"Every woman likes healthy looking faces, but women who are [or may be] pregnant are particularly keen on healthy looks.”

Previous studies by psychologists at the University of St. Andrews’ Perception Lab found that women at their most fertile times (where the hormone progesterone is high) are more particular about their choice of possible mate -- women tended to choose masculine-looking men, presumably in the hope of securing strong genes.

The new study, shows that women with high levels of progesterone -- but low levels of fertility -- are also choosy. It occurred not just with pregnant women, but women with similar hormonal states -- progesterone is raised in women who are pregnant, taking the oral contraceptive pill, or in weeks three and four of their monthly menstrual cycle.

All About Protecting Baby

While previous studies have shown the part hormones play in women’s preferences for masculinity in men, this new study also demonstrates how hormones influence their preference for health in men and women.

“Pregnant women and women with raised progesterone levels were more attracted to the men who appeared healthier. Our findings suggest that pregnancy -- or when a woman is in a similar hormonal state -- trigger strategies within the body for avoiding illness during social interactions," says Dr. Ben Jones, the first author of the academic paper published today. Dr. recently completed his PhD at St. Andrews and is now a lecturer at the psychology department of the University of Aberdeen.

"These could compensate for weakened immune system responses at these times and reduce the risk of maternal illness disrupting the development of the unborn child," Dr. Jones suggests.

The work, conducted at the Perception Lab at St. Andrews, involved collaboration across the University with researchers from the Schools of Psychology, Computer Science, Medicine and Biology.

Fertile Women Go for Masculinity

The team carried out a series of studies involving hundreds of women stating their preference for computer-generated images of faces. Participants were shown two pictures of the same face side by side and were asked to state their preference. The team had made tiny tweaks to the synthesised faces to suggest one was healthier than the other, giving one an unhealthy pallor and the other a healthy glow.

Women with elevated progesterone levels were drawn to the healthier face more than women with relatively low levels of progesterone.

“Most theories of attraction predict that women will be keen on macho men," says Professor David Perrett, Head of the Perception Lab, who led the team of researchers. "This is because such men are good bets to produce healthy kids with a tough immune system. The prediction turns out to be partly true.

"Five years ago, we discovered a link between hormones and facial attraction: Masculinity counts most for male allure when hormones controlling women’s fertility were highest.

“Our new findings reveal different and stronger hormonal influence that controls women’s attraction to health cues in men. We show that women’s choice of both friends and possible romantic partners is also guided by apparent health. Every woman likes healthy looking faces, but women who are [or may be] pregnant are particularly keen on healthy looks,” says Professor Perrett.

Pregnant Women Go for Health

The study showed that the preference for healthier faces was stronger for women during the low fertility phase of their menstrual cycle as opposed to those in their fertile phase. This preference was also stronger in pregnant women, compared with non-pregnant women, and stronger in women using oral contraceptives, than in women with natural menstrual cycles.

The health preferences in UK faces were strongest for women from the UK. This may reflect familiarity with health cues in one’s own culture. The researchers also discovered that the changes in attraction to healthy men occurred when the women considered a hypothetical short-term relationship but not a hypothetical long-term relationship.

"The effect of the relationship for which the male faces were judged is particularly interesting," Dr. Jones adds. "It indicates that women do not prefer the healthier men because these men would make good long-term partners, but are choosing them to avoid illness during short-term social interactions.

Hormonal Effect Is Short-Lived

“Perhaps more importantly though, it indicates that the effects of pregnancy, pill use and menstrual cycle phase on preferences for healthy men are unlikely to pose a risk to the stability of long-term relationships," Dr. Jones concludes.

The group’s paper, "Menstrual cycle, pregnancy and oral contraceptive use alter attraction to apparent health in faces" is published by Proceedings of the Royal Society of London, Biological Series.