From Princess Diana to Tiger Woods and Jesse James, why do men and women risk losing everything for illicit sex when the stakes are so high? It's about more than sexual gratification, reports Jane Phare.
It's everywhere, this sex business. The lure of it, the lust, joy, thrill and pain of it, ready and waiting to pounce.
Stay within a relationship's rules and boundaries and all goes well. Stray outside and there's a potential timebomb ticking.
It's not a matter of if, it's a matter of when. Ex-president Bill Clinton, Prince Charles and Princess Diana, Hugh Grant and Sandra Bullock's ex Jesse James all learned the hard way.
So why is it so rife when the risks associated with affairs and illicit sex are so high?
Auckland husband-and-wife sex therapists Nic Beets and Verity Thom, who run a fulltime practice, deal every day with people trying to figure out why they had affairs and trying to salvage their relationships.
They debunk the myths that men are the main instigators - that high-profile, high-powered men are more likely to have affairs.
"It's a total fallacy, that one," scoffs Thom.
"I could give you four prominent names [of women] right now that would blow that theory out of the water."
Women, it seems, are just better at covering their tracks.
"Women are better at hiding affairs than men. No question," says Beets.
Men aren't as good at assessing risk, and therefore take greater risks than women.
Take ex-president Bill Clinton. He liked the conquest - "It gave him some sort of payout" - and for a time, he got away with it, Beets says.
"If you have done it hundreds of times and got away with it, the ability to judge risk is further compromised."
Quite why Clinton risked everything for sexual liaisons, culminating in a date with a White House intern and a cigar, shocked and mystified the American public. But risk it he did.
Thom rejects any suggestion that men in powerful positions have more to lose as superficial and sexist.
The mother-of-three who has a fling with her personal trainer while her husband is away on business has just as much to lose if she has an affair, she says.
She risks losing a home for her children, a partner and provider, and a lifestyle. Yet she, too, will take that risk.
The question of why is complex. First up, it's not just about sex, therapists say.
Hormones and MP Shane Jones' "red-blooded male" excuse for watching pornographic movies are greatly over-rated, they say.
Therapists report that clients often tell them the extra-marital sex was no better than at home. While hormones and sex drive might be part of the story, they are not the main chapters.
Instead, pyschologists say, it's more about "emotional grounding" - or a lack of it. Wrapped up, the reasons why people stray from their relationships and, in some cases, become hooked on forbidden sex, are insecurities and a lack of emotional awareness.
Until those issues are faced, experts say, people may never feel satisfied. Instead they will continue to look for short-term gratification - a quick fix - from a love affair or a one-off fling.
Clinical psychologist and director of Sex Therapy NZ, Robyn Salisbury, says those with a high sex drive driven by emotional needs will increasingly use sex to stave off feelings of inadequacy or insecurity.
People who are emotionally secure and confident are less likely to look for sex outside a relationship.
Although having an affair might bring some anxious moments, for many that's preferable to the anxiety caused by facing up to emotional pain from the past, Salisbury says.
Just what causes a lack of emotional maturity is complex, too, but therapists say it more often than not dates back to childhood - a lack of bonding with a mother, a dominant father who only noticed and praised a son when he achieved, growing up in a dysfunctional household and without decent role models.
Talk to those who have had affairs and they tend to see it more simply.
Broadcaster and columnist Paul Holmes, whose affair with Fleur Revell in 1997 broke up his marriage with Hine Elder, doesn't see his reasons any differently from Tiger Woods.
"It boils down to one thing: He wasn't happy at home."
Holmes shrugs off suggestions he may have had emotional issues or insecurities that needed working through.
Holmes says those who stray may be happy enough in their lives but "significantly unfulfilled in their marriage".
"Sometimes things happen and they happen unstoppably. It is extremely sad, but sometimes there is no going back and people have to move on through the pain they've caused."
And, he points out, he is no different from thousands of others who seek love and affection outside their relationship. "It's gone on since time began and it will go on until time ends."
