Commonly quoted statistics suggest that more men are unfaithful to their partners than women. But how reliable are the figures and, if it takes two to tango, is it even mathematically possible?

On the face of it, the evidence does not look good for men.

The 2006 American General Social Survey found that nearly twice as many married men as women admitted to having had sexual relations with someone other than their spouse.

The UK's last major study of sexual behaviour - the 2000 National Survey of Sexual Attitudes and Lifestyles (Natsal) - found that 15% of men had had "overlapping" relationships in the previous year, but only 9% of women.

Dr Catherine Mercer, head of analysis for the Natsal study, says the gender gap may in part be because women are less likely to own up to cheating than men.

"We can't directly observe unfaithfulness so we have to rely on what people tell us and we know there are gender differences in the way people report sexual behaviours," she says.

But that's not the whole story.

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One explanation that may spring to mind for this difference between the male and female statistics is that fewer women are being unfaithful, but they are doing so more often. But all the evidence of the Natsal research suggests women generally have fewer sexual partners than men, not more.

Another reason for the difference could be to do with age.

"We know that on average men tend to be slightly older than their female partners," says Mercer.

"If you imagine a scenario where a married man has an affair with a younger woman, who is perhaps more likely to be single because she's younger, he would have engaged in unfaithfulness but she wouldn't have done."

Whether or not you think a single woman who has sex with a married man is complicit in adultery will depend on your own moral code.

Problems measuring infidelity The first problem is how do you pose the question. Some surveys do it in a very judgmental way. The second thing is when you are talking about cheating, what do you actually mean? People have different boundaries. Do you draw the line at kissing? Some people say looking at pornography or chatting on Facebook is being unfaithful. Whenever we talk about relationships and sex, we seem fixated by gender. We set up these clunky measures. We could look at age, we could look at relationship status, we could look at hair colour. Why are we so interested in trying to suggest that men cheat more than women and why does there have to be an either or about this? This taps into this negative stereotype that men are more sexual, more promiscuous and less reliable, and women are more virtuous. But men may well exaggerate and women underplay it. Historically it was sometimes dangerous for women to admit being unfaithful, and in some parts of the world this is currently still the case for women and men.

It's also worth noting that some relationships are open, and in such relationships, sex with other partners would not be considered cheating. But there is no room for value judgements in the data.

In fact, Dr Mercer does not even use the word "infidelity" in her research, preferring to use the more neutral terms "overlapping" or "concurrent" relationships.

"Infidelity is quite a loaded word, whereas thinking about overlapping partnerships is more appropriate when we are thinking about the epidemiological context of these data," she says.

"We're thinking, for example, about the transmission of sexually transmitted infections and HIV."

Men who pay for sex could also explain their higher rates of "overlapping relationships". The last Natsal study found that about 4% of men had paid for sex in the past five years. If we assume there are fewer women selling sex than there are men paying for it, then prostitution could also explain some of the discrepancy.

That assumes, of course, that far fewer women pay for sex. Previous studies have not asked women that question, so there is no firm data, but it will be asked in the next Natsal study.

There is another weakness in the methodology. Previous studies did not explicitly ask whether respondents had overlapping relationships. Instead respondents were asked for the dates of their first and last sexual encounters with their most recent partners.

Experts then studied the dates to look for overlaps. As Mercer points out, however, this method can give the impression of infidelity where none has occurred.

"Imagine a scenario where a couple get together at school and then they split up and then they get back together years later.

"They have each had other partners in the meantime. So their date of first sex may well be when they were at school. Their date of most recent sex may well be last week. But then their other sexual partner dates would suggest that they had been unfaithful when in fact they had not."

Image caption Far from home

Can these studies tell us anything about which kinds of men are most likely to cheat?

The head of the American General Social Survey, Tom Smith, identifies several factors. "Among the groups that are more likely to be unfaithful are the less religious and people who are separated from their spouses for extended periods - such as by travelling or working away from home."

Mercer says young people report more overlapping relationships than other age groups.

So if you are the female partner of a young man with no religion who spends a lot of time away - be warned!