Tenth grade, at the lockers just before homeroom. My good friend Julie and I put away our coats.

“How are you?” she asks.

“Ugh,” I say. “I have a soccer game today but” – and here my voice drops to a whisper – “I got my period this morning.”

Julie looks back at me, wide-eyed. “Oh my god, I have my period too!”

“When did you get it?”

“Three days ago!”

“Oh my god!”

Yes, that is what we really said, and you have to imagine it with a trace of Boston accent. And mind you, this is before the days of OMG.

We close our lockers and head to homeroom slowly, our backpacks heavy with textbooks. I am awed by our biology. It must be our friendship, all the time we spend together in class and after school, that has led to our menstrual cycles being so aligned.

* * *

Menstrual synchrony is one of those ideas that seems to be confirmed in our daily lives. If we are menstruating at the same time, or close to the same time as a friend, the coincidence takes on a greater meaning, a sign of our time spent together, or closeness as friends or partners. It’s a private way to celebrate our bond. Perhaps for others it serves as another indicator of the ways in which girls and women are ruled by their reproductive biology.

And there are species who do have synchronous cycles. Some cycle at about the same time of year because they are seasonal breeders, like sheep, some marsupials and even some primates. Others do use social cues like pheromones from fellow females, as in rats: the urine from one female can get the cycle of another to synchronize with her.

But the study of human menstrual synchrony has suffered from three major problems: first, whether a mechanism exists that can produce menstrual synchrony, second, methodological issues with existing papers and third, statistical artifacts in how one analyzes synchrony. A related issue is that, from an evolutionary standpoint, many struggle to find a reason women would want to have synchronous cycles, and an ancestral environment in which it could even happen.

The evolution of human menstrual cycles

These days in industrial and post-industrial populations, women can expect to have around 400 menstrual cycles in their lives, give or take a few depending on the number of pregnancies they have and degree of energetic constraint they experience from breastfeeding or exercise (Strassmann 1997). Family planning and contraceptive use increases the number of cycles as well, so we are looking at a biocultural phenomenon. With so many women cycling so often, there is a higher chance of menstrual cycles aligning, or at least appearing to align.

Contrast that with the number of cycles women have in forager populations, which is probably around 50. Forager women tend to have higher energetic constraint from the amount of physical work they need to do to acquire food. This work isn’t necessarily substantial in the sense that they are sprinting all day long, but contrast walking ten miles slowly over the course of one day while carrying food and a kid with an American sitting at a desk, and you can see how the calories burned add up.

Perhaps most important of all, forager women are usually natural fertility, which means overall they aren’t actively limiting their fertility (though again, it’s important to note that they still might try to limit number of offspring). This means that within a few years after their first period they are having their first baby, breastfeeding for many years, then maybe cycling a few times before getting pregnant again. Six to eight pregnancies with four years between each one and you’re almost to menopause! You can see how rare synchrony would be in a population where women are breastfeeding or pregnant through most of their reproductive years.

Maybe we should look to our primate relatives for evidence, then: in fact two papers have come out this year testing this hypothesis in primates! Setchell et al (2011) observed semi free-living mandrills, which is a kind of Old World monkey, a group to which the Great Apes belong. Out of ten observation-years of data, they found a single year that had significant synchrony… only to have that one year fail to be significant once they corrected for multiple testing. Multiple testing corrections are important because of the chance that if you test a hypothesis enough times you will get a spurious significant result (and for a brilliant take on this, see this xkcd comic).

The other equally interesting paper to come out this year on this topic is by Fürtbauer et al (2011), entitled “You Mate, I Mate: Macaque Females Synchronize Sex Not Cycles.” Their study population was wild Assamese macaques, also Old World monkeys. Fürtbauer et al (2011) observed behavioral receptivity and measured fecal ovarian hormones (yes, that means they measured hormones in poop) in order to assess behavioral and hormonal synchrony. They found long periods of behavioral receptivity that synchronized well across individuals, but that actual estrus cycles were randomly distributed within the receptive period. I thought this paper did a great job at providing an evolutionary framework for why mating might evolve to be synchronized, but not cycles, and because the paper was published in PLoS ONE, you can read it yourself for free.

This paper resolves a question I’ve had for a long time about menstrual synchrony, which is how in the world it could actually be beneficial to females, particularly those with covert ovulation. Why would you want all the females, or even a subset of them, to be fecund and receptive at the same time? And the answer is, you probably wouldn’t. Humans, other primates, even some cetaceans like dolphins have mating that is largely decoupled from reproductive cycling. That is, we don’t only mate at the time in our cycle when our chances are highest to conceive, though we might find ourselves slightly more proceptive or receptive at that time. Sex is not just about making babies, but is an affiliative behavior, promoting bonding but also plain old enjoyment. Instead, it may in some circumstances make sense to have extended periods of synchronous receptivity, as within a promiscuous species like the Assamese macaques (Fürtbauer et al. 2011). But this isn’t necessarily an adaptive feature of the entire primate lineage.

You sniff, I sniff

The other issue that has plagued the study of menstrual synchrony is that we don’t have a good idea for how it could actually work. For most, the entire claim for synchrony rests on the presence and ability to detect pheromones. When I first heard of the idea of menstrual synchrony I read Beverly Strassmann’s “Menstrual synchrony pheromones: cause for doubt” and was sufficiently convinced to stop paying attention to the issue for many years (Strassmann 1999). Since then, I have been told by those more knowledgeable than I that humans don’t have a functional vomeronasal organ (VNO), which is necessary for detecting pheromones. Yet, people keep studying menstrual synchrony, and they even study other behavioral phenomena that would require some kind of pheromone or odor detection ability in humans, like mating preferences around immune complementarity. And some of these studies get significant results.

