Researchers have carried out the study and found that women who have high levels of oestrogen not only look and feel beautiful but are also more likely to be unfaithful to their partners, the 'Biology Letters' reported.

According to them, these women engage in “opportunis- tic serial monogamy” -- being open to flings and moving on to a new relationship if a higher-quality mate becomes available.

“Physically attractive women receive more male attention and when in relationships are more likely to be the targets of mate poaching. Attractive women also have specially high mating standards.

“Because it's difficult to obtain a partner who is a good provider and also has good genes, women often have to trade off between having a long-term mate who provides continual material resources and more physically attractive, short-term sexual partners with good genetic resources.

“However, highly attractive women demand greater amounts of both types of resources in a partner, in addition to good parenting and partner skills. Thus, physically attract -ive women may not only have more alternatives but also high standards that are difficult to satisfy.

“Accordingly, they may have fewer reasons to be committed to any particular partner if higher quality poten- tial mates are available,” 'The Daily Telegraph' quoted lead author Dr Kristina Durante of Texas University as saying.

In their study, the researchers took saliva samples from fifty-two female students aged 17 to 30 at two stages of their menstrual cycle to measure their hormone levels.

They were also questioned about their sexual histories and asked to rate their own looks before two male and seven female undergraduates blind to the study were invited to score their attractiveness from full-body photos.

Durante said: “Women with higher oestradiol reported a greater likelihood of flirting, kissing and having a serious affair with someone other than their primary partner and were marginally more likely to date another man.” But while high-oestradiol women had significantly more long-term relationships the hormone wasn't related to the likelihood of one-night stands.