Elevated levels of an important sex hormone could be causing sexy looking women to cheat on their partners, according to a recent study.
University of Texas psychologists Kristina Durante and Norman Li found that women with high concentrations of the hormone oestradiol were likelier to flirt, kiss and have a serious affair outside an established relationship.
In a study published in the British journal Biology Letters, the duo described the behaviour as "opportunistic serial monogamy" and not related to one-night stands.
Instead, they suggested, such women were more probably being prompted to trade up in their relationships to find a better partner.
Other studies of oestradiol have shown that higher levels of the hormone are closely linked to enhanced fertility among women.
As a result, say some researchers, men have an evolved response that prompts them to pick up on telltale signs of the hormone's presence.
High concentrations of oestradiol are associated with big breasts, facial attractiveness and low waist-to-hip ratio, with the result that men solicit such women.
Durante and Li measured hormone levels at two different stages of the menstrual cycle among 52 women aged 17 to 30 who were not taking the pill.
The women were asked to rate their own attractiveness and sexiness "compared to other women," their satisfaction with current partners, their willingness to engage in uncommitted sex and about their past relationships.
Separately, a panel of men and women unaware of the experiments evaluated photos of the women, rating them for attractiveness.
The women with the highest levels of oestradiol scored the highest across the board, and reported a greater likelihood of having a serious affair.
They also reported a greater number of long-term relationships in the past.
The study says that oestradiol-related attractiveness provides opportunities in the sexual hunting ground.
Men are keen to find a sexier woman and women want to find a partner who is a better provider and looker.
"Women 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," write Durante and Li.
Women with higher oestradiol levels "may have fewer reasons to be committed to any particular partner if higher quality potential mates are available," they concluded.