HOLLY Petraeus looks like a woman who has relaxed into her life.

She wears sensible shoes and possibly elasticated waistbands. Her hair is natural grey. Why not? Her life is about rather more important things than holding back the years.

She is the mother of two children who has moved home 23 times to support her celebrated husband, General David Petraeus. She's also the assistant director of an organisation which supports military families with debts in their fightback against the banks. She protects them from the threat of illegal foreclosure.

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I doubt, however, that she works out. She seems to have plumped up her skin with good food instead of Botox. Now, some tweets have greeted news of her husband's affair with cries of, what does she expect?

After 37 years of marriage, Holly Petraeus probably expected loyalty. Maybe she thought her husband loved her for who she is, not what she looks like. And maybe she was right. In his letter of resignation from running the CIA, he wrote: "I showed extremely poor judgment by engaging in an extra-marital affair. Such behaviour is unacceptable as a husband and as a leader of an organisation such as ours.'

It suggests that – like the majority of errant husbands – Gen Petraeus is on his way back home to Holly.

So why did he cheat? Because he could, but also for the reason many men cheat after they become successful. The biographer Claire Tomalin once said that over a certain income level a lot of men think they deserve more than one woman in their life. I quoted her to a politician who agreed and added: "Also over a certain level of power."

The contention is that the man doesn't want his mistress instead of his wife. He wants his fun and his family. I'd also hazard that it has less to do with his wife's appearance than tweeters think.

If beauty is the deciding factor, what would explain Diana, Princess of Wales, losing out to Camilla, Duchess of Cornwall? Why would Tiger Woods have strayed so relentlessly from Erin Nordegren? Even the nation's one-time sweetheart, Cheryl Cole, was let down.

Jack Kennedy's faithlessness knew no bounds, despite Jackie's famed beauty. Then there's Jerry Hall, Elizabeth Hurley, Sienna Miller, Eva Longoria and Halle Berry – all lovely, all careful of their appearance and all betrayed.

Statistics about infidelity vary but some suggest almost 50% of men cheat at least once. It can't all be because partners cease to be looks-conscious.

Lots of the happily married-and-faithful have wives who don't focus on appearance. Look at Gillian Edwards, the unglamorous but charming wife of Ken Clarke. They met at Cambridge and have remained together for half a century despite her being another natural grey and some way from size zero. Mr Clarke, reputedly a contented and loyal husband, is a clever man who knows when he has a good thing.

Perhaps we could have said the same of Gene Petraeus in every year but this one. Maybe we could say it most of the time for all but serial philanderers. So what changes? What triggers an otherwise loyal and faithful husband and father into a reckless affair?

If we take strong sexual attraction as a given, what factors make them more susceptible when they're at the top of their game.

It could be entitlement as Ms Tomalin suggested. In his book, The Hour between Dog and Wolf, John Coates says success triggers a rush of testosterone in men. It's empowering, an ego boost and it increases risk-taking.

Getting to the top also brings a sense of being above the herd, not bound by normal rules and strictures. And of course a high salary makes it all so much easier and affordable.

Then there is loneliness. A man at the top can't confide in those who work for him. He can talk to his wife but high-powered jobs involve long hours away from home and they demand a level of expertise some wives won't possess.

Success separates couples.

Now add temptation to the mix. Let's say that, like Gen Petraeus, he's thrown together with a younger, good-looking and intelligent woman, someone who understands the trials and tribulation he faces, a woman who clearly admires him. They may spend time together, travel together and have the opportunity to stray.

Power is an aphrodisiac. A man considered unattractive in a junior role becomes more desirable with each rung he ascends. When he reaches the top he has the pulling power of a Greek god. If you doubt it, recall that Nicolas Sarkozy is married to Carla Bruni and 81-year-old Rupert Murdoch's third wife is the protective Wendi Deng who launched herself at the man who assaulted her husband with a plate of foam.

Men at the top find themselves presented with sexual opportunities they might only have dreamed of. And because they have reached the top they think they deserve it.

It's exciting, risky, rejuvenating, an adrenaline rush. It's an escape from real life and the daily demands of career, home and family.

It's easy to see why men like Gen Petraeus might want both.

So why don't successful women follow the same behaviour pattern?

I dare say a few do, but not enough to be a similar phenomenon. In part it's because women look upward to find a mate. They seek a partner who is more successful, brighter, more prestigious. The higher they climb themselves, the fewer the candidates.

Men are less fussy. When they are at the top, a forest of possibilities lies below. Maybe the phenomenon of the cougar will be commonplace as more women succeed. Somehow I can't see it.

For now it remains a man thing – whatever his wife looks like.

Holly Petraeus – every woman with a successful partner – could be a slave to diet, surgery, injections, gym classes and fashion and the upshot would be just the same.

But if a man is open to temptation, while he remains a success his mistress will have to beware that competition never sleeps: that she may one day be the biter bit.