The “ovulatory preference-shift hypothesis” is the idea that what women seek in a partner changes over the course of their menstrual cycle. Around ovulation, about two weeks after menses begins, women tend to up their preferences for masculine traits, like a robust jawline, a muscular body and an aggressive personality.

The explanation for this so-called preference-shift is that masculine men have good genes, and if a woman sleeps with these men when she is most fertile, she’s more likely to become pregnant. Masculine men aren’t preferred at other times because they tend to make worse long-term partners than feminine men, who are more prepared to invest time and resources in a relationship.

Put simply, there may be two broad types of man—masculine and feminine—and women’s evolved psychology allows them to target each type at the times when it most makes sense to do so.

This implies that women may have historically fared better if they secured a long-term partner who was feminine-and-committed and then sneakily slept with sexy masculine men on the side. If this is true, we should find that women’s preferences for unfamiliar men become greater around ovulation.

This question is addressed by a study by Steve Gangestad, Randy Thornhill and Christine Garver-Apgar, scientists who’ve led the way in this field of research for more than a decade. They gave women a questionnaire to complete when they were at the fertile phase of their cycles, around ovulation, and then again during the later part of the cycle, the luteal phase, when conception is less likely. The survey revealed that, when fertile, women experience greater arousal at the thought of attractive men, and express a stronger interest in sex with those men. This was true even if the men are strangers.

It’s often thought that only men are opportunistic when it comes to selecting short-term partners, but this research show that women can be opportunistic too, at times when it suits them to be. If you think about it, this makes sense because every time a heterosexual man has a short-term fling, a heterosexual woman is a having a short-term fling at the same time, so a short-term strategy is obviously not a male preserve.

Gangestad and colleagues have been especially busy these last few months because they also published a second paper on this topic, which appeared in the same recent issue of the journal Evolution and Human Behavior. In that paper the researchers found that when women are in the fertile phase of their cycle they express stronger preferences for men who are not their primary partner if their primary partner is feminine-faced.

So, if a woman’s partner looks like gerrard butler she may be less inclined to go hunting for a secret short-term partner. If her primary partner is more feminine, like, say, Orlando Bloom, then she’s more likely to look elsewhere once she hits ovulation. Again, this makes sense if we think of short-term relationships as women’s way of securing good genes for their offspring.

If women’s partners already have these good genes — that is, if they are masculine — then there’s less need for women to seek those qualities elsewhere.