Women's sexual preferences change over the course of their menstrual cycle, with partnered women being most attracted to single men during their fertile stage, according to a new study.
Researchers at the University of Padua in Italy suggest that partnered women, who want to have an affair, or even think about having a fling, subconsciously choose single men when they are fertile.
On the other hand, their attraction shifts to coupled men when they become infertile.
The researchers say that women select a man who is more likely to be a willing partner when they are fertile.
They explain that having an affair with a coupled man may be both a waste of time, and risky, 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," Nature quoted Paola Bressan of the University of Padua in Italy, as saying.
"These subconscious preferences are apparently still with us," she added.
The reason for women choosing coupled men during their non-fertile stage, according to Bressan, is that the women unconsciously consider 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 said.
For the study, 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 percent higher than attached men; infertile women scored them 8 percent lower.
Bressan said that the effect of fertile women's liking for single men was most noticeable when those men had particularly 'manly' features, such as strong jaws.
The results also revealed that single women showed no variation in their preferences, they usually showed no regard for whether a man was single or taken when expressing sexual preference, at any point in their cycle.
Bressan said that the findings boost other studies looking at physiological changes and sexual behaviour associated with reproductive hormone cycles in women.
"These findings provide additional evidence that oestrus in women, long believed to have been lost in human evolution, is alive and well," said Eli Finkel, a psychologist at Northwestern University in Evanston, Illinois.