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There are pretty much only two reasons periods ever get discussed on prime-time television: first, to draw attention to their lateness, thus introducing a pregnancy story line. And second, to note that two characters’ menstrual cycles have synced, indicating that they’ve been bonding or spending a lot of time together. In an episode of Jane the Virgin that aired earlier this year, for example, Jane and her fiancé, Rafael, briefly compete over which of them has a closer relationship with another character, and Jane boasts to Rafael that “Petra and I are on the same cycle.” Sex and the City and GLOW have employed synced-up periods for a similar purpose, while other shows—Modern Family, The Office, Community, New Girl—have invoked the idea as a punch line.

For a phenomenon that’s highly unlikely to be real, period syncing has enjoyed an impressively long life in the popular imagination. Every now and again, news stories and listicles pop up to inform the public that no, actually, period synchronization as a result of prolonged proximity is not a thing, but the fictional story lines and offhand jokes persist nonetheless.

TV and movies certainly help maintain the popularity of the period-syncing myth. But to some extent it survives because so many people want it to be true. No matter how inaccurate the myth of period syncing may be, the idea that women’s bodies can fall into collective rhythms carries a certain mysterious, otherworldly appeal and, lending the myth more inertia, gives women a way to feel connection, empathy, and collective empowerment with other women.

Period syncing—or, more formally, “menstrual synchrony”—was introduced into the popular consciousness in 1971 by a researcher named Martha McClintock. Her study on the menstruation patterns of students at a women’s college, published in the journal Nature, tracked the period start dates of 135 women who lived together in a dormitory over a time frame of about six months. It claimed to find “a significant increase in synchronization (that is, a decrease in the difference between onset dates)” among roommates and among groups of women who independently identified one another as a “close friend.” At the beginning of the study, these friends averaged about six and a half days’ difference between period start dates. By the end, they averaged a little less than five. (McClintock conducted this research while she was still an undergraduate at Wellesley College.)

By 1978, McClintock’s study had been cited more than 40 times in other journals and academic publications, according to the late H. Clyde Wilson Jr., a former anthropology professor at the University of Missouri who presented this statistic in a 1992 paper poking holes in McClintock’s research. He pointed out various methodological oversights and noted that while two similarly designed follow-up studies had comparable findings, four other subsequent studies did not replicate the results McClintock had gotten.

Other researchers published further critiques, including Beverly Strassmann, an anthropology professor at the University of Michigan. In a 1999 paper in the journal Human Reproduction, Strassmann pointed out a fundamental flaw in the period-syncing logic:

Given a cycle length of 28 days (not the rule—but an example), the maximum that two women can be out of phase is 14 days. On average, the onsets will be 7 days apart. Fully half the time they should be even closer. Given that menstruation often lasts 5 days, it is not surprising that friends commonly experience overlapping menses, which is taken as personal confirmation of menstrual synchrony.

In other words, as Strassmann explained to me in an interview, women have their periods at the same time as other women an awful lot, so it’s common to mistake menstrual overlap for menstrual synchrony: “A quarter of the time, [two] women’s menses should be overlapping, based on random chance,” she said.

Confirmation bias—or the practice of consciously or unconsciously discarding evidence that doesn’t support a desired hypothesis—can also contribute to this mistaken impression. For one thing, given that menstruation is usually kept private, it’s rare for women to be aware of just how many strangers around them are menstruating at the same time they are. Plus, when two women are close enough to tell each other when they’re menstruating, an alignment of cycles can inspire what might feel like a mystical, sisterly connection.

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.

So when Fahs presented some research on women’s attitudes toward the idea of synchrony at a conference on menstrual research and advocacy a few years ago, she encountered aggressive resistance to the suggestion that period syncing wasn’t scientifically supported. Other attendees at the conference insisted, Fahs told me, that on this particular matter, the scientists were biased. “Women are dismissed all the time for what they believe about their bodies,” Fahs said. “So people want to believe in this.”

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