American researchers say women with high levels of estrogen feel more attractive and are far more likely to indulge in affairs.
The researchers from the University of Texas say the female hormone estrogen which affects fertility is known to make women feel more attractive and flirtatious and they may act on those feelings by having affairs with different men.
According to Dr. Kristina Durante young women feel more attractive when they have high levels of estradiol, and they acted on those feelings by flirting, kissing and having a serious affair with someone other than their primary partner.
Dr Durante's team say the results of their study provide support for the relationship between physical beauty and fertility and suggests that women high in reproductive health engage in opportunistic serial monogamy - they have affairs and move on to a new relationship if a higher-quality mate becomes available.
For the study the researchers tested 52 female undergraduates aged 17 to 30 who were not taking hormone contraceptives and took two estradiol samples at two stages of their menstrual cycle to measure their hormone levels.
The women were questioned about their sexual histories and asked to rate their own looks before two male and seven female undergraduates blind to the research who were invited to score their attractiveness from full-body photos - women with high-estradiol levels were considered significantly more physically attractive by themselves and others.
The high-estrogen women also reported more sexual behaviour - especially outside of a relationship, which was not linked to one-night stands.
The researchers say their results are consistent with the possibility that highly fertile women are not easily satisfied by their long-term partners and are especially motivated to become acquainted with other, presumably more desirable, men and adopt a strategy of serial monogamy.
Dr Durante says physically attractive women receive more male attention and when in relationships, are more likely to be the targets of mate poaching - they also have especially high mating standards and also more alternatives, which gives them fewer reasons to be committed to any particular partner.
Research has established that estradiol, which is similar to testosterone in men, fuels a lust for power with single women and those who are not on the Pill are particularly vulnerable to the effects of the sex hormone.
The study is published in the Royal Society's journal Biology Letters.