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What is the McClintock Effect and do periods actually sync?


Does the term “cycle sisters” sound familiar to you? Have you ever been around other women and heard them discuss how their periods have synced up? Typically, this discussion happens amongst women that live in close proximity, they are around each other a lot and have even developed the same routine. Perhaps you are able to think of a time when you lived in your childhood home and noticed the different occasions when you and your sister would experience the same excruciating cramps that come every month, only to realize that the both of you have experienced what has been labeled as menstrual synchrony.


One must wonder, where and when did the idea of menstrual cycles syncing come about? In 1971, the first paper about “menstrual synchrony” was published by Martha McClintock, a University of Chicago psychologist. McClintock began to notice that close friends in an all girls dormitory were getting their periods at the same time every month. She hypothesized that because these girls were living in such close quarters along with the combination of pheromones being released would lead to them eventually “syncing” with each other.


McClintock tested her hypothesis by asking 135 college girls that live in dorms to give their period start dates at 3 times throughout the school year. Though her findings were not rooted in and backed by scientific evidence, she found that close friend groups had periods significantly closer together in April in comparison to October. This finding meant that the average days apart between close friend groups went from 6.4 days apart to 4.6 days-- the McClintock Effect was born.

The McClintock Effect (“an alleged process whereby women who begin living together in close proximity experience their menstrual cycles becoming more synchronized together in time than when previously living apart”) was held as the first example of pheromones, which are said to be unconscious chemical signals that influence behavior and physiology among humans. Eventually, people and doctors begin to wonder if this synchronization was an actual thing? Do periods actually sync?


The research surrounding menstrual synchrony can be conflicting because it’s mostly based off of personal experiences from women. In fact, research and studies have accumulated over the years to debunk McClintock’s findings-- most of them making the point that menstrual cycles don’t converge, but they actually diverge! Most researchers would say that periods syncing amongst women can simply just come down to basic probability and simple mathematics. If enough cycles go by, whether it’s every 27-30 days, there’s going to be a chance you “sync” periods with your friend.


What do you think? Could menstrual synchrony just be some age old myth that continues to go around or could it be a definitively real thing?


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