Mayday, Mayday, Are We Syncing?

Period-synchronization: Fact or fiction?

There is something funny, maybe even enjoyable, about the camaraderie many people seem to feel with their “period-buddies”. After all, we all know the old cliché: misery loves company.

Things get a little wacky, though, when you start suggesting that shared cycle times among friends is a result of their closeness, rather than just an amusing coincidence. That this the fundamental idea of period-syncing, or what is more formally referred to as “menstrual synchrony”.

If you have ever lived in a group of people who menstraute, you have probably heard jokes about how the timing of their periods will line up, with some people even believing that there is a social hierarchy element, where an “alpha” female can induce others to join her cycle timing.

It sounds pretty cool, actually, as if this is a biological mechanism of solidarity, something induced by social interaction that stands as a strange, yet scientifically objective record of friendship.


But where did this idea come from, and is it actually backed by empirical evidence?

I decided to do a little sleuthing, and this is what I found out.

The concept of menstrual synchrony was first introduced into scientific literature in 1971, in a study of college-aged women by McClintock. In the published paper, it was noted that menstrual synchrony was already something reported informally among groups of women, and that previous research with mice had found it possible for social factors, including pheromones and crowding, to affect the endocrine system and hormone secretion (McClintock 1971). With this foundation, the study set out to see if period-syncing was measurable on a large scale. 135 women in a college dorm were asked to report the dates of their previous two periods 3 times over the course of a school year, and data designed to determine how close the individuals were to each other socially was obtained by asking them to report lists of  ten women they considered to be their closest friend. They found that there was statistically significant evidence of synchrony between friends; specifically, period onset dates got about 2 days closer over the course of 4-6 months for pairs of friends.

This sparked more interest into the subject. If period-syncing was happening, how did it work? And what evolutionary purpose could it possibly serve?

One proposal suggested that a mechanism for syncing might be related to human pheromone secretion. Pheromones are chemicals that the body secretes that act like signals, eliciting some behavioral response in the other individuals of the same species that receive it. This is easily observable in some organisms, such as termites, who leave and follow path/food-finding pheromones to help each other navigate their complex underground tunnels. There isn’t much known about the role of pheromones in humans, however. Another study by McClintock did find  evidence that compounds taken from the armpits of women could influence menstrual cycle lengths (Stern and McClintock 1998), and these quickly became the most popular hypothesis as to how intrapersonal interactions could leads to synchrony. There were other proposals, of course, including the idea that direct exchange of chemical compounds between women (via sweat, physical contact, the swapping of clothes, saliva, and other mean) could be responsible, and research has been conducted between many different groups to test these ideas, including investigations that predicted athletes and lesbian couples would be more strongly in synch because of the likelihood of increased physical contact (Graham 1991). It was also suggested that the physiological effects of different emotions, like heart rate, blood pressure, and hormonal secretion levels, could be involved, perhaps making the person more sensitive to olfactory signals (Graham 1991). Many of these are speculative explanations, though, and an exact pathway hasn’t yet been identified.

Lets assume for a moment, though, that there is a working mechanism driving the synching of periods between companions. What biological function would that serve? Is there any reason why it might be advantageous for this part of the reproductive cycle to occur simultaneously between females in close proximity?

One suggestion was that it evolved because it increases the chances of conception for females living together with a single male, maybe by increasing the total pheromonal signal that could motivate mating in the  for the male (Ziomkiewicz 2006). Another proposal said that it may aid in the post-pregnancy child-rearing; if many women were lactating at the same time it could ensure that there would be someone able to care for the babies if a mother died. As Ziomkiewicz points out in a 2006 review article, however, all of these theories assume that period synchrony would correlate directly to ovulation or fertility synchrony, which isn’t actually something we know to be true (Ziomkiewicz 2006). In fact,  a review of McClintock’s 1971 data, where the period outset timing only changed by a few days over the course of months, found that this change would have no significant impact on conception timing at all (Strassmann 1997).

Without an impact on fertility timing, it seems as though period-synching would be… kinda pointless.

