Do Period Cycles Really Sync?

By Eva

You’ve probably heard that when people with periods live together, their menstrual cycles begin to sync. Is this true?

In 1971, Nature published the first study looking at over 100 women living in college dorms to see if their cycle synchronized. They found that a significant number of women living in proximity to each other experienced a decrease in the onset of their period or greater synchronization. The author concluded that menstrual synchronization is due to an increase of pheromone communication.

Evolutionary arguments sprouted to support why period synchronicity is beneficial to reproductive success. The most popular one is that the synchronization of sexual receptivity in groups prevents the monopolization of one dominant male. In other words, if all the women are on the same cycle, it makes it harder for one male to gain sexual domination.

However, since this study came out, a series of other research studies were published looking at this phenomena with inconsistent results. Some supported this finding and others revealed no synchronization or raised questions to the methodological procedures of the original study itself.

Recently, Clue, the period and fertility tracking app, teamed up with researchers from Oxford to conduct the largest study done on this phenomena. It included about 350 pairs of women and analyzed their cycles. Interestingly, their cycles actually diverged instead of synchronized.

Clue’s data scientist, Marija Vlajic, told the Guadian, “Menstrual syncing amongst the sample we had did not exist. We’ve also done some statistical tests and found that the difference in cycles actually grows. This doesn’t mean that pairs go out of sync — it means they were never in sync in the first place. It’s the nature of two mathematical series that keep repeating: the series will diverge as the numbers grow”. 

It is safe to conclude that period synchronization is probably a myth, unfortunately. But when you are syncing with your best friends or roommates, it is still fun to embrace the random coincidence and eat chocolate ice cream together.

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