When it comes to relationships, many men lust after women they can't - or shouldn't - have. That sounds like a recipe for disaster, so why do men do it?
It happens to all of us some of the time, and some of us most of the time. What we want more than anything else is the thing we can't - or shouldn't - have.
Actress Gwyneth Paltrow has expressed sympathy for those conducting affairs
Even Hollywood actress Gwyneth Paltrow - famously married to Coldplay frontman Chris Martin - recently defended infidelity. The married mother of two said: "Life is complicated and long and I know people that I respect and admire and look up to who have had extra-marital affairs ... It's like we're flawed - we're human beings and sometimes you make choices that other people are going to judge."
The allure of forbidden fruit is well documented. We secretly lust after a best mate's girlfriend, even though we would never do anything about it. We love the idea of a sordid affair with a married woman - the perfect mix of danger, excitement and eroticism.
But wouldn't life be much simpler if we were hardwired to be attracted to only those who were freely available? But the real world isn't like that, so why do we keep falling for those who are tantalisingly out of easy reach? MSN Him searches for clues.
Do we really go for forbidden fruit?
Part of the attraction of women we can't have seems to be the very fact that we can't have them
Ask your mates, and have a root around in your own fantasies. Has there been a teacher or lecturer in there? A comely married neighbour or colleague? A friend's girlfriend?
Or maybe you've made the classic male mistake of dumping a girl who was all over you, and then finding her insanely desirable as soon as she's moved on and barely gives you a second glance.
The number of people happy to have affairs is testament to the allure of forbidden fruit, even though study after study has shown that people who ignore or who are inattentive to women other than their partners have better, more stable relationships in the long run.
And a new study published in the Journal of Personality and Social Psychology found that as soon as you start viewing someone as forbidden fruit that person becomes instantly more desirable.
The authors say that just as people want jobs, salaries, cars and houses out of their reach, they "may desire attractive alternatives more and desire their current relationship partner less" when those alternatives are perceived as forbidden fruit.
In a nutshell, research has shown that, even though it can be bad for us, we are all turned on by the idea of forbidden fruit. When we perceive that something or somebody is not for us, we want them even more.
Why do we want what we can't - without serious consequences - have?
There are a couple of theories as to why we desire forbidden fruit in relationships. The first is tied up with what psychologists call 'ironic process theory', which simply means that when we're told not to think about something we invariably do.
Students told not to think about pink elephants will, in all likelihood, think of pink elephants
Psychologists might tell their students not to think about a pink elephant, which of course means the students then start thinking about a pink elephant.
The same may be true in our relationships, they say. If you tell yourself not to desire someone because society says it's wrong (a married woman, say, or your friend's new girlfriend), that puts the desire to the forefront of your mind.
Another theory has been put forward by Helen Fisher, an anthropologist from Rutgers University in the UK. She thinks that levels of dopamine - a chemical that helps us experience pleasure - rise in the brain the longer it takes for a pleasure to be fulfilled.
So waiting a long time for a delicious meal makes it taste better. Waiting for forbidden fruit in relationships can take an age, but the anticipation can be exquisite.
Should we pursue forbidden fruit?
Be careful you don't get what you wish for, or the attraction may vaporise
It may be common to desire what you can't or shouldn't have, but that doesn't mean you should take it one step further and act on that desire.
The fact is, forbidden fruit relationships rarely end well. You might think that the best possible result is that the woman of your dreams registers your interest and leaves her boyfriend or husband to be with you, but even that unlikely scenario has one glaring downside. Once she's yours she's no longer forbidden fruit, and you might find your desire starting to wane.
But though we shouldn't act on such desires, it's also not a good idea to suppress them. There's some evidence that allowing both partners in a relationship to at least notice forbidden fruit is one way to make sure they don't take it any further.
Research published in the Journal of Personality and Social Psychology also found that when you stop people noticing attractive members of the opposite sex, they remember them more and have a more positive attitude to infidelity and a more negative one to commitment.
In other words, the allure of forbidden fruit seems to be natural, and by suppressing the desire we make it more alluring still (it becomes more forbidden, and thus more desirable). By giving our fantasies free rein, we're more likely to leave them in the safe realms of our imagination.
So if you want to stay on the right side of your partner, and your own conscience too, indulge in the fantasy and you'll give vent to your inner desires without threatening your relationship.
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