The Science of Synching Up

It’s a tale as old as time…when two women live together, their menstrual cycles begin to sync up. We heard it with college roommates, mothers and daughters, and even friends who spend a lot of time together – are getting their periods around the same time each month. 

In a Strike Magazine Instagram poll of 78 people, 97% said they had experienced this cycle sync. 

This phenomenon gained great attention in 1971 when Martha McClintock published the first study examining cycles of 135 women living in Wellesley College dormitories, suggesting that “social interaction can have a strong effect on the menstrual cycle” and hypothesizing that pheromones or the lunar phases could play a role in this event.  

Because of her study, cycle syncing became known as the McClintock effect. Another study published in 2023 found that 54% of the women involved had closer onset dates of their cycle than when the study started, suggesting synchrony. However, eight pairs of women had greater onset differences by the end of the study. 

And while women everywhere can tell stories of their cycles syncing with another woman, science keeps telling us it’s just a coincidence. 

A 2006 study of 186 Chinese women determined that any cycle syncing was due to chance or “mathematical coincidence.” 

And in recent years, data from period tracking apps have debunked the theory even further. As reported by the Guardian, an Oxford study using data from the period-tracker Clue found that 273 out of 360 pairs of women living together had a greater difference in cycle onset dates than when the study started. 

There are lots of different studies either suggesting synchrony or disproving it. It has been a widely debated topic in science ever since that first study in 1971, with many questioning the methodical and statistical errors in the study. Part of the reason why it’s so hard to nail down an answer is the lack of research surrounding it. 

The theory that pheromones could play a role in cycle syncing is little known. Pheromones are chemical signals released by humans and other animals to signal different things like arousal, attraction, fear, and more. The study from 2023 notes that “despite ongoing research, the cycle-altering pheromone has yet to be chemically identified and isolated.” 

The best explanation science can give us rests on mathematical coincidence. Say, for example, you have a three-week cycle, and your roommate has a five-week cycle. Over time, your periods might sync up for a month every now and then, but the difference in onset dates isn’t actually getting smaller. 

Other external factors can influence your periods, such as contraceptives, stress, and diet, among other things. 

Another 1999 study showed that of 122 women, 84% were aware of cycle syncing, and 70% experienced it personally. It seems curious that women everywhere say one thing, but science says another. Some scientists even suggest women hold onto the belief that their cycles sync to feel closer to one another or believe we have a stronger bond with those close to us. 

In a world where women's reproductive health is extremely overlooked and under-researched, science can only tell us results from the research that’s been done. There are still many more gaps to fill in the research surrounding cycle synching and the hormonal release women experience on a monthly basis. 

Strike Out, 

Haley Dockendorff 

Boca Raton

Haley Dockendorff is a Content Writer for Strike Magazine Boca. Loud and proud, this Virgo loves writing just about anything that will cause a bit of commotion. If you can’t find her, she’s probably somewhere on a beach with a camera in hand. 

You can reach her at haleydockendorff143@gmail.com