in sync

The synchronization of periods might feel like a rite of passage for some friend groups. But is this proverbial proof of friendship fact or fiction?

By Raven Lawson

If you’ve ever been to a sleepaway camp, had female roommates, or even hung out with the same girls every day, chances are, at one point or another, you’ve been convinced that your periods are in sync.

Known as “the McClintock effect,” the notion that women’s menstrual cycles synchronize first appeared in an academic paper by Martha McClintock in 1971. In her study of college girls living together in a dorm, she found that social interaction impacted one’s menstrual cycle.

Suppose you start breaking out and getting similar abdominal cramps, cravings, or any other symptoms all around the same time. In that case, it’s only natural to think your cycles must be in sync. Some of you reading this right now might be thinking, “oh yeah, this totally happens with me and my friends.”

Well, you’d be wrong…because period syncing is a myth.

“There may be some similarities with people’s menstrual cycles; however, there’s no scientific evidence to support that [they sync],” says Dr. Lisa Nicholas, OBGYN and professor at UCLA School of Medicine.

Since McClintock’s original study, several other reports have denounced the period syncing idea. One such study conducted by period tracking app Clue in partnership with Oxford University (2017) found that women’s periods do not sync over time and are actually more likely to diverge than come together.

One of Clue’s data scientists, Marija Vlajic, states in an article for The Guardian, “it’s very unlikely that cycle syncing is a real phenomenon.”

There’s a sense of unity and community in knowing you’re simultaneously on your period as a close friend. A shared struggle. But at the end of the day, any synchronization boils down to coincidence.

Our brains naturally look for patterns to make sense of things around us. So, when a friend or roommate expresses that they, too, are on their period, what we actually experience is a confirmation bias.

This feeling of collective menstruating “probably came from people sharing information with one another,” says Nicholas. “Communication regarding what their patterns are like likely causes people to make anecdotal assumptions.”

While period syncing might not be real, and you, unfortunately, do not have a super special bond with your bestie because you (coincidentally) menstruate at the same time, that bond of sisterhood is not entirely lost. There are plenty of other shared struggles to unite over like the outrageous prices of feminine hygiene products or the attack on women’s right to choose. Or you can ignore this article and continue to live in the delusion that “in-sync” periods signify true friendship.