the American script goes into overdrive. 'Well, there's the one-strike-and-you're-out rule: an affair, even a one-night stand, means a marriage is over. That's a very American and British idea. I spoke to women who, on discovering that their husbands had cheated, immediately packed a bag and left, because that's "what you do". Not because that's what they wanted to do - they just thought that was the rule. They didn't even seem to realise there were other options. And then - all those people who discover an affair, and then say: "It's not the cheating, it's the lies I can't stand!" I mean, really, like they're reading from a script!'

The coda to the American script - and, increasingly, in the UK - is the inevitable recourse to therapy. 'This idea that the only way to mend the relationship post-affair is through therapy, is unique to the American script,' says Druckerman.

Druckerman talks about the 'entrepreneurs' who build a business on the aftermath of infidelity - the therapists and couples counsellors. She points out that there's an entire industry with a serious financial stake in upholding the idea that cheating is desperately serious, a symptom of a deeply flawed marriage, of two people who need to be cured.

Does Druckerman think that couples therapy is an exploitative, harmful waste of money? 'I'm not totally cynical about it. I know a lot of people who have been helped by it. But I do think at the heart of it there's an idea that the only way to heal is with total transparency, by revealing exactly what was involved in the affair, blow job by blow job, and I've seen no evidence that this openness helps anyone. And I mean, this can go on and on and on - 80-year-old women who have the moral high ground on their marriage, because of a one-night stand their husband had 40 years earlier ...'

Druckerman insists that Lust In Translation is not a self-help volume; rather, that she's just making a series of observations. However, it's hard not to read some level of criticism into her perspective on the American and British attitude towards cheating. Druckerman has lived in Paris for four years, and while she says that received wisdom on the French and affairs is wildly exaggerated, they do nevertheless 'consider affairs to be part of the fairytale of a marriage, and not, as we do, a complete rupture in it. They haven't mastered infidelity by any means, but I do think their approach might be healthier'. Maybe, Druckerman thinks, if we acknowledged from the beginning that infidelities can and do happen, we'd be better equipped to repair a relationship afterwards. 'Because we often do recover in the end. We may start out the door at first, but we come back.'

Druckerman also thinks the French approach to infidelity is superior, because it encourages the adulterer to at least try to enjoy the affair a little. 'I mean,' she says, 'if you're going to do it anyway, at least enjoy the sex. Have you seen that Kate Winslet film, Little Children, where she's having an affair and, while she's actually having sex with this guy, he starts crying out: "What am I doing? What am I doing?"' Druckerman laughs. I tell her I have heard of comparable real-life incidents. 'Oh right - people who get naked with their affair partners, but won't have sex because they're too guilty? I mean, talk about ruining it!'

Halfway through writing Lust In Translation, Pamela Druckerman got married. 'I was juggling adultery stats and florists' numbers,' she says. The book didn't put her off? 'It changed the way I went into marriage. A lot of Americans I know would assume that they and their partners were going to be perfectly faithful. I still meet people who say: "We're so solid, it's not even an issue for us." And that makes it all the more devastating when it happens. Part of the trauma is thinking that it never will happen, and so when it does, all your assumptions about the world are turned upside-down.

'So while I still very much aspire toward fidelity, for me and my husband - ha! if one can [aspire] for someone else? - if one of us cheated, it would be less of a shock. Although probably not less of a devastation. And I hope that having lived in France for four years, my assumptions would be more French. I wouldn't assume that the marriage was automatically over, or that I had to pay someone to usher us through the pain.'

Ultimately, Druckerman does think that infidelity is a big deal. She says that, wherever she went, whatever the local script on infidelity dictated, the one recurring motif was heartbreak, if the infidelity was uncovered. 'It always hurts,' she says. 'People might expect it, they might not be surprised, they might not leave the relationship because, hey, the next guy's gonna do it too, right? ... but they will still be hurt.'

She is, however, unconvinced that it's as big a deal as received wisdom in the US and UK would have us believe. Ultimately, Druckerman invokes Hillary Clinton, who reveals in her new autobiography that, although she considered leaving her husband on hearing about his affair with Monica Lewinsky, she realised that 'worse things happen in a marriage'. Druckerman thinks this attitude might permeate the American script on infidelity, in the fullness of time. 'I hope it will,' she says. For her part, she's going to keep researching.

She doesn't have a choice. Druckerman says she can't go to a dinner party without someone tracking her down. 'They'll say: "you remember what happened when I saw you last time?" ... and I'll be thinking, now, are you the one who's considering running off with her gynaecologist, or was that someone else ...? Because of course, for lots of us, affairs are the biggest, most exciting thing that ever happens to us ...' And that is why we'll keep having them.

· This article was changed for legal reasons on July 16 2007.