Menstrual synchrony: fact or fiction?

Menstrual synchrony is sometimes referred to as the dormitory phenomenon or the McClintock effect.


Sonia Chartier

02 May 2016

Whether it’s between a mother and daughter, between students in residence or between lesbian partners, it’s something people love to talk about and it even makes headlines in scientific journals…

So what’s it really about?

The dormitory phenomenon (menstrual synchrony) is when the menstrual cycles of women living in close quarters (for instance, in a dormitory, house or military bunker) shift over a period of a few months and become synchronised.

Menstrual synchrony: Fact or fiction?

The first study of the phenomenon was published in 1971 by Martha McClintock, a doctoral student in psychology at Harvard, who had observed menstrual synchrony in the university’s dorms. Her study, published in the journal Nature, was followed by several others conducted in a range of settings and with very varied results. Some showed that menstrual cycles do in fact sync up, whereas others showed the opposite to be true!

Ms. McClintock had hypothesized that the phenomenon could be explained by the secretion of pheromones, hormonal signals transmitted through smell. Several studies have attempted to reproduce the findings, but none have succeeded.

The synchronization phenomenon was once observed in female hop pickers in Belgium. Apparently, their periods started without warning at the beginning of the harvest, regardless of their normal cycle. While this legendary effect was once called the “hop devil,” we now know that a compound called hopein (or 8-Prenylnaringenin for the technical types among you) is a phytoestrogen that can mimic the effect of natural estrogens. Since the advent of mechanical harvesting, the hop devil has been exorcised.

With such a powerful effect, you’d think that the huge amounts of beer students consume—after all, beer is made with hops—would be enough to explain the dormitory effect. But as it turns out, the quantity of hopein in beer is 1/400th the amount needed to have any estrogenic effect on health.

In fact, any observable synchronization would be better explained by the fact that women each have “their” cycle, whose duration varies and whose frequency is more or less regular. A so-called “normal” cycle lasts 21 to 35 days, with bleeding lasting for between five to seven days. It would therefore be quite amazing if there weren’t certain times when the menses of women living together overlapped. Furthermore, the fact that many women have an irregular and unpredictable cycle makes this dormitory phenomenon even less likely. This type of irregularity can be corrected by another plant, vitex, which is not used to make beer or wine.

Come to think of it, it’s probably a good thing that all our friends and colleagues don’t have PMS at the same time as us!


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