Menstrual Synchrony - Jazlyn Kendall

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Oct 30, 2023

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Menstrual Synchrony Literature Shows That a Healthy Scepticism is Needed It has been a widespread myth that women who spend extended lengths of time in proximity will align the onset of their menstrual cycles (MC). After more than 50 years of research, menstrual synchrony (MS) remains a mystery unsolved. In 1971 Martha McClintock reported evidence of MS in women living in college dormitories and hypothesized that pheromones influenced these changes in MC. Aiming to replicate these results in varying groups of women, researchers focused on roommates in a same-sex or mixed-sex college (Graham & McGrew, 1980; Wilson, 1992; Ziomkiewicz, 2006), lesbian couples (Weller & Weller, 1992), as well as a natural fertility community (Strassman, 1997). Since her findings there has been a significant amount of research claiming to have replicated her results (Cutler, 1987; Graham & McGrew, 1980; Law, 1986; Weller & Weller, 1992); however, some did not and contradicted the findings of MS were due to methodological artefacts (Strassman, 1997; Wilson, 1992; Yang & Schank, 2006; Ziomkiewicz, 2006). As more studies dispute McClintock’s hypothesis of pheromones being the contributing factor for MS, researchers began postulating on other influences, such as lunar cycles (Cutler, 1987; Law,1986; Strassman, 1997) and social affiliation (Graham & McGrew, 1980; Weller & Weller, 1992). In this review, scientific research over the past 50 years will be examined; evaluating the methodology and hypothesises surrounding MS, and evaluating why, after 50 years there is still no conclusive evidence of MS. Pheromones The influence of pheromones on MC synchronization was first postulated in 1971 by Martha McClintock, an undergraduate at Wellesley College at the time. Her findings were later published in Nature, a highly reputable journal, and have been cited more than 1200 times. She observed 135 female students living in campus dormitories at a same-sex college. The woman aged 17-22 were sorted into pairs of close friends and roommates, and into groups of friends of 5-10 women. All participants were asked the same set of questions: the date of onset for their last MC and second to last MC, whom they affiliated with the most from their dormitory, and how often they interacted with males. The results from McClintock’s (1971) study showed significant MS, between the close friends and roommates’ groups, however, there was no synchronization when participants were randomly paired. Interestingly, one participant claimed to have a MC lasting more than 6 months with no male interaction, once

she resumed contact with males her MC shortened to an average of 4.5 weeks. As male interaction ceased again, her MC lengthen, this finding gave rise to McClintock’s hypothesis that pheromones influence MS. Using the Wilcoxon Matched-pair signed-ranks test, McClintock tested for changes in MC onset dates for all participants. Her Methodology was undermined by Wilson (1992) and Yang & Schank (2006), who declared that hers and other studies (Graham and McGrew, 1980; Weller and Weller, 1992) had methodological errors that were biased toward MS. These errors included: Too short of an observation time; incorrect calculations of MS onset differences and the assumption that MC length is the same for all women. It is worth mentioning that McClintock had a controversial inclusive criterion. Women who were taking birth control pills; which was later noted as a possible confounding variable (Wilson, 1992; Weller & Weller, 1992) were included as the effects of their MC on another woman were still unknown. Surprisingly there were no exclusion criteria noted, so women who may have had any reproductive conditions which would alter their MC length were still included; again, this could have led to results biased towards MS. Almost 10 years after McClintock’s study, Weller and Weller (1992) aimed to replicate her findings by observing 20 lesbian couples. They hypothesized that due to high female interaction in lesbian couples if pheromones or social affiliation were influencing MS it would be pronounced to a greater extent in this group. They aimed to utilize a population that had low male interaction, as it had been identified to increase ovulation, changing MC length (Weller & Weller, 1992); and a population that doesn’t use oral contraceptives as previously stated as a possible confound (Wilson, 1992; Weller & Weller, 1992). Contrary to McClintock’s study, Weller and Weller did not observe participants over a length of time, instead giving them a questionnaire on their social affiliation and menstruation. They calculated the degree of synchrony from their answers on the basis that all women had a MC length of 28 days. The results showed the greatest synchrony of other studies (Cutler, 1987; Law, 1986; Graham & McGrew, 1980), which they predicted. However, their methodologies appeared biased toward MS, as noted previously (Wilson, 1992; Yang & Schank, 2006) by their assumption that all women have the same MC length and that their study had no observational time to achieve accurate results. It is also worth noting the limitation of this study, is the possibility of inaccurate reporting on MS onset, as it cannot be expected for all participants to accurately recall their Menstruation dates. Studying the reproductive profile of women from a natural fertility community, Dogon, from West Africa in 1997; Strassman evaluated her data for menstrual synchrony. The Dogon population segregate menstruating women from the rest of their village for the

