Women at peak fertility tend to have a stronger preference for sexually desirable men, many past studies have shown. An open question, however, is whether these variations affect women's long-term relationships. Psychologists at the University of California, Los Angeles, gave 65 women in committed relationships a questionnaire to assess their feelings about their partnerships at different times of the month. Results indicate that on high-fertility days, women who considered their partners less sexually desirable felt less close to them and were more critical of their faults. Women with more sexually desirable partners, on the other hand, felt more satisfied with their relationship and closer to their partner on high-fertility days. Women do not seem to act on these fluctuating feelings, however: the surveys revealed no change in their intention to stay in the relationship depending on the time of month.

The researchers say their results are consistent with the well-supported theory that women select mates by balancing their desires for men with “high-fitness” genes—the sexy guys who are also more likely to stray—and men who are more reliable and likely to be committed fathers. Many men have a combination of both qualities, so a lot of women probably experience only subtle changes in attraction and satisfaction during their menstrual cycle.