Men find women more attractive near ovulation, when they're most fertile, suggests the largest study yet to look at whether a gal's allure changes over the course of her menstrual cycle.
The findings are plausible, the scientists note, since the ratings of attractiveness were related to hormonal shifts, which may cause facial and vocal changes in women.
The research, detailed online Nov. 15 in the journal Hormones and Behavior, adds to the idea that a women's cycle is linked with various physiological and behavioral changes. For instance, earlier studies have found that when fertile, women's sexual desire increases, as does their preference for strong-jawed men. Past studies have also shown men find fertile ladies' dance moves more attractive, as well as her voice and smell, with one well-known 2007 study showing erotic dancers brought in better tips during the fertile phase of their cycle.
In the new study, researchers took photographs of 202 women's faces and made recordings of their speaking voices at two points in their menstrual cycles. They also took saliva samples to measure hormone levels during both sampling sessions. More than 500 men rated the attractiveness of the women's faces and voices from one of the two sessions. The ratings from the first session were averaged for each woman and then compared with ratings for her second session.
Men rated faces and voices as more attractive when women's progesterone levels were low and estradiol (estrogen) levels were high.
"The only time in the cycle when estradiol levels are high and progesterone levels are simultaneously low is the late follicular phase, near ovulation when fertility is highest," said the study's lead author David Puts, an assistant professor of anthropology at Pennsylvania State University. [10 Odd Facts About a Woman's Body]
A group of more than 500 women were also asked to rate women's attractiveness across their cycles. (The two groups of women did not overlap.) They scored the photographs and vocal recordings based on two measures: flirtatiousness and attractiveness to men. Women rated the subjects higher on both measures when the subjects were in their more fertile phase.
"We learned beyond a reasonable doubt that women's faces and voices change over the menstrual cycle, and that both men and women perceive this as changes in attractiveness," Puts told LiveScience.
Nathan Pipitone, a psychologist at Adams State University in Colorado who studies human mating and voice attractiveness, agreed: "This paper establishes conclusive evidence for how men and women rate other women as a function of their hormonal status." Pipitone, who was not involved in the research, said the study's large sample size and measure of hormone levels strengthen its conclusions.
Hormones and sexiness
Research has suggested hormones, indeed, alter facial and vocal features.
The larynx, or voice box, has estrogen and progesterone receptors, and puberty, pregnancy, menopause, hormone replacement therapy and hormonal contraceptive use have all been shown to change women's voices, the study authors said. A 2011 study co-authored by Pipitone found that men could predict when women were menstruating based on vocal features, such as its mood, pitch and quality. [7 Surprising Facts About the Pill]
In the new study, women's hormonal state was linked with the perception of attractiveness, but no acoustic changes were found. "In evolutionary terms, it's the perception of attractiveness that matters to humans, not the proximate mechanisms (i.e., acoustics) that allow us to try and quantify what is and what is not an attractive voice," he wrote in an email.
Puts is studying facial changes over a woman's cycle that could make her appear more or less attractive. "There could be changes in blood flow that would result in color changes in the face, changes in acne, or changes in puffiness due to water retention," he said.
Most scientists believe such cyclic changes, known as fertility cues, are "leaked," meaning they are a byproduct of female reproductive biology rather than traits that evolved to advertise fertility. Unlike female chimps and other mammals, women conceal their ovulation, giving them more control over their reproduction. "Many researchers favor the hypothesis that concealing ovulation afforded our female ancestors the ability to cheat on their mates, because their mates couldn't concentrate mate guarding near ovulation if they couldn't tell when it occurred," Puts said.
Men who could pick up on women's subtle fertility cues, even if they were unaware of it, may have had more reproductive success. The same could be true for women, researchers theorize: Women who were able to subconsciously spot a fertile rival might have better guarded their mates from them, keeping their partner's investment focused on them and their children.
Researchers from the University of Missouri and the University of Sterling in Scotland also contributed to the study.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.