While men have always had a worse reputation for being unfaithful, recent studies show that women are catching up fast - but we are a lot more likely to lie about it, and a lot less likely to get caught.
Simply put, it seems that women are better at having affairs than men.
The news that Farrah Fawcett had a secret affair for 11 years without telling a soul is a classic example of the way a woman cheats: discreetly, in secret, and while carrying on with the rest of her life as normal.
It has been met with hot denials by Ryan O'Neal, but - and I'm sorry to break it to you this way, Ryan - you'd be the last to know.
According to Dr David Holmes, a psychologist at Manchester Metropolitan University, women are having more affairs than ever - recent studies say the figure is around 20 per cent for men and a bit over 15 per cent for women - but they behave very differently from men when they cheat.
'The biggest difference is that women are much better at keeping their affairs secret,' he says. 'If you look at the studies into paternity, even conservative figures show that between eight and 15 per cent of children haven't been fathered by the man who thinks he's the biological parent.'
That's a lot of women keeping a lot of secrets. One woman I know - let's call her Juliet - has been married to Adrian for ten years, and having an affair with Mark for the past two.
She has told me and her sister - otherwise, she says, she's '100 per cent certain' that nobody else knows.
'I won't allow Mark to call me on my mobile or email me, ever. Mobiles and email leave a trail. I insist we talk on our office landlines, and when we meet for drinks or dinner, we pay with cash.
'We work in the same field, so meeting for lunch doesn't look suspicious. If we meet in the evening, I tell Adrian I'm with my sister, and meet Mark a few miles away, where we won't bump into anyone.'
Does she feel guilty about the affair? 'Not so long as Adrian doesn't find out. You know the 11th commandment: don't get caught.'
Women have always had affairs, but over the past 20 years that number has risen dramatically.
Jobs outside the home - with the ready-made excuse of working late or business travel - financial independence and changing social attitudes mean that modern women simply have more opportunity to meet other men and start affairs.
Mobile phones, internet chat rooms and email also make it easier to fuel intimate encounters.
But while women's lives and sexual behavior might have changed, their willingness to be honest about it hasn't.
The truth is that we have always lied about our sex lives. British men consistently claim to have had more partners than women - the current average is 13, while women claim to have had only nine.
Plainly, someone is lying here. While men might exaggerate their sexual conquests, the bigger liars are women.
When studies about sexual partners or fidelity use a mixture of face-to-face interviews and anonymous computer questionnaires, men will give the same answers to both, but women will report much higher numbers when the answers are anonymous.
Why do women lie? Because we must, and because we can. In spite of apparent equality and a more sexually open society, we are still more harshly judged for our sex lives than men.
Can you imagine a leading female politician having an affair and her husband standing loyally by her?
Or the reaction to a female CEO having a public affair with her young male assistant?
There is something particularly humiliating for a man about being made a cuckold. So we lie to protect ourselves from the judgments of others, and because sexual 'reputation' still matters more to women, whether we like it or not.
But we also lie naturally and instinctively, as a way to manage and control our relationships, to protect our partners and our families, and to keep our options open.
In fact, we lie so much and for so many reasons that often we don't even think of it as lying at all, but as 'relationship management'.
Women are taught to lie from childhood. Those simple, altruistic lies such as saying we've had a lovely time when we haven't, that someone looks nice when she doesn't, or that we're delighted with a gift we don't really like, are just some of the small ways that lying oils the wheels of our social lives, keeps the peace, and makes other people happy.
Girls will lie to protect someone's feelings or to build a relationship. Honesty, in these circumstances, looks highly overrated, and we quickly learn the value and power of being economical with the truth in relationships.
Many women also develop a habit of secrecy about smaller issues in their relationship as a way of maintaining some kind of control.
It's common, for example, for women to lie to their partners about how much money they have spent - even when they don't need to.
It's not just a question of avoiding the inevitable row over the 'worth it' factor of a pair of killer heels, but of holding something back in order to maintain a sense of emotional independence, and to balance out the power we might feel our partner has in other aspects of the relationship.
A recent survey by a leading building society also found that women are much more likely than men to have a 'secret' account with money their partner knows nothing about.
