Dr Mairi MacLeod: Can you spot a born cad? Here are some vital clues

WOMEN through the ages and across the world have had to put up with it, from Napoleon's wife Josephine to the likes of Natalie Cole and Sarah Jessica Parker. Infidelity by their husbands, that is. What is it about some men that makes them feel compelled to sample exotic delicacies instead of sticking to good old home cooking?

New research published by scientists from the Karolinska Institute in Stockholm suggests that the tendency for a man to stray may lie partly in his genes, and is related to how sensitive he is to the bonding hormone vasopressin. Regular readers of this column may remember me going on a few weeks ago about how male prairie voles are sensitive to vasopressin and remain faithful to their mates, while their promiscuous cousins, the montane voles, have far fewer vasopressin receptors in their brains.

Well, the Swedish scientists have found that men also vary in their genes coding for vasopressin receptors; men with two copies of part of the gene (allele 334) were less likely to be married than other men, and if they were married they were more likely to be in the midst of a marital crisis, or facing divorce.

Clearly a man's personality will also have some bearing on whether or not he's likely to be a philanderer, and it can be measured using five personality factors which are groups of traits which tend to be linked together. So which factors are associated with infidelity?

According to personality expert Daniel Nettle, extrovert men are more likely than others to indulge in extra-pair relationships. Their sociability and positive mood tends to put them more often into an ideal situation for meeting new partners.

Other traits may also be needed in the mix, though. The classically promiscuous man will be high in extraversion, low in neuroticism and fairly low in agreeableness as well, according to Nettle. "The extraversion gives you the desire to do it, the low neuroticism means you don't worry too much about doing it and the low agreeableness means you kind of don't really care if you mess someone around or cheat on your wife," he says.

But, regardless of personality traits or genetics, most men are capable of infidelity given the opportunity and the motivation. The classic scenario for the man straying from the marital bed is where his wife is exhausted by work or preoccupied with the baby, and hubby is left feeling rejected. Right or wrong, it's not surprising if he's tempted by another woman who welcomes him with open arms and makes him feel desirable and valued.

That's a bit different from the men that bed-hop with abandon and perhaps they are the ones with some innate propensity for infidelity. If you'd like more clues about a man's philandering potential, check out his appearance. Men who have masculine faces with big jaws and heavy brows also have high circulating levels of the hormone testosterone, which itself has been linked in many studies to high numbers of sexual partners, marital discord and divorce.

A study by Matthew McIntyre and his Harvard University colleagues demonstrated that while most men's testosterone levels drop when they get into a long-term relationship, some maintain a high level of the hormone, and these are the men who tend to have affairs. Testosterone channels energy into maintaining muscle mass and encourages risk-taking behaviour, both of which may be useful for competing for access to women. So, basically, men who keep their levels high are also keeping their options open for a bit on the side.

It's hard to know the direction of causality here, though. Do testosterone levels stay high because a man is playing away, or does a constantly high level of circulating testosterone compel a man to go out looking for more women?

Anyway girls, the take-home message seems to be: if you meet a man with two copies of allele 334, Neanderthal brows, who likes to crack jokes and be the centre of attention, you'd better watch out.

Still, it has to be said that we are humans after all, and not montane voles. If we want to have good relationships, then we have the choice to behave accordingly, no matter what personality or genes we're born with.