Speaking of that original data from McClintock (1971), let’s re-examine research supporting the idea that period-syncing occurs. That original study has actually come under a great deal of criticism and scrutiny by follow-up studies. First of all, many scientists have renounced the paper’s application of statistical methods. Wilson (1992) found three major issues with this study, and subsequent papers on the topic, stating that the studies:

  • Didn’t account for the probability of period onset dates aligning by random chance
  • miscalculated the differences in initial menstrual onsets, in a way that lead “to an erroneous impression of synchronization over time” (Strassmann 1999)
  • excluded data of particular subjects, creating a sampling bias

In addition, others found discrepancies in the lists they used to rate the level of “closeness” between women, saying that overlaps between lists of roommates and friends resulted in the repetitive inclusion of some data, or “double-dipping” data (Yank and Schank, 2006).

With such substantial errors in methodology, the initial studies and its statistically follow-ups, are questionable at best.

So, back to square one: Does research show that period-syncing is real?

The answer seems to be no, not really. Other studies have found no statistically significant evidence of such a phenomenon.

One such study took data from 29 rooms of women living together in college dorms in China. There were 4-8 people per room, and data was gathered for more than 1 year (Yank and Schank, 2006). Using each room group as a unit of closely-interacting women, they analyzed the data for rooms that showed “statistical clusterings” of cycle onsets around similar dates. They found that only 9 of the 29 groups showed any clustered onset dates, and that this diverged again over time. According to their analysis, this is not greater than the expected convergence frequency you would find with a random variable. They concluded that there was “no onset convergence and no synchronization of cycles” in the course of their study (Yank and Schank, 2006).

Another study of college women, this time in Poland, used questions about the amount of time spent together (eating, sleeping, studying, exchanging clothes, spending free time, ect.) to create a indexed score of closeness/interaction between the 200 participants (Ziomkiewicz 2006).They found similarly inconclusive results, with no trend between time spent together and period-onset dates.

Yet another experimental approach decided to test a population of native Dogon women. The Dogon people are largely millet farmers in the Sahel of northern Africa, and presented a unique research opportunity as a predominately pre-industrialized society. Because the use of contraceptives or family planning is not common, it is what anthropologists call a “natural birth” population, a society where medicinal regulations of natural fertility cycles isn’t a factor (Strassmann 1997). Within this population, Strassmann (1997) selected 3 increasing selective subsets to measure the women’s level of social interaction, where the “village” group included all individuals in the village, “lineage” included those who lived with a line of related males, and “economic unit” referred to those who typically worked and ate together. Once again, no evidence of synchronization was found between any of the groups (Strassmann 1997).

A  large-scale study from the menstrual period tracking app Clue took a very different approach, examining user data, and also found no relationship between individuals’ cycle schedule and the time spent together (Gunther 2019).

And I could go on and on- Strassman’s 1997 review article lists at least three more studies with inconclusive results- Jarett 1984, Wilson, Hildebrandt Kiefhaber, and Gravel 1991, Trevathan, Burleson, and Gregory 1993 (via Strassmann 1997).

With such an abundance of research unable to find evidence of period-synching, and a corresponding dearth of well-received studies detecting it, I think it is safe to conclude that period-synching, at least on the scale the public accepts, simply isn’t true.

And yet, it is a remarkably common belief. According to one study (Arden et al 1998), over 80% of women believe that menstrual synchrony occurs. That isn’t surprising to me, given the years of casual bantering and bathroom chitchat I’ve heard about it. It is a staple in pop culture, and nearly every woman I know has observed it in their own experiences.

So if there isn’t actually something scientifically provable happening, why do we keep noticing it? It seems like a strange concept to fixate on.

There may be an simple explanation for that, though- people just don’t know how likely it actually is to have overlapping cycles. There seems to be a widespread idea that being “timed” together is a rare, or statistically abnormal event, when in fact, mathematical models have shown that it should be expected in half of the studied cases just by random chance (Wilson 1992, Ziomkiewicz 2006). As one researcher notes, if two women both have 30 day cycles (already improbable, due to the large intrapersonal cycle variability), the maximum number of days they could be apart is only 15, and they are expected to be an average of 7.5 days apart. This means that at least half the time, they will be closer together than that, and therefore pretty likely to overlap by a few days anyway (Strassmann 1997).

