Are you a married woman aged between 40 and 45? I hate to tell you, but you're at the peak age for adultery. And if you're a married man, it's 55 to 65.

Human beings seem cursed with contradictory impulses. We search for true love, find him or her and settle down. Then, if the spell begins to fade, the mind begins to wander, the Daily Mail reported.

Most people, it's true, will keep to their marriage vows, but a surprising number will not.

This seems to be the case throughout much of history, whatever moral and religious codes prevailed at the time.

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In the 1920s, Gilbert Hamilton, a pioneer in sex research, interviewed 100 men and women. Of these, 28 per cent of the men had committed adultery and 24 per cent of the women.

And that was when far more people were practising Christians and contraception was less available or reliable.

By the late 1950s, a survey of more than 13,000 men and women found that a third of husbands had been unfaithful and 26 per cent of the wives.

A couple of decades later, there was an interesting shift: both sexes were starting to have extra-marital trysts at a younger age.

It's not hard to understand why. The Pill had become widely available in the Seventies and the so-called sexual revolution was encouraging the young to experiment.

And today? There's barely any change since the 1920s in the number of us who are having extra-marital affairs.

So there's no getting round the fact that a quarter of married men and women cheat on their spouses. And they do so despite all the risks to family, friends and livelihood.

There is no culture on the planet - or in history - in which adultery is unknown, as I have discovered in the course of my work as an anthropological biologist and a senior research fellow at the Kinsey Institute for Research in Sex, Gender and Reproduction at Indiana University.

In ancient China, a man who slept with the wife of another man was burned to death; in Japan, he was told to commit suicide; and an Indian man who dared sleep with the wife of his guru could be forced to sit on a red-hot iron plate, then chop off his own penis.

Note the punishment was solely for sleeping with another man's wife. In a great many societies, it was taken for granted that married men would be unfaithful with prostitutes or single women.

Not so women. In ancient times, a married woman caught sleeping with anyone other than her husband could be executed or have her nose chopped off.

But even dire punishments couldn't stop either women or men from playing with fire. Adultery, it seems, is an innate part of human behaviour.

At least the majority of us don't cheat; for other mammals it's very different. Thought foxes mated for life? Think again: their pair bond lasts only through the breeding season and then they move on.

The same applies for at least half of birds. Why? Because it's not normally to a male's advantage to remain with one female when he can have sex with several and pass more of his genes onto posterity.

As for females, bearing young with different fathers increases the likelihood that more of them will survive. A case, if you like, of not putting all your eggs in one basket.

So there is an evolutionary imperative at play.

But why do modern husbands and wives have affairs? Most adulterers questioned by scientists say it's 'lust', 'love' or - pathetically - 'I don't know'.

What we know is that some use their escapades as an excuse to leave a spouse. Others want to feel special, desired, more masculine or feminine, more attractive or better understood. And some want more communication, more intimacy or just more sex.

Others want to solve a sexual problem or just get high on all the secrecy involved in an affair.

A few seek revenge. Some are just bored. Or they crave drama, excitement or danger.

Among men, infidelity is less dependent on the state of their marital relationship. Photo / 123RF
Among men, infidelity is less dependent on the state of their marital relationship. Photo / 123RF

Digging deeper, scientists have examined the impact of religion and social class on rates of adultery. Religion, it turned out, made no difference at all - no matter what people professed to believe, it failed to stop some philandering.

Class is a more nuanced issue. The famous Kinsey sex report of the Fifties found young working-class men indulged in a great deal of cheating, but much less so in their 40s.

Meanwhile, white-collar, university educated men tended to philander less in their 20s, then increase their dalliances to almost once a week by the age of 50.

Scientists have found gender differences, as well. Married women tend to have a greater emotional connection with their illicit lovers and seek more intimacy.

Perhaps unsurprisingly, the intensity and frequency of women's affairs are linked to the degree of dissatisfaction they feel with their husbands.

Among men, however, infidelity is less dependent on the state of their marital relationship.

Astonishingly, a 1985 study found that 56 per cent of male adulterers rated their marriage as 'happy' or 'very happy'. For women, the figure dropped to 34 per cent.

So, many men and some women jeopardise happy marriages for the sake of a tumble or two. Why?

Scientists have found some clues by studying prairie voles - one of very few mammals that generally mate for life. The breakthrough came when they discovered that the few philandering prairie voles carried a particular gene that influenced the vasopressin system.

Vasopressin is a hormone, formed and stored in the pituitary glands, before being released into the bloodstream and possibly directly into the brain, where it is believed to play a role in social behaviour, sexual motivation and pair bonding.

And what do you know - humans carry similar genes governing their vasopressin system.

Swedish scientists investigated whether one of these genes affected men's sexual behaviour. The results were remarkable. Men who had inherited this gene scored significantly lower in a questionnaire that measured their degree of attachment to a mate.

Men who had two copies of the gene had the lowest scores.

Both categories of men admitted they had experienced more marital crises during the previous year, including threats of divorce.

And those who did not carry the gene? They turned out to be the most attached to their partners.