duration of their MC, which made observational studies of MC highly accurate. She also utilised a population without oral contraceptives, which has been noted as a possible hormonal confound (Weller & Weller, 1992; Wilson, 1992). Strassman noted no MS in her findings, despite adjusting her methodology to fix the methodological errors noted (Wilson,1992) in McClintock’s study. Although this study was undertaken over 2 and a half years, women from natural fertility populations often spend their reproductive lives pregnant, and therefore the median number of MC for those two years was 6.0, which is significantly less than western populations. Possible implications that may have swayed these results are that MC can be affected by Lactation (Wilson, 1992), male interaction (Weller and Weller, 1992), and reproductive age (defined as the difference between their actual age and onset of puberty) (Strassman, 1997). A more recent study to highlight is from Yang & Schank (2006), who collected data over 460 days from 186 students living in a college dormitory. Adapting Wilson’s revised methodologies for calculating onset differences, they implemented a technique to detect clustering of cycle onset; the Kuiper-Stephens non-parametric test for deviation. When analysing the data, they used a program to randomly shuffle the order of women’s MC onset and analysed this data using the Kuiper-Stephens test. No MC onset clusterings remained constant, and as such, no MS occurred. Yang & Schank also revised McClintock’s original study adopting their revised methodology to her data. Using their revised methodology the results coincided with Wilson’s findings, that MS is a methodological artefact, and that McClintock’s method for testing MC onset is biased towards MS. Lunar Cycles When researchers investigated Lunar cycles influencing MS, studies from Cutler (1987) & Law (1986) both agree MS is influenced by lunar cycles, however, both studies conflict as Cutler claims MS coincides with a full moon cycle whereas Law argues at the new moon cycle. In 1987, Cutler observed a cohort of 312 women, for 14 weeks; of that 229 women were noted to menstruate at or near a full moon. It is worth recognising that their exclusion criteria were suitable for their clinical trial design. Women who used contraceptives (oral or IUD) were excluded from this study due to possible variable confound, as well as roommates; as previous research (Graham & McGrew, 1980; Law, 1986; McClintock, 1971) claimed that

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roommates are likely to synchronize their MC, therefore may be a possible confound to a true representation of lunar cycle influences. Conflicting Cutler’s results, Law observed 28.3% of participants’ MC onset aligned with a new moon cycle. Law observed 826 females over a time frame of “four lunar months in different seasons”. Although this cohort is of a considerable size, statistically, we can expect a percentage (3.5%) of women to initiate the onset of their MC each day in a 28-day lunar cycle. With the assumption that approximately 3.5% of participants will begin their MC each day, we can see significant MS (28.3%) on the day of a new moon cycle. In Strassman’s study of natural fertility populations, as previously stated, she hypothesized that if MS is influenced by the lunar cycle then it would occur village-wide. Due to this population having no electrical lighting and spending their nights outdoors, they were considered an ideal population to observe (Strassman,1997). She implemented new methodologies in her study using Cox’s model; which is used to determine the probability of an event happening at a particular time; in this case whether menstruation onset occurs at a specific lunar cycle. Although the Dogan population were ideal to observe lunar cycle influences, in this study it was not observed, and as such scrutinizes the hypothesis that lunar cycles influence MS (Cutler, 1987 & Law, 1986). Strassman also emphasized that the failure to observe MS in such a population seriously weakens the conjecture of MS, as even now there is no conclusive evidence of MS mechanisms. Although Cutler and Law both observed MS, and claimed lunar cycles being the underlying mechanism involved, neither author agreed on which lunar phase this occurs. Strassman’s observation of villages in a West African population with higher exposure to the moon than other western population studies undermines this hypothesis as no MS was observed at all. Social Affiliation As researchers follow up on McClintock’s observations of an association between time spent together and MS, they pioneer an enthralling hypothesis for sociobiologists as they are given a glimpse of a possible imbricate between female and evolutionary psychology (Fahs, 2016). In 1979, Graham and McGrew, bring forth one of the first observational studies introducing the hypothesis that MS is influenced by social affiliation. Once considered one of the most likely hypotheses to explain the underlying mechanism behind MS (Weller and Weller, 1992;

