A woman's relationship satisfaction changes as she nears ovulation, when she's most fertile.
But whether or not she's more or less happy with her guy depends on his sex appeal.
In a new study, heterosexual women who rated their guys as highly sexually desirable felt closer to their partners and more satisfied with their relationships just before ovulation, as compared to their less-fertile days.
The opposite was true for women who said their partners were less sexy; they felt less close to their male partners and were more critical of their mate's faults as they approached ovulation.
Previous research has shown that the type of man a woman prefers tends to change across her ovulatory cycle, as she becomes more attracted to masculine faces and bodies, and bilateral symmetry, when she's fertile.
Another recent study showed that heterosexual women actually look and sound more attractive to guys on the women's fertile compared with non-fertile days.
"This is the first research to show that these changes have implications for relationship functioning," said study researcher Christina Larson, a psychologist at the University of California, Los Angeles.
Hormones likely influence these fluctuating preferences and evaluations. However, the researchers didn't directly measure hormones.
"So we can't say exactly which hormones were responsible or how exactly they change women's behaviors," Larson said. Estrogen, which peaks at ovulation, is a strong candidate, she said.
Sexiness and satisfaction
The study, detailed online Dec. 3 in the journal Hormones and Behavior, involved 108 heterosexual women who had not used birth control in the past three months and who were not pregnant or breast-feeding.
The women, who had been in committed relationships for an average of two years, answered the same questions during two sessions, one during their high-fertility phase and one during their low-fertility phase.
The subjects self-reported where they were in their cycles, and the researchers confirmed the high-fertility session with an ovulation test. [10 Odd Facts About a Woman's Body]
The questionnaires included prompts such as, "How desirable do you think women find your partner as a short-term mate or casual sex partner, compared to most men?" and, "How sexy would women say your partner is, compared to most men?"
The women also answered questions about their closeness to their partners, their partners' faults and virtues, and the women's commitment to and satisfaction with their relationships.
The good news for men: Although a woman's time of the month seemed to influence her feelings about her partner and relationship satisfaction, her commitment to the relationship stayed constant throughout the cycle.
The findings are in line with the so-called dual-mating hypothesis, which suggests that women have two mate-choice mechanisms: "one leading to preferences for sexually desirable men who have high-fitness genes, and one leading to preferences for men who are able to invest in a woman and her children," the authors wrote in the journal article.
Choosing mates
Though the researchers can't say that hormones caused the mate-preference changes, there's reason to think the two might be related from an evolutionary perspective.
Dissatisfaction with a less sexually desirable partner when a woman is near ovulation may have encouraged cheating among our female ancestors, thus increasing the likelihood of conceiving children with sexually desirable partners, Larson told LiveScience.
Because sexually desirable traits like masculine appearance in men are thought to have indicated genetic quality in ancestral environments, these couplings outside the primary partnership might have provided an evolutionary advantage for ancestral women.
"All else being equal, a woman who conceived children with men who possessed high fitness genes (e.g., relatively free of deleterious genetic mutations) probably had children who were more likely to survive and later reproduce than the children of a woman who chose a less genetically fit partner," the authors wrote.
Sexually desirable men would have benefited, as well, but "if men were sexually undesirable, these behaviors were likely to be reproductively disadvantageous if they caused their partners to conceive children with other men," Larson said.
Jealousy and mate guarding — actions that men perform more frequently when their partners are fertile, according to research — may have coevolved to counter cheating. Larson’s lab previously showed that men identified as not very sexy were more jealous and attentive to their mates on the women's high-fertility days.
In the future, Larson plans to study whether or not women actually change their behavior — treating less desirable partners differently than sexier guys, or even cheating on them — when fertile.