Menstrual Synchrony...Or Menstrual Illusion?


~ MOLT thanks Dr. Jeffrey Schank, Associate Professor at the University of California, Davis, for bringing the article by Lisa Belkin, “The Odds of That,” to our attention. Visit his webpage at


Quite commonly, menstrual synchrony is included in a list of other synchronous phenomena. As an example, the following from is typical:

“…electric synchrony among cardiac pacemaker cells, flashing synchrony among swarms of fireflies, crickets chirping in unison, synchronization of menstrual cycles…”

But as we have seen from the foregoing sections of this MOLTXIBIT, perhaps the "WMBS phenomenon" belongs in a different category, that of “coincident phenomena.”

In Amber Kuhn’s email, she argued:

I have never been great with statistics, but I am pretty certain that the odds are stacked against the fact that my three roommates and I experience onset within just a few days of each other, month after month.

An answer can be found in “The Odds of That,” a lengthy article by Lisa Belkin examining a string of post-9/11 deaths thought by some to not simply be coincidences, but rather, a conspiracy to kill bioterror experts.

Belkin writes:

What are the odds?  The mathematician will answer that even in the most unbelievable situations, the odds are actually very good.  The law of large numbers says that with a large enough denominator – in other words, in a big wide world – stuff will happen, even very weird stuff.  “The really unusual day would be one where nothing unusual happens,” explains Persi Diacomis, a Stanford statistician who has spent his career collecting and studying examples of coincidence.  Given that there are 280 million people in the United States, he says, “280 times a day, a one-in-a-million shot is going to occur.””

In terms of menstrual “synchrony,” then, we would reason that approximately half the US population is female, that is, 140 million; and among these 140 million females, there are millions who are having regular menstrual cycles.  Therefore, in this “big wide world” of menstruating women, the odds are “actually very good” that “[Kuhn’s] three roommates and [she] experience onset within just a few days of each other, month after month.”

Later in the same article by Lisa Belkin, she describes an experiment performed by researcher Ruma Falk.  Falk collected birth dates from a large group of students, and then “wrote…on the blackboard” “the handful of birth dates that students had in common.”  Falk found that “those in [the subgroup of students whose birth dates were written on the board] were consistently more surprised by the coincidences than the rest of the students.  “It shows the stupid power of personal involvement.”

Certainly a menstruating woman is “personally involved” with her own menstrual cycle, for two reasons:  1) You can’t get much more personal than a bodily function, and 2) because of the cultural silence so often surrounding menstruation, it can feel even more “personal” than it is, due to its so obviously not being “public.” 

So if personal involvement leads to “over-interpreting” coincident phenomena, and further, a belief that there must be some “cause” for it, this would be a powerful motivator for believing menstrual synchrony is real, regardless of how contradictory the last 30 years of menstrual synchrony research findings have been.

Belkin discusses the work of researcher Joshua B. Tenenbaum.  Belkin writes:

Finding connections is not only the way we react to the extraordinary, Tenenbaum postulates, but also the way we make sense of our ordinary world.  “Coincidences are a window into how we learn about things,” he says.  “They show us how minds derive richly textured knowledge from limited situations.””

To put it another way, our reaction to coincidence shows how our brains fill in the factual blanks.  In an optical illusion, he explains, our brain fills the gaps, and although people take it for granted that seeing is believing, optical illusions prove that’s not true.  “Illusions also prove that our brain is capable of imposing structure on the world,” he says.”  “One of the things our brain is designed to do is infer the causal structure of the world from limited information.”

So perhaps it makes more sense to reinterpret menstrual synchrony as menstrual illusion, best listed alongside other illusions, such as the Poggendorf illusion, Staircase illusion, Vase/Faces Illusion, and so on.  (From  

But that’s not the only reason why menstrual synchrony is so popular – perhaps there’s another – click here to read about menstrual synchrony and the desire for menstrual camaraderie. 

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