Graham & McGrew, 1979), it is, however, considered not so likely today (Ziomkiewicz, 2006). One of the first studies to replicate McClintock’s study was published by Graham and McGrew in 1979. The study focussed on 79 female college students, aged 17-21, living in campus dormitories at a mixed-sex college. With the aim to examine how social interaction affects menstruation, the study looked for a relationship between cycle length and duration of menstruation, as well as male interaction. Limitations of this study are similar to other limitations mentioned: a small cohort, the study was for only 4 months, required participants to document data themselves, and included females on an oral contraceptive. In September of that year, they collected the data from their participants and organised each person into groups of close friends, neighbours, and random pairs (Graham & McGrew, 1979). Surprisingly they found no significant correlation between male exposure, neighbours or random pairs, and menstruation variation. They did, however, note MS in close friends, which suggests that MS is due to social affiliation and not pheromonal; if it was then it would be expected that MS would have occurred throughout the dormitory (Graham & McGrew, 1979). In Weller and Weller’s previously mentioned study they also explored social affiliation contributing to MS. Exploring the socio-biology of lesbian couples, they questioned the level of friendship as well as the amount of time spent together. Although the limitations of this study have been previously acknowledged; regarding social affiliation, Weller and Weller have included various pertinent questions which they examined as a whole, unlike previous studies. Results coincide with their hypothesis that social affiliation affects MS, with significant synchrony seen in mutual activities, a feeling of closeness and stressful life events (Weller and Weller, 1992). A more recent study from 1998, revised in 2006, by Anna Ziomkiewicz, investigated 99 women from a mixed-sex college in Poland. With reference to McClintock’s arguably biased methodology (Strassman, 1997; Wilson, 1992; Yang & Schank, 2006) Ziomkiewicz adopted Wilson’s (1992) method to calculate the initial onset differences in pairs of women, and as such, no MS was found. Interestingly at the time of this study the known effects oral contraceptives have on menstrual cycles were better known (Ziomkiewicz, 2006), and as such the 10 participants taking oral contraceptives were included and results were calculated with and without their data. Despite knowledge of oral contraceptives as a potential hormonal confound were noted, interestingly there were no significant differences between users and non-users of oral contraceptives. Ziomkiewicz also appeals to the idea that cycle variations

between individuals can give the perception of MS. Once seen as an enthralling hypothesis for socio-biologists, social affiliation being the mechanism for MS is now more frequently disputed as cycle variation. Although the idea that cycle variation gives the perception of MS proves to be a promising conclusion to this mystery, it cannot disregard the existence of MS yet. It appears scepticism of MS is crucial still, as the abundance of scientific literature still shows no conclusive evidence to prove nor disprove this theory. Since McClintock’s first study, there has been a startling rate of researchers who argue not only the mechanism behind MS, and whom they synchronize with but also MS in its entirety. Fellow researchers were inspired to replicate her results, however, it isn’t until 1992 that Wilson scrutinizes McClintock’s study, as biased toward MS. Thus began a new wave of studies, implementing and adapting Wilson’s Methodology into their own MS studies, and affirming McClintock’s results as null. In fact, most hypothesized mechanisms of MS have been scrutinised and considered unrealistic as more recent research identifies that cycle variations can be perceived as MS. Even though there has been an abundance of literature having observed menstrual synchrony, there is also an abundance claiming not. So, whilst it seems no one can agree on the existence of MS, it can be agreed that MS remains a mystery, even today.

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References Cutler, W. B., Schleidt, W. M., Friedmann, E., Preti, G., & Stine, R. (1987). Lunar influences on the reproductive cycle in women. Human Biology , 59 (6), 959–972. Fahs, B. (2016). Demystifying menstrual synchrony: women's subjective beliefs about bleeding in tandem with other women. Women's Reproductive Health , 3:1, 1-15. DOI: 10.1080/23293691.2016.1150132 Graham, C. A., & McGrew, W. C. (1980). Menstrual synchrony in female undergraduates living on a coeducational campus. Psychoneuroendocrinology , 5 (3), 245–252. https://doi.org/10.1016/0306-4530(80)90028-1 Law S. P. (1986). The regulation of menstrual cycle and its relationship to the moon. Acta Obstetricia et Gynecologica Scandinavica , 65 (1), 45–48. https://doi.org/10.3109/00016348609158228 McClintock, M. K. (1971). Menstrual synchrony and suppression. Nature , 229 (5282), 244– 245. https://doi.org/10.1038/229244a0 Strassmann, B. I. (1997). The biology of menstruation in homo sapiens: total lifetime menses, fecundity, and nonsynchrony in a natural-fertility population. Current Anthropology, 38 , 123 - 129. Weller, A., & Weller, L. (1992). Menstrual synchrony in female couples. Psychoneuroendocrinology , 17 (2-3), 171–7. Wilson H. C. (1992). A critical review of menstrual synchrony research. Psychoneuroendocrinology , 17 (6), 565–591. https://doi.org/10.1016/0306- 4530(92)90016-z Yang, Z., & Schank, J. C. (2006). Women do not synchronize their menstrual cycles. Human Nature: An Interdisciplinary Biosocial Perspective , 17 (4), 433–447. https://doi.org/10.1007/s12110-006-1005-z Ziomkiewicz, A. (2006). Menstrual synchrony: fact or artifact? Human Nature: An Interdisciplinary Biosocial Perspective , 17 (4), 419–432. https://doi.org/10.1007/s12110-006-1004-0