Published online 26 February 2008 | Nature | doi:10.1038/news.2008.625

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For partnered women, a manly man with no attachments seems sexiest when she is fertile.

Whom do you fancy? When in a relationship, women's preferences change over the course of their menstrual cycle. GETTY IMAGES

Women beware: instinctive preferences might up the odds of getting pregnant when cheating on a partner.

In a study looking at the ever-interesting (and ever-mysterious) question of why women are attracted to certain men, researchers found that sexual interest shifts with a partnered woman’s menstrual cycle. When fertile, women in relationships are most attracted to single men; when infertile their attraction shifts to coupled men1.

The reason, the researchers suggest, is that coupled women who are thinking of having an affair (even when asked to think about it by researchers) subconsciously select a man who is more likely to be a willing partner when they are fertile. Courting a coupled man may be both a waste of time — as he is less likely to participate in an affair — and hazardous, as there is a greater chance of getting caught. “Ancestral women who felt more attracted to a single man than to an already coupled one would have been more likely than others to succeed and transmit this preference to their daughters,” says Paola Bressan of the University of Padua in Italy. “These subconscious preferences are apparently still with us.”

As for why women in relationships prefer coupled men during their non-fertile stage, Bressan thinks that the women are instinctively assessing these men as potential replacement partners. “A coupled man obviously has the skills to maintain a long-term relationship, whereas a single man is an unknown. Indeed, women tend to be suspicious of men who are still on the mating market,” Bressan says.

Single women showed no such variation in their preferences; they habitually showed no regard for whether a man was single or taken when expressing sexual preference, at any point in their cycle.

A man for all seasons

In many monogamous animals, including marmosets and humans, males of high genetic quality are less likely to invest time in paternal care than are those of lower genetic quality. The theory behind this is that females view males with good genes as so desirable to the quality of their offspring that they are willing to sacrifice help with the rearing, letting the men get away with not being around. Lower-quality males make up for their poorer genes by being supportive and aiding in child rearing.

Given these realities, one strategy for a female is to develop a long-term relationship with a lower-quality male while secretly breeding with single high-quality males. The only problem then is getting caught. Long-term partners will often attack an adulterous female in the animal world. In humans, the penalties can be equally stiff.

Bressan and colleague Debora Stranieri wondered whether these factors would drive women to be most interested in single men during their window of highest fertility.

Manly men

The team asked more than 200 women, half of whom were single, the other half in relationships, to rate the attractiveness of men in photographs described as single, married, in love, or with a girlfriend. When the same man was labelled as single rather than being in any kind of attachment, fertile women in relationships scored single men 13% higher than attached men; infertile women scored them 8% lower. The effect of fertile women’s preference for single men was most noticeable when those men had particularly ‘manly’ features, such as strong jaws, says Bressan.

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The findings add to other studies looking at physiological changes and sexual behaviour associated with reproductive hormone cycles in women. Some work has shown that women are more amorous, for example, when at the fertile stage of their menstrual cycle — similar to the way that animals are ‘in heat’ near the fertile stage of their oestrus cycle.

“These findings provide additional evidence that oestrus in women, long believed to have been lost in human evolution, is alive and well,” says Eli Finkel, a psychologist at Northwestern University in Evanston, Illinois. They also have the alarming implication that adulterous women, instead of trying to avoid getting pregnant, will subconsciously tend to do the opposite. However, actual romantic behaviour is not the same as instinctive sexual preferences, says Finkel: both need to be intensively studied to better understand all of this.