When my girlfriend’s cycle synced up with mine during the pandemic, I wasn’t surprised. For me, syncing periods has been an inevitable perk of dating other menstruating people — we can give each other a heads up when the bloodbath is on its way and buy a family-sized bag of chips for our shared time of need. Since my girlfriend is quite literally the only person I see these days, I (correctly) assumed that we would start bleeding on the exact same day each month. Of course, my belief in syncing periods rested on my own anecdotal evidence and the phenomenon’s prevalence in popular culture. It’s a punchline on sitcoms. It’s a given on GLOW. Supposedly, folks who go together flow together. But does the science of syncing periods hold up?
The subject of “menstrual synchrony” first hit scientific journals 1971. While still an undergrad at Wellesley College, future psychologist Martha McClintock hypothesized that pheromones could influence period synchronization, so she rounded up some college pals and studied the start dates of their cycles. McClintock didn’t factor in ovulation or cycle length — she only tracked the onset of bleeding — and her study of 135 college women claimed “a significant increase in synchronization” among roommates and among groups of people who considered one another “close friends.” In other words, over the course of the study, the start dates of the subjects’ periods grew closer together. Thus, menstrual synchrony (later dubbed “the McClintock effect”) was born, setting a cultural narrative of bleeding BFFs into motion.
After McClintock’s study garnered attention, other researchers set out to disprove it, and — surprise, surprise — male researchers came after her with the most venom. In 1992, anthropology professor H. Clyde Wilson came down on McClintock with the academic version of a “well, actually,” correcting methodological errors which stripped her results of their validity. Wilson justly noted that in McClintock’s initial study, she excluded subjects with irregular cycles, but instead of bowing out, McClintock doubled down with a new study in 1998. She found that people who were exposed to underarm secretions collected from donors in the first and second phases of their cycles reacted with significantly altered menstrual cycle lengths, but her triumph was short-lived.
Countless menstrual synchrony papers have been published since McClintock’s second study. Most findings sit firmly in the anti-synchrony camp, chalking up period syncing to mathematical coincidence. In recent years, thanks to the rising popularity of period tracking apps, researchers have even more data at their fingertips. A 2017 study by the menstrual health app giant Clue even found that the cycles of people who live together are more likely to diverge, rather than sync, over time. But the ethics around period surveillance apps are murky, raising questions about the studies that use their data. Many of these apps aren’t made with with queer and trans menstruators in mind, which could affect the population sample, and since this data is primarily collected and sold for marketing purposes, some take this research with a grain of salt. McClintock doesn’t seem to trust any attempts to disprove her work, and in an interview with Scientific American, she remained resolute. “I don’t think there is any doubt that social interaction among women and body compounds from women can change the way the ovary functions,” she said.
In recent months, the pandemic has forced many of us into close quarters with roommates and partners, marking the perfect moment to determine (once and for all!) which way the PMS-craving-cookie crumbles. I spoke to ten menstruating people who are currently quarantined with menstruating partners. Two respondents noted that the stress of the pandemic has altered the length and duration of their cycles, making the possibility of syncing less likely, but most reported that their periods are not and have never been synced. Still, the vast majority of interviewees said they believe that period synchrony is a real phenomenon. Some cited past experiences of syncing with former partners, friends or coworkers, but most just assumed that menstrual synchrony had to be real — after all, periods are magic, aren’t they? As one cisgender lesbian from a small Midwestern town said, “I can’t tell if it’s something I once had evidence for and just forgot what that evidence was, or if it’s something I just accepted as a truism and assumed I was the exception because it doesn’t happen to me.”
The way some of us feel about period synchrony is the way most people feel about aliens — we want to believe. Dr. Breanne Fahs, a Women and Gender Studies professor at Arizona State University, can tell us why. In 2016, Fahs conducted a study examining the widely-accepted period synchrony narrative. She found that despite the lack of conclusive evidence supporting synchrony, most of her subjects believed in its existence, and they had all kinds of unscientific theories to back it up. One common belief was that some people have “alpha uteri” that draw other bleeders into their cycle. An Autostraddle writer who had previously synced with friends and partners said, “We’ve always joked that some periods are ‘tops’ and some are ‘bottoms,’ and that’s how our bodies decide whose cycle to follow. I really like that idea — that like some periods are so powerful that they pull everyone else’s cycle into their orbit!”
According to Fahs, being a menstrual “top” or “bottom” is most likely a myth, but the desire for solidarity among menstruating people is very real. Rather than assuming that synchrony believers are hysteric menstruators who don’t believe in science, Fahs argues that belief in menstrual synchrony operates as a social tool, pointing to the need “for more social spaces to feel solidarity with each other, to express collective anger, to feel connected to the magical and scientific aspects of their bodily existence, and to reckon with the patriarchal scripts of women’s competition and dominance over each other.” Perhaps belief in menstrual synchrony also serves as a “fuck you” to the medical industry, which has a rich history of ignoring reproductive health and women’s pain. Fahs’ study reveals why the belief in menstrual synchrony endures, and perhaps it explains why McClintock has defended it so fiercely.
OK, fine — occurrences of menstrual synchrony are most likely happy accidents, but McClintock’s belief that social interaction “can change the way the ovary functions” is worth further investigation. My girlfriend didn’t experience many PMS symptoms before we started dating. Now she’s jumped on board my “gotta catch ‘em all” PMS symptom train, and we’re not alone. Autostraddle’s Fundraising Director Nicole reported, “I’ve noticed since living with my partner that sometimes I’ll get some PMS symptoms when she is having PMS symptoms even though the timing isn’t right for me.” Another interviewee explained, “With my wife, menstruating roommates I’ve lived with in the past, and the only other woman in my MFA cohort, I have often experienced cramps/ breast pain in my own body when they’re menstruating or premenstrual.” Is it science? Is it the magic of the big, gay moon? Perhaps the resulting comradery matters more than the facts.