A woman's face is most alluring at the height of her cycle.
Women who make the room light up with their good looks may have a secret up their sleeve - it may be down to their menstrual cycle. Both men and women consider a woman's face to be at its most attractive when she is at the peak of her fertility, according to new research.
Craig Roberts from the University of Newcastle and colleagues looked at how female facial attractiveness varies during their menstrual cycle, to see if that might convey the level of their fertility. Other animals have more obvious ways of letting their mates know when they are fertile: for example, female chimps' genital areas swell and turn pink.
Roberts and his team selected about 50 women aged between 19 and 33 years in both Newcastle and Prague, the hometowns of two of the group members. They took two pictures of each subject. The first picture showed them when they were fertile, 8 to 14 days after the first day of their last menstruation. The second was taken 14 days later.
Roughly 125 women and 125 men were then asked in which picture the women looked more attractive. The picture showing a fertile woman was chosen by 51-59% of each group - a statistically significant result, says Roberts. Interestingly, female viewers appeared to be more sensitive to the effect.
Roberts does not yet know what hints people are extracting from the pictures in order to judge their attractiveness. The colour and condition of their skin may be important, he says. Previous studies have shown that skin tone becomes lighter during ovulation. "But there may be more factors; we have to look into that more," he says.
A few women had changed their hairstyle between pictures, so Roberts wondered if that was playing a role in the viewers' decisions. The team covered up the hair and ears on the pictures and asked people to rate them again.
Covering up the hair did not make much of a difference to a woman's looks as far as the men were concerned. But, while women still picked the picture of the fertile woman more often, they did so less reliably when the hair was disguised.
The results indicate that men and women rely on different cues to judge another woman's attractiveness, says Roberts.
Women in particular may have evolved to be sensitive to other women's cycles, adds Ian Penton-Voak, a psychologist at Stirling University, UK. This would let them assess their biggest competitor when vying for mates, he says.
Other researchers have focused on how women rate men's looks during their cycle, says Penton-Voak; asking men about women instead is "really interesting", he says.
Penton-Voak's own work has shown that women prefer masculine-looking men when they are ovulating. At other times, he says, they prefer softer features that are associated with more social and caring behaviour.
Earlier studies have indicated that men might use other clues, such as female body odour, to help them pinpoint their partner's fertility. Others have shown that a woman's ears and breasts actually become more symmetrical in the days leading up to ovulation.
Roberts, S. C. . et al. Proc. R. Soc. Lond. B (Suppl.), published online, doi:10.1098/rsbl.2004.0174, (2004).
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Kramer, D. Women look best once a month. Nature (2004). https://doi.org/10.1038/news040329-6