I Thought We Were Moon Sisters, But Apparently Menstrual Synchrony Is A Myth

by Olivia Loperfido

The myth of menstrual synchrony was first introduced to the scientific community by Harvard psychologist Martha McClintock in 1971, when her study ‘Menstrual Synchrony and Suppression‘ was published in Nature magazine. In it, McClintock hypothesized that women’s’ pheremones might impact the menstrual cycles of their female peers. After studying a group of women living together in a college dorm, McClintock concluded that ‘synchrony and suppression among a group of women living together… suggest that social interaction can have a strong effect on the menstrual cycle.’ 

But there have been skeptics of the synched period myth since it was introduced.

McClintock’s study was heavily criticized for its methods, which excluded subjects with irregular periods and assumed random variation between the menses onsets of paired subjects. A 2005 study by Anna Ziomkiewicz at the Polish Academy of Sciences concluded that ‘social interactions… [were] unrelated to any difference in menstrual cycle onsets,’ that ‘menstrual onset difference was influenced by… body mass and menstrual cycle irregularity,’ and thus that ‘women do not synchronize their menstrual cycles.’ A 2006 study published by U.C. Davis tracked 186 Chinese women living together in dorms for a year, and found no evidence that their periods had become synchronized. 

Most recently, period-tracking app Clue teamed up with Oxford University to study menstrual synchrony.

In the largest study of its kind, Oxford’s data scientists tracked the cycles of 360 pairs of closely-related volunteers (friends, sisters, roommates, partners) for three months via the Clue app. None of the volunteers used hormonal birth control. 100 out of 273 pairs lived together. Researchers concluded that over time, the volunteers’ menstrual cycles were more likely to diverge than to sync. Approximately 76% of studied pairs had a larger difference in their cycle start dates at the study’s conclusion than at the beginning of the study. Clue’s data scientist, Marija Vlajic, told the Guardian, ‘It’s very unlikely that cycle syncing is a real phenomenon.’

So why is menstrual synchrony such a popular and enduring myth?

To begin, menstrual health was overlooked by science, psychology, and research communities until the 1970s. Until McClintock’s study in 1971, menstruation was an almost invisible feature of womanhood; the notion that menstruation might connect us in some lofty, inexplicable, lunar way is a tempting one. Evidencing this, a study published in 1999 showed that 80% of sampled women believed in menstrual synchrony, and 70% of the sample enjoyed it. Popular culture ideates the synched period as a symbol of friendship and female solidarity and, of course, as material for a good joke. Take this scene from the 2011 rom-com No Strings Attached

‘I love when we’re all on the same cycle,’ one of the characters says. ‘We all get to be passive aggressive and fight!’

In short: it’s fun to be moon sisters. But scientifically, it doesn’t hold up. Scientist Marija Vlagic says, ‘… when I say to my friends that I have my period and they have theirs too I don’t conclude that we are syncing. I just think it’s information bias; our brains looking for patterns.’

Top photo: Instagram.com/clubclitoris

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