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When women live together, do their menstrual cycles tend to synchronize? It’s been a long time since I first heard that claim. I didn’t believe it, for a number of reasons. I had never observed it myself, I saw no plausible mechanism to explain how it could happen, I thought the statistics to prove it would be problematic and complicated, and I suspected that confirmation bias and selective memory might have persuaded people that a spurious correlation existed. How often do women say “Oh, look! We’re having our periods at the same time”? How often do they say “Oh, look! We’re having our periods at different times”? Now that many years have passed since my first encounter, I thought it would be fun to revisit the claim and see whether science has supported it or rejected it.

A perusal of PubMed and other Internet sources left me confused and amused.

Synchrony Is Difficult to Define

Consider that the normal menstrual cycle can vary from 21 to 35 days and can last 2 to 7 days. Consider that some women are regular and consistent, while others have variable patterns, even “regularly irregular” patterns. Consider that anovulatory cycles and other conditions often lead to menstrual irregularities that fall outside the normal range. Consider that strenuous exercise and other life events can affect menstruation. Put all that together, and you can see that often cycles will overlap simply by chance, and that it is difficult to define synchrony.

If two women have regular 28 day cycles and 7 day periods, the maximum number of days they could not overlap is 14. On average, their periods will be 7 days apart, and half the time they will be closer.

How could a 21 day cycle ever “synchronize” with a 35 day cycle? For example if you compare a woman with a regular 35 day cycle who starts on January 1 to a woman with a 21 day cycle who starts two weeks later on January 15, their next periods will coincide almost perfectly (Feb 4-10 and Feb 5-11) but they will diverge after that. Would it count if the last day of one woman’s period overlapped with the first day of another woman’s? What if half the periods coincide and half don’t? The whole thing is problematic.

What Does the Literature Say?

It all started with Martha McClintock. In a paper published in Nature in 1971 she found that “social interaction” in a college dormitory setting could have a strong effect on the menstrual cycle. A follow-up study in 1998 tended to support the hypothesis that pheromones were involved: smelling armpit secretions of other women could either lengthen or shorten cycles depending on what part of her cycle the donor was in.

I’ll summarize rather than trying to cover everything published on the subject. A Scientific American article did a good job of reviewing the literature as of 2007. Suffice it to say that about half the published papers support the synchronization hypothesis and half don’t; and the half that do have been harshly criticized for their poor design and poor statistical analyses. So we haven’t reached a consensus, but it’s looking more likely that synchronization is a myth.

A study in a nursing journal assumes that synchronization occurs and addresses the subjective meaning of the experience to

assist nurses to understand the holistic aspects of this everyday experience of women and to design effective strategies and techniques to help women gain knowledge about their cycle functions, promote healthy attitudes toward menstruation as a process, and acknowledge and honor this natural, healthy aspect of their menstrual cycle.

I will be kind to those nurses and apply Thumper’s rule. (In Bambi, he said “If you can’t say something nice, don’t say nothing at all.”)

What About Non-Humans?

It was originally thought that other primates and other mammals demonstrated menstrual synchrony due to pheromones, but recent studies have shown that it doesn’t occur in chimpanzees, hamsters, mandrills, or golden tamarins.

Do Human Pheromones Exist?

The existence of human pheromones is controversial. Pheromones are chemical signals released by one individual that affect the behavior of another individual of the same species. They can be useful for alarms, food trails, sex attractants, and other purposes. They have been well documented in insects and even in plants. In one memorable demonstration, a drop of oleic acid was all it took to persuade other ants that a live ant was dead; they dragged it back to the trash heap over and over, ignoring the fact that it kept protesting its trips to the ant graveyard by vigorously kicking, and that it kept “coming back from the dead.”

In humans, pheromones have been postulated and even sold as sexual attractants. But there is little or no peer-reviewed evidence to suggest that any pheromone influences human behavior. No human pheromones have been identified, and the vomeronasal organ that detects pheromones in other mammals is rudimentary and nonfunctional in humans.

Other Explanations

Not knowing if it occurs hasn’t stopped people from speculating about why it occurs or about why it doesn’t. The evolutionary reasoners have chimed in with just-so stories both about why it would and wouldn’t offer a survival benefit. For instance, if everyone ovulated at the same time, it would be harder for a woman to get a man. Or being banished to the menstrual hut at the same time would allow women to bond and collaborate on social enterprises. I was particularly amused by one chain of reasoning:

Synchronization also doesn’t make sense because during their mentrual cycles, women often experience lack of energy. These spells of low energy among women in a tribe would be detrimental to a hunter-gatherer lifestyle. Furthermore, if all of the women get pregnant at the same time, they won’t be able to help each other. Given our anthropological history, evolution should select that women do not cycle together. (errors in the original)

And this comment on the Scientific American article left me rolling on the floor:

Astrology explains this for better than modern science. Normally close friends have similar sun, moon positions in their horoscopes. The moon and mars determine the menstrual cycles. Astrological harmony makes them friends than living together which in turn is the cause of synchronization. Even if they are living far in different continents they will be in sync. In India any normal astrologer can check a woman’s horoscope and can tell the date of her menstruation time ( especially used when they have to attend auspicious ceremonies) and plan the events. (errors in the original)

Conclusion

Do women’s menstrual cycles tend to synchronize when they live together? As so often in medicine, science doesn’t provide a clear answer. Even if the phenomenon occurs, and even if human pheromones cause it, so what? It’s a matter of curiosity, not of clinical importance. When there is a reason to manipulate ovulation or menstrual timing, pharmaceutical hormones work very well. The paucity of evidence for human pheromones suggests that if they do exist, their effects must be too small in magnitude to be very important.