This field of work is relatively new, but scientists think other 'adultery genes' are probably involved. In another recent study, for instance, a direct link was found between specific genes in the dopamine system (dopamine is a neurotransmitter that controls the brain's reward and pleasure centres) and a higher frequency of sexual infidelity.

So some married men, it seems, may be biologically programmed to be more open to temptation.

Can they rise above this dubious heritage? Of course they can.

The world is full of people who are faithful to their partners - a testament to the triumph of culture, love and personality over natural predisposition.

You won't be surprised adultery is top of the list of all the reasons that marriages end in divorce. And not just in our culture: a study of 160 different societies came to the same conclusion.

Some married men, it seems, may be biologically programmed to be more open to temptation. Photo / 123RF
Some married men, it seems, may be biologically programmed to be more open to temptation. Photo / 123RF

Intriguingly, the reason for divorce was more likely to be the wife's adultery than the husband's.

The next most common reasons were a spouse's inability to produce children; cruelty, particularly by the husband; and various unappealing aspects of a spouse's behaviour (such as nagging, disrespect and temper tantrums).

Today, the divorce rate in Western countries is staggeringly high: up to 50 per cent of marriages will end in the courts.

There are various reasons, but many observers home in on one - the modern phenomenon of women who earn their own living.

Back in the days when most couples farmed for a living, they point out, hardly anyone got divorced. A woman depended on her husband to move the rocks, fell the trees and plough the land, while her husband needed her to sow, weed, pick, prepare and store the crops.

More important, if one of them wanted to leave the other, they left empty-handed.

Neither could dig up half the wheat and easily relocate.

A woman with a salary coming in, on the other hand, will usually be far less prepared to put up with an unhappy marriage. She leaves because she can. Thus career women are responsible for some of the rise in divorces.

They're also most likely to divorce young, usually before the peak of their careers at 25 to 29.

For men, the peak divorce age is 30 to 34.

After that, women don't hang about: 80 per cent of those who divorce before the age of 25 remarry before turning 35. If they divorce after 25, 44 per cent of women remarry before they're 40 and 55 per cent of men.


What about the seven-year itch? I have news for you: it doesn't exist. What most certainly does, however, is the three to four-year itch.

Since 1947, records show that - despite massive upheavals in society - the highest number of divorces take place after three or four years of marriage.

This peak time for splitting has remained roughly the same in each decade, even when the divorce rate doubled between 1960 and 1980. After doing research, I found that the same applies in more than 60 countries.

Some of their citizens are bankers, others herd cattle - but their divorces all cluster around the three to four-year mark.

The only exception is in traditional Muslim societies, where divorces occur most frequently after the first few months of marriage. And that's largely because so many marriages are arranged.

In the West, one reason may be that so many young people expect to marry a soul-mate in perfect harmony with themselves.

When it turns out they haven't, they bail out soon after the end of the infatuation stage.

But there's another reason buried deep in our past. In many traditional hunting-gathering tribes, the woman breast-feeds until the child is three or four. This suppresses ovulation.

Our earliest ancestors almost certainly did the same. And when the child was old enough to join a pack of children looked after by other adults, the father's role as food gatherer and protector became largely redundant.

In other words, human pair bonds originally evolved to last just long enough to raise a single child through infancy. So the modern divorce peak of three to four years after marriage may, in fact, be a biological phenomenon.

Interestingly, 29 per cent of all divorces occur among young couples with one child.

If they have two children, the number falls to 18 per cent and only 5 per cent of parents with three children ever head for the divorce court.

People without children are the most likely to divorce (43 per cent), thus freeing them to have children with someone else. And the couples least likely to split have four or more offspring. So, the more children there are in a family, the less likely it is that their parents will divorce.

What about middle-aged and older couples? You may expect some to grow bored with each other or to end unhappy marriages after the children have left home. Indeed, these age groups are divorcing more frequently than in the past. But don't be fooled into thinking age is a major factor in divorce. The overwhelming majority of these spouses remain content with their choice.

In fact, if a couple get past the age of 34, they're statistically less likely to divorce. And the ones who do leave a first marriage will almost all remarry.

Most tie the knot three or four years after divorcing. This is a pattern that has remained stable for decades.

They may have been buffeted by adultery and quarrels. But hope reigns eternal that the next spouse will remain faithful.

How to spot a cheat

Could your partner be tempted to stray? Scientists say people who have affairs are likely to fall into at least one of the following categories:

• Men and women who regard themselves as more socially desirable than their spouses (they tend to cheat soon after the wedding).
• Wives who report they get their way during disagreements.
• People who weren't securely attached to their parents during childhood.
• Those who don't feel their spouse supports and loves them.
• Men and women who are more open to new experiences.
• Alcoholics and those who are clinically depressed.
• Spouses who aren't equally open and conscientious.
• Women who are more educated than their husbands.
• Individuals with a high income.
• Spouses who work outside the family home.
• Men and women whose jobs involved touching clients.
• Anyone who works alone with a co-worker.
•Those who have spouses with chronic illness.
• Anyone who thinks their sex life is poor - especially men with frigid wives.
• A man whose wife is pregnant.

Adapted by Corinna Honan from Anatomy Of Love: A Natural History Of Mating, Marriage And Why We Stray by Helen Fisher