Strassmann considers the idea of period syncing to have been “debunked,” but of course much of the general population remains convinced that it’s a real thing, thanks to what she calls “an appealing narrative that overrides the science.”

In 2016, Breanne Fahs, a women- and gender-studies professor at Arizona State University, published a study examining that narrative. For the study, Fahs spoke with 18 women who believed they had experienced menstrual synchrony, and a few key themes repeatedly came up.

Fahs wrote that some of the women she interviewed believed “that menstrual synchrony happens because of biological, animal-like, or hormonal reasons”—in other words, that it was a natural, primal phenomenon programmed into humans to promote the survival of the species. Other women in Fahs’s study believed that menstrual synchrony was real and simply transcended scientific explanation. Some, she wrote, even likened it to a mystical, invisible connection between two women.

Fahs also found that some women believed that when a group of female friends spent significant time together, their periods all synced to one particular woman’s cycle, sensing her “dominant or ‘alpha’ status” and adjusting accordingly, like heliotropic flowers turning toward the sun. This idea “seems really outlandish to me—but it also fits with the ways that we think about group settings and social dynamics,” Fahs told me. “We’re a culture that really does think about things like dominance and hierarchy and social groups a lot.”

What Fahs views as the main takeaway from her research, however, is that the idea of suffering through periods together satisfies an appetite for community among women. “Women … expressed that menstrual synchrony allowed them to express anger together with other women; anger served as a platform for solidarity as women by allowing them to be more demanding or forceful,” Fahs wrote in her study.

One woman Fahs interviewed spoke fondly of the feeling she had when she and her best friend got their periods around the same time, describing it as a sort of “don’t mess with us” solidarity. That feeling can be particularly empowering, Fahs noted, given that women’s public expression of anger, particularly as a group, is often met with hostility or ridicule.

Another factor that enables the myth of period synchrony to remain in circulation, Fahs found, is that women are used to feeling as if the medical establishment has dismissed what they believe to be true about their own bodies. Over the past few years, for example, media outlets have been covering the widespread problem of “health-care gaslighting” (physicians downplaying or trivializing women’s pain or discomfort, or dismissing it as all in their heads). Similarly, women (and men) have for decades believed that hormonal birth control can cause weight gain and an imminent period can cause irritability—based on firsthand experience, observations from their own lives, and some support from scientists—even as a number of physicians and researchers have questioned whether these phenomena are actually real.