Add in the psychological appeal of “sharing” the experience, and it is probable that people are more likely to notice and remember instances of close period-onsets than the ones that aren’t near each other. The conformation bias, or our tendency to ignore evidence that doesn’t support  our ideas, is well known in psychology, and is definitely a potential player in contributing to the long-lasting effect of this myth (Fetters 2019).

It’s also important to note that cycles aren’t actually that consistent, between women and for an individual person. Cycle lengths differ significantly person-to-person, and it is mathematically impossible to synchronize rhythms with different frequencies (Strassman 1997, Schank 2000). Other factors including, such as physical health, stress, and lactation can also induce changes. As Traloar et al. (1967) succinctly put it, the menstrual cycle is “characterized by variability rather than by regularity”.

So, unfortunately, no, sharing the suffering with your roommate doesn’t prove your relationship or represent the amount of time you spent together. As cruel as it is to strip away what very well may be the only “fun” part of “that time of month”, it is important to investigate truths behind our collective beliefs about female reproductive health.

I think it is telling that period-syncing is still popularly accepted, after the original study came under so much fire and the continued research has continued to refute its existence. It seems to indicates a general dismissiveness towards sexual and reproductive health, that while pretty harmless and quirky in this case, is concerning. It is important to note that our misunderstandings have contributed to unfortunate consequences in the past. For example, the incorrect belief that menstrual fluid was partially developed embryo to which semen gave final form, and menstruation before marriage/pregnancy was therefore equated with  child-murder (Finn). This contributed to the practice of child brides to prevent the perceived loss of life in pre-marital menstruation.

There are many aspects of reproductive health that we still don’t understand much about,  but having mystery and stigma around menstruation doesn’t clarify anything.

That said, keep enjoying the solidarity of shared cycles- commiserating by chance is still something to bond over, if that’s your thing. Just remember to treat your reproductive health with scientific precision and respect rather than superstition.

Works Cited

Arden, M. A., and L. Dye (1998 ).The Assessment of Menstrual Synchrony: Comment on Weller and Weller (1997). Journal of Comparative Psychology 112:323-324.

Fetters, A. (2019, September 24). Why the Myth of Period Syncing Won’t Go Away. The  Atlantic. Retrieved from

Graham, C. A. (1991). Menstrual Synchrony: An Update and Review. Human Nature2(4), 293–311. doi: 10.1007/bf02692195

Gunter, J. (2019, June 6). The Myth of Period Syncing. New York Times. Retrieved from

Mcclintock, M. K. (1971). Menstrual Synchrony and Suppression. Nature229(5282), 244–245. doi: 10.1038/229244a0

Schank, J. C. (2000). Menstrual-Cycle Variability and Measurement: Further Cause for Doubt. Psychoneuroendocrinology 25:837-847.

Stem, K., and M. McClintock (1998). Regulation of Ovulation by Human Pheromones. Nature 392:177-179.

Strassmann, B. I. (1997). The Biology of Menstruation in Homo sapiens: Total Lifetime Menses, Fecundity, and Nonsynchrony in a Natural-FertilityPopulation. Current Anthropology 38:123-129.

Strassmann, B. I. (1999). Menstrual synchrony pheromones: cause for doubt. Human Reproduction14(3), 579–580. doi: 10.1093/humrep/14.3.579

Treloar, A.E., Boynton, R.E., Behn, B.G. et al. (1967) Variation of the human menstrual cycle through reproductive life. Int. J. Fertil., 12, 77–126.

Wilson, H. (1992). A critical review of menstrual synchrony research. Psychoneuroendocrinology17(6), 565–591. doi: 10.1016/0306-4530(92)90016-z

Yang, Z., & Schank, J. C. (2006). Women do not synchronize their menstrual cycles. Human Nature17(4), 433–447. doi: 10.1007/s12110-006-1005-z

Ziomkiewicz, A. (2006). Menstrual synchrony: Fact or artifact? Human Nature17(4), 419–432. doi: 10.1007/s12110-006-1004-0

Alex Gurgis
Alex is an undergraduate student at Georgia Tech passionate about learning how and why things work. She loves reading, writing, and traveling, and spends her spare time listening to podcasts, playing and composing music, and telling bad puns.