Does Your Period Sync With Your Friends' Cycles? The "McClintock Effect" Might Be A Myth, According To Science

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Sorry to burst your bubble, but new research suggests that period syncing is a myth. In fact, the idea that our periods sync up with the cycles around us doesn’t have much evidence beyond its establishing study — which is now decades old. Looks like this belief's scientific foundation fell about as easily as the lining of your uterine wall does every 28-ish days.

The idea of “syncing up” originally comes from a study done in 1971 by Martha K. McClintock. After tracking the menstrual cycles of 135 college students, the study concluded that interpersonal relationships and proximity likely had an effect on the subjects’ cycles. The study suggested the idea of a “dominant uterus” that hormonally influenced the cycles of less dominant uteruses around it, also known as the McClintock Effect.

Is the uterus powerful? No doubt. Is it "harness the strength of the moon and change the cycles of all surrounding uteruses" powerful? According to a more recent study, probably not.

This new study comes from the University of Oxford and Clue, a period-tracking app. Researched asked Clue users if they thought their cycles synced up with the people menstruating around them. From an initial data pool of about 1,500 responses, they looked at 360 pairs of period-havers with at least three cycles over a similar time. Their main finding? Only about 22 percent of the pairs (79 of the 360) had data that suggested period syncing, with their cycle start dates coming closer over the course of the study. However, the majority actually experienced the opposite: 273 pairs of the 360 had a larger difference in their cycle start date at the end of the study than they did at the beginning. This study’s stats suggest that “cycles are actually more likely to diverge, rather than sync, over time,” writes Clue.

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The Clue study also looked at the effect of proximity among people who have periods. They found that living together didn’t increase the likelihood of having periods that synced up. Of pairs with cycles that diverged, 37 percent lived together. Of those that converged, 24 percent lived together. The study’s scientist acknowledge that their sample size is small, but state there is little evidence to suggest that period syncing actually occurs.

Marija Vlajic, a data scientist from Clue, spoke to the Guardian about the study. Vlajic says the growing difference in cycle start dates doesn’t imply that periods go “out of sync”; it implies they were never synced up to begin with. Vlajic tell the Guardian, “It’s the nature of two mathematical series that keep repeating: the series will diverge as the numbers grow.”

If you were convinced your period synced with your friends’ cycles, you aren’t alone. As cited by The Guardian, a 1999 survey found that 80 percent of women believed their menstrual cycles synched with the people around them, with 70 percent saying they liked that their periods syncing up. The idea that our cycles sync is one of many period myths we've believed throughout time.

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Our brain is programmed to find patterns, and there is comfort in finding meaning in what may just be coincidence. Like Vlajic tells the Guardian, “It feeds into a feeling of connection, support, and sisterhood. ...Periods are personal and the thought of sharing [yours] with someone makes the idea powerful.”

So, while periods don’t connect us in an “alpha uterus syncing our cycles” kind of way, there is something certainly powerful in the shared experience. And based on a scientific study by me of my own life, being saved by someone with a spare tampon brings people together. Period.