And go on it does, with golf star Tiger Woods topping the list in recent times as the most prolific and startling philanderer - again with an enormous amount to lose: a beautiful wife and two children, a supportive home life, a multi-million-dollar career, the adoration of fans and millions of dollars in sponsorship.
What was he thinking?
Salisbury says someone like Woods could not afford to stop and think. "The crazy behaviour is about avoiding thinking."
Beets and Thom agree. In all probability, they say, the more Woods did it, the better he felt - for a time. When the effect of sex wore off, he did it again, all the time feeding some insecurity or emotional need to which only Woods and his therapist are privy.
Those who display compulsive sex behaviour - what some label as sex addiction - are disguising a "core of shame", Beets says.
"They do something to make themselves feel better, but the act makes them feel shameful so it is a vicious cycle."
And Woods is by no means alone in his transgressions.
A former New Zealand sports coach says many top, high-performing sports people are driven by an underlying insecurity.
They use sport to fill the gap, but often unsuccessfully.
"The other thing that fills the gap is adulation - and sex, adulation and love are fairly close," she says.
"People think they [sporting stars] have big egos. It's not a big ego, it's a fragile ego, an insecure ego."
Screeds have been written about why Charles and Diana sought love and sex outside their marriage, and they're in revered company: Clinton, JF Kennedy - forever linked to movie star Marilyn Monroe - and then there's Henry VIII who either divorced or beheaded his wives when the new mistress came along.
Princess Anne's first husband Mark Phillips strayed while he was in New Zealand and fathered a daughter. And the great Civil Rights leader Martin Luther King became renowned not only for his vision and leadership, but also for his extramarital affairs.
Hollywood's streets are littered with the debris of publicly humiliating sex revelations - Hugh Grant being caught with a prostitute having the highest cringe factor, and Oscar-winner Sandra Bullock's betrayal by husband Jesse James raising the greatest sympathy.
British politics has been riddled with it through the decades. Italy is up to its eyeballs in it.
Closer to home, New Zealand politicians were protected for years by an unwritten code of secrecy in the Press Gallery that their affairs, so to speak, were their own business.
Nowadays their dalliances are likely to appear first as innuendoes in a gossip column followed a full outing when the story breaks.
The late David Lange, Prime Minister of New Zealand, had an affair with his speech writer Margaret Pope, whom he later married. Two years ago former Labour minister Dr Michael Bassett, in his book Working With David, blamed Pope's influence on Lange as the single biggest factor in the collapse of the Lange Government.
Former National Party leader Don Brash broke up his marriage by having an affair, and the political career of former Internal Affairs Minister Richard Worth came to abrupt end after allegations that he sent sexually explicit texts, and made phone calls, to women.
Behaviour that ended in tears. Beets says humans on the whole are not good at assessing risk and probability.
"By and large people who have affairs know the risk and do it anyway."
But it's not a good time to make rational decisions.
A high state of arousal - be it driven by sex, anger or fear - means the brain's risk and reasoning functions are not working as well. So a decision made sitting at a desk about whether to have unprotected sex with a foreigner might be quite different from one made in the heat of the moment.
"If your heart rate is over 100 beats-per-minute your brain is not functioning properly," Beets says.
One high-profile Aucklander whose marriage broke up after his affair with another woman hit the headlines now says he paid "a very high price".
The extra-marital relationship had been "very satisfying", but he was no longer with the woman.
"It didn't work for her or me."
The businessman conceded the attraction may have been to feed an emotional need but did not discount the strong pull of sex.
"It's not a coincidence that some of the most celebrated affairs in recent history have involved very high profile people with enormous responsibilities - people you simply do not expect to put their entire careers and life's work at risk but have done, in some cases not once but repeatedly.
"So there's obviously something extraordinarily powerful going on," the businessman says.
"It's not something that you sit down and think through rationally. I think it is quite primal."