'My mother always used to say that you should never tell a man everything,' says Clare, 38, 'and I thought it was rubbish. I believed in total honesty.
'But now I agree with her. Why explain yourself over every little thing? Why have a huge row over something he needn't even know about? I never tell him about my girlfriends' secrets, because he'd be judgmental and disapproving.
'I don't have a secret account, but I'm not completely honest about how much money I spend. If he doesn't know, he doesn't miss it.'
Harmless? Maybe. But it's easy to see how a climate of withholding and keeping smaller secrets can easily lead to bigger ones.
'Women are better liars because they're more psychologically sophisticated,' says Dr Holmes. 'You can argue about whether it's nature or nurture, but women are programmed to be much more relationship and people orientated than men.
From a young age, he says, women will start to consider the kind of life they want, what kind of job or house they aspire to, how many children they'd like and what kind of partner.
'Most men don't really develop that facility until they're much older. They live, psychologically, in the moment, while women think around corners and two steps ahead.'
Hillary stood by Bill Clinton when he had an affair, if it had been the other way around, would he have done the same?
We're also better at mental multi-tasking. 'You can keep all the different strands of a lie in your minds, remember them and make them convincing.'
Men lie, too, but they are much more likely to forget something, or make a mistake and get caught out.
When men have affairs, he says, they tend to be bigger risk-takers and naïve about how obvious their cheating is to an emotionally astute partner. They will also be bursting to tell someone about it.
'Men have more "trophy affairs" than women. They will often want to brag about it, or be so taken up by the sexual thrill of the affair that their behavior is a giveaway.'
Women, on the other hand, are much more cool and rational, even when they're in the grip of passion.
'It's a myth that they're more romantic. They may confide in one or two very close women friends, but they compartmentalize their emotional lives and don't let the affair bleed into the rest of their lives.'
Like Farrah Fawcett, a former colleague of mine, Susan, now 50, had a long-term affair with Brian, a salesman she met, ironically enough, when she was buying her husband a car as a surprise 40th birthday present.
'I was clear from the start that I'd never leave my husband and family. Brian kept me sane during some of the tougher years of my marriage, but he was strictly for snatched afternoons off and the odd "working weekend" away,' she says.
Susan adds: 'We made each other laugh and our sex life was wonderful. But Brian was a permanent "man-child" - a great lover, but not the kind of man you'd marry.
'He was always overdrawn, always teetering on the edge of some crisis that he'd just laugh off. I never wanted to live with him.'
The affair ended amicably after eight years, with her husband none the wiser.
Cheating is never pretty, but lying about it doesn't necessarily make it worse. Often, our lies are meant to shield our partners and families from pain. We tell ourselves that if nobody knows, nobody gets hurt.
Dr Holmes says: 'Often, women keep the secret because they're much more conscious of how much there is to lose, and to protect the other people involved. If she has children, she'll be especially careful that they aren't hurt in any way by the knowledge of what she's doing.'
Like my friend Juliet, who has become so good at lying to her husband that she surprises even herself.
'Adrian and I have drifted apart, but he's a nice man, a good father. He doesn't deserve to be hurt, and I have no intention of letting my son grow up without his father.'
Biologically, too, women are programmed to be more careful about revealing an infidelity.
In purely evolutionary terms, men are programmed to sleep with lots of women, and women are programmed to attract and retain a quality mate.
Dr Holmes adds: 'She will be very canny and keep her options open until she decides which of the men she is going to choose.'
Another reason women lie more about their affairs is simply that they can get away with it.
Historically, a man has put a higher value on the fidelity of his woman because he needs to be sure that any children she has are really his.
This seems to produce a strange kind of emotional blindness in many men, once they've selected a partner to settle down with.
'Whenever we question men about fidelity,' says Dr Holmes, 'they will very often admit to their affairs.
'But if you then ask the same men if they think their wives or girlfriends have had affairs, too, they will be shocked at the very thought, or laugh and call it ridiculous.'
In other words, men want to think women don't cheat, and women want men to think they don't cheat. No wonder we're better at secret affairs.