So, I read up on it. Turns out that while we don’t have as many scent glands as New World monkeys, humans have pretty big ones compared to other Old World monkeys. And we may not need a VNO to detect pheromones. Hays has a thoughtful, well-written review that outlines the several types of pheromones as well as the signaling pathways that could lead to human detection of pheromones, and only one of them is through stimulants to the VNO (Hays 2003). Further, certain odors are associated with some pheromones (which are themselves odorless) and genetic profiles, so the mechanism for pheromone detection could be related to the sense of smell.

That said, the mechanism for detection of pheromones related to menstrual synchrony is a weak one, not really supported by evidence. So far the only proposed mechanism does require stimulants to the VNO or pheromones in axillary sweat, and no studies have convincingly demonstrated that women respond to other women’s pheromones in this way (Hays 2003).

Fight, fight!

The last two issues around menstrual synchrony are related to methodology and statistics. And this is where the story starts to get fun.

I found a series of articles and letters in the Journal of Comparative Psychology between Schank, Weller and Weller, and Graham in the early 2000s. Schank is someone who contends there is no support for menstrual synchrony, where Weller and Weller and Graham study it and support it. The subtext of each piece was that those on the other side are flaming idiots: Schank makes parenthetical comments that question Weller and Weller’s motives, where Weller, Weller and Graham imply Schank is intentionally obtuse. It’s rollicking good fun!

Schank’s main criticisms are that many researchers who study synchrony are examining a synchrony score (where the closer the cycles are the smaller the score). A low synchrony score, then, is not actual synchrony. Further, Schank points out that synchrony scores increase over time in every paper he reviews, meaning that women become less synchronous over time in all existing study populations.

In both of Schank’s papers and his letter (Schank 2000; Schank 2001; Schank 2002), as well as Strassmann’s piece (1999) and others by Wilson (Wilson 1992; Wilson et al. 1991), the authors point out additional methodological statistical biases. Those who perform menstrual synchrony research fail to control for recall biases or random synchrony, and they inflate the initial difference in cycle onset, which creates a stronger appearance of later synchronization. Further, Strassmann points out that those who do control for these issues find no evidence for synchrony (Strassmann 1999). Finally, most critiques of synchrony research also point out that synchrony is pretty hard to come by when natural menstrual cycles are highly variable – which they are.

Weller and Weller and Graham each address these issues. Graham claims Schank is overstating the problem – in fact, she claims his statements are “inappropriate and misleading” – because his particular gripe about recall bias isn’t in the literature (Graham 2002). That is, Schank claims most synchrony studies are based on recall data, which means researchers ask women about when their last periods are, where Graham claims most of these studies are prospective, which means they do longitudinal assessments that actually ask women over time when their periods are. The answer appears to be somewhere in between, where even largely prospective studies rely on one, sometimes two previous menstrual cycle dates to create their synchrony scores.

The other Graham-Schank beef appears to center around Schank finding fault with the use of menstrual cycle calendars, where Graham contends they have been validated and used in many areas of research. Again, this is a place where the answer is somewhere in the middle. Menstrual cycle calendars are surely imperfect, but likely not enough so to warrant the heavy criticism Schank lodges at them.

Instead, frankly, I find the absolute lack of synchrony evidence in non-human primates as well as in well-controlled human studies pretty darn compelling.

Personal intuition

What about all that great anecdata, like what I describe in my opening story? Strassmann is the one who actually states the issue most accessibly:

“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” (Strassmann 1999: 579).

Um, yeah. So. The fact that menses overlapped between Julie and I? Maybe that would lead to a low synchrony score, for that one cycle. But it is far more likely that this overlap – not synchrony, just overlap – was a result of random chance. This doesn’t reduce the bond I had with my dear high school friend. It just means there wasn’t any substance in her sweat telling me when to ovulate.

References

Fürtbauer I, Mundry R, Heistermann M, Schülke O, and Ostner J. 2011. You Mate, I Mate: Macaque Females Synchronize Sex not Cycles. PLoS One 6(10):e26144.

Graham CA. 2002. Methods for obtaining menstrual-cycle data in menstrual-synchrony studies: Commentary on Schank (2001).

Hays WST. 2003. Human pheromones: have they been demonstrated? Behavioral ecology and sociobiology 54(2):89-97.

Schank J. 2000. Menstrual-cycle variability and measurement: further cause for doubt. Psychoneuroendocrinology 25(8):837-847.

Schank JC. 2001. Menstrual-cycle synchrony: Problems and new directions for research. Journal of comparative psychology 115(1):3.

Schank JC. 2002. A multitude of errors in menstrual-synchrony research: Replies to Weller and Weller (2002) and Graham (2002).

Setchell JM, Kendal J, and Tyniec P. 2011. Do non-human primates synchronise their menstrual cycles? A test in mandrills. Psychoneuroendocrinology 36(1):51-59.

Strassmann BI. 1997. The biology of menstruation in Homo sapiens: Total lifetime menses, fecundity, and nonsynchrony in a natural-fertility population. Current Anthropology 38(1):123-129.

Strassmann BI. 1999. Menstrual synchrony pheromones: cause for doubt. Human Reproduction 14(3):579-580.

Wilson HC. 1992. A critical review of menstrual synchrony research. Psychoneuroendocrinology 17(6):565-591.

Wilson HC, Kiefhaber SH, and Gravel V. 1991. Two studies of menstrual synchrony: negative results. Psychoneuroendocrinology 16(4):353-359.