Awkward news from the world of science: Women with less-masculine husbands or boyfriends are more likely to lust after other men during the fertile part of their cycle than women partnered with butch guys.

A new study reveals that heterosexual women whose partners have less-masculine faces report more attraction to other men during ovulation. Women with masculine-looking partners said their eyes wander less, perhaps because the traits women tend to find sexy when they're fertile are already present in their partners. However, while those women weren't looking elsewhere, they also weren't more attracted to their own manly partners while fertile, suggesting that women's reasons for wanting sex, not overall desire, might be what varies throughout the cycle.

Baby-faced men can breathe easy: The findings, reported in the November issue of the journal Evolution and Human Behavior, apply only to women's ratings of men as short-term partners, not as lifelong mates.

"When they rate men's sexiness, in a sense, that's when they show the shift," study co-author Steven Gangestad, a University of New Mexico evolutionary psychologist, told LiveScience. "If they rate men's attractiveness as a long-term partner, then they don't show it."

Ovulation and attractiveness

For many years, researchers believed that female Homo sapiens had evolved to hide their fertile periods, unlike other primates whose swollen genitals signal fertility to males. But the past decade of research suggests women aren't such stealthy ovulaters after all. Studies have shown that men rate women's smells and looks as more attractive during fertile periods of a women's menstrual cycle. Other studies have shown that women walk differently when ovulating and may pay more attention to grooming and dress.

A number of studies have found a peak in women's preferences for masculine, muscular men during fertile times, but many of those studies are lab-based, Gangestad said. He and his colleagues wanted to see how preferences shift in real couples.

The researchers recruited 66 monogamous heterosexual couples. Women took a hormone test to determine their phase in the menstrual cycle. Next, over the course of a month, the women came to the lab three times to answer a questionnaire on their sexual attractions and fantasies. One of the sessions was scheduled for each woman's fertile period, while the other two were during the luteal, or non-fertile, phase of the menstrual cycle.

The men in the couples reported their college entrance exam scores and took a test on pattern-finding to measure their intelligence. The men's photos were rated for attractiveness and measured for masculinity. (A strong jaw, chin and brow are masculine traits.)

Macho, macho man

"What we found was that, indeed, women who are with less facially masculine men — so more feminine men — they're the ones showing a shift toward men other than their partner," Gangestad said. For instance, while ovulating, women were more likely to have sexual fantasies about a non-partner.

The findings fit with the theory that men who are more masculine would have produced fitter offspring in the ancestral period when reproductive hormones first evolved.

"Things that may have been important ancestrally may not be all that important now, but the idea is you see the preferences that would have evolved ancestrally still show up," Gangestad said.

Facial attractiveness wasn't a statistically significant factor in women's lust for other men, though women with more attractive mates (regardless of whether they had masculine or feminine faces) did tend to get a boost in sexual interest for them while fertile. Masculine men didn't provoke an increase in sexual interest from their partners. It may be that women want sex for intimacy outside fertile periods, while they're motivated by looks when ovulating, Gangestad said.

A man's intelligence, on the other hand, made no difference in his woman's wandering eye. Those findings are surprising in that evolutionary psychology theory would predict that women would want their offspring to have genes for intelligence, Gangestad said. However, he said, research on fertile women's preference for intelligence throughout the menstrual cycle has been mixed.

"The take-home message of that is that intelligence is probably always pretty important for a woman," said Martie Haselton, a University of California, Los Angeles, psychologist who was not involved in the current study, but who has studied changes in women across the menstrual cycle. "Low intelligence is not really a turn-on for anybody."

Lust uptick

Haselton said the hypothesis that humans are at least partially driven by evolutionary forces makes many people feel uncomfortable. Be that as it may, she said, the current study is "another piece of evidence building toward that same general story."

It remains to be seen what the consequences of women's mid-cycle burst of lust might be, Haselton said. Another study by Gangestad in the same issue of Evolution and Human Behavior finds that women surveyed during their fertile periods report more lust based on physical attraction and more willingness to hook up with an attractive stranger. Other studies suggest that men sense women's fertility and react by becoming more attentive, Gangestad said, which can cause tensions in some couples.

On the other hand, an uptick in lust may be a non-issue for most couples, Haselton said.

"Most women, it probably happens and it's gone after a matter of days, and the relationship goes on as it was before," she said.