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I live with four other girls. A few weeks ago at dinner, one of my flatmates mentioned that our periods did not seem to be synchronised.

Huh? What? How? I said. (Putting aside that this is a weird enough topic for the dinner table)

Apparently it is a ‘thing‘, and apparently everyone else knew it but me. Also, apparently the “How” is “Pheromones!”

My spidy-skeptic-senses were tingling.



When I first went hunting through the Google Scholar articles, I was surprised to find quite a lot of studies in support of this.

For example, in this 1992 study: (Bolding mine)

Menstrual synchrony was examined in three groups of women: 1) mothers and their daughters; 2) women sharing a room in a private residence; and 3) women sharing a room in a dormitory. The intracouple difference in menstrual onset dates was the dependent measure. Mothers and daughters living in the same domicile displayed a significant degree of synchrony. Roommates in private residences were also synchronous, although not significantly more than roommates residing in dormitories.

This seemed interesting. Other articles also reported menstrual synchrony, but only in close friends, rather than just people who lived in close proximity. For example in this 1995 Study,

Menstrual synchrony was found among roommates who were close friends. Synchrony was not found for roommates who were not close friends, nor by housing units. The phenomenon of menstrual synchrony may more likely occur among close friends and women with intensive social contact than under the conditions common to university dormitories.

And this 1979 study:

The amount of time that individuals spent together, and not similar living conditions, was the significant factor in synchrony.

Both of these studies were conducted on young females in University housing.

Pretty interesting right? It seemed I was wrong.

Until I found this study, published in Psychoeuroendocrinology (One of the studies cited above was also published in this journal) in 1992, entitled A critical review of menstrual synchrony research. (Again, bolding mine)

Two experiments and three studies reported a significant level of menstrual synchrony after subjects had been treated with applications of axillary extract from a donor subject or after subjects have spent time together. Four studies failed to replicate these results. A comparison of the studies shows the only consistent difference is that those studies not finding menstrual synchrony reported problems with subjects who had irregular cycle lengths, while those finding menstrual synchrony reported no such problems. All experiments and studies were based on the methods and research design introduced by McClintock (1971). Three errors are inherent in research based on her model: (1) an implicit assumption that differences between menses onsets of randomly paired subjects vary randomly over consecutive onsets, (2) an incorrect procedure for determining the initial onset absolute difference between subjects, and (3) exclusion of subjects or some onsets of subjects who do not have the number of onsets specified by the research design. All of these errors increase the probability of finding menstrual synchrony in a sample. One or more of these errors occurred in the experiments and studies reporting synchrony; no significant levels of menstrual synchrony occur when these errors are corrected. Menstrual synchrony is not demonstrated in any of the experiments or studies.



So the gist of that is: flawed methodology, menstrual synchrony doesn’t exist. Maybe.

So why do so many women who live together report having their periods in sync?

Well, a possible reason for this is suggested in this article, Menstrual synchrony pheromones: cause for doubt

Popular belief in menstrual synchrony stems from a misperception about how far apart menstrual onsets should be for two women whose onsets are independent. Given a cycle length of 28 days (not the rule—but an example), the maximum that two women can be out of phase is 14 days. On average, the onsets will be 7 days apart. Fully half the time they should be even closer (Wilson, 1992; Strassmann, 1997). Given that menstruation often lasts 5 days, it is not surprising that friends commonly experience overlapping menses, which is taken as personal confirmation of menstrual synchrony.

Confirmation bias! What a surprise!

That article also had something to say about pheromones, the supposed mechanism for menstrual synchrony.

In the absence of a theoretical reason for expecting menstrual synchrony to be a feature of human reproductive biology, and until a cycle-altering pheromone has been chemically isolated, it would appear that scepticism is warranted.

Good line, that. Scepticism is warranted. They haven’t managed to isolate any reason why cycles would be altered, so it seems much more likely that this is a case of confirmation bias.

tl;dr Is menstrual synchrony a modern myth or not? Most likely. It seems to be a case of women falling for confirmation bias, and flawed methodology in scientific studies of the phenomenon. But, as always, scepticism is warranted.

Featured Image Credit: Google Images