Idea that the menstural cycle synchronizes with those of other women in close proximity
Menstrual synchrony, also called the McClintock effect, or the Wellesley effect, is a contested process whereby women who begin living together in close proximity would experience their menstrual cycle onsets (the onset of menstruation or menses) becoming more synchronized together in time than when previously living apart. "For example, the distribution of onsets of seven female lifeguards was scattered at the beginning of the summer, but after 3 months spent together, the onset of all seven cycles fell within a 4-day period."
Martha McClintock's 1971 paper, published in "Nature", says that menstrual cycle synchronization happens when the menstrual cycle onsets of two or more women become closer together in time than they were several months earlier.
After the initial studies, several papers were published reporting methodological flaws in studies reporting menstrual synchrony, including McClintock's study. In addition, other studies were published that failed to find synchrony. The proposed mechanisms have also received scientific criticism. Reviews in 2006 and 2013 concluded that menstrual synchrony likely does not exist.
Overview.
Original study by Martha McClintock.
Martha McClintock published the first study on menstrual synchrony among women living together in dormitories at Wellesley College, a women's liberal arts college in Massachusetts, US.
Proposed causes.
McClintock hypothesized that pheromones could cause menstrual cycle synchronization. However, other mechanisms have been proposed, most prominently synchronization with lunar phases.
Efforts to replicate McClintock's results.
No scientific evidence supports the lunar hypothesis, and doubt has been cast on pheromone mechanisms.
After the initial studies reporting menstrual synchrony began to appear in the scientific literature, other researchers began reporting the failure to find menstrual synchrony.
These studies were followed by critiques of the methods used in early studies, which argued that biases in the methods used produced menstrual synchrony as an artifact.
More recent studies, which took into account some of these methodological criticisms, failed to find menstrual synchrony.
Terminology.
The term "synchrony" has been argued to be misleading because no study has ever found that menstrual cycles become strictly concordant, nevertheless "menstrual synchrony" is used to refer the phenomenon of menstrual cycle onsets becoming closer to each other over time.
Status of the hypothesis.
In a 2013 systematic review of menstrual synchrony, Harris and Vitzthum concluded, "In light of the lack of empirical evidence for MS [menstrual synchrony] "sensu stricto", it seems there should be more widespread doubt than acceptance of this hypothesis" (pp. 238–239).
The experience of synchrony may be the result of the mathematical fact that menstrual cycles of different frequencies repeatedly converge and diverge over time and not due to a process of synchronization, and the probability of encountering such overlaps by chance is high.
Evolutionary perspective.
Researchers are divided on whether menstrual synchrony would be adaptive. McClintock has suggested that menstrual synchrony may not be adaptive but rather epiphenomenal, lacking any biological function. Among those who postulate an adaptive function, one argument is that menstrual synchrony is only a particular aspect of the much more general phenomenon of reproductive synchrony, an occurrence familiar to ecologists studying animal populations in the wild. Whether seasonal, tidal, or lunar, reproductive synchrony is a relatively common mechanism through which co-cycling females can increase the number of males included in the local breeding system.
Conversely, it has been argued that if there are too many females cycling together, they would be competing for the highest quality males, forcing female–female competition for high quality mates and thereby lowering fitness. In such cases, selection should favor avoiding synchrony. Divergent climate regimes differentiating Neanderthal reproductive strategies from those of modern "Homo sapiens" have recently been analysed in these terms.
Turning to the evolutionary past, a possible adaptive basis for the biological capacity would be reproductive levelling: among primates, synchronising to any natural clock makes it difficult for an alpha male to monopolise fertile sex with multiple females. This would be consistent with the striking gender egalitarianism of extant non-storage hunter-gatherer societies. When early Pleistocene hominids in Africa were attempting to survive by robbing big cats of their kills, according to some evolutionary scientists, it may have been adaptive to restrict overnight journeys—including sexual liaisons—to times when there was a moon in the sky.
Media attention.
The question of whether those who live together do in fact synchronize their menstrual cycles has also received attention in the popular media.
Traditional myth and ritual.
The idea that menstruation is – or ideally ought to be – in harmony with wider cosmic rhythms is one of the most tenacious ideas central to the myths and rituals of traditional communities across the world.
 ;templatestyles src="Template:Blockquote/styles.css" / ;String was first made by the two Wawalik Sisters at Mudawa, near Buckingham Bay... The sisters sat down, looking at each other, with their feet out and legs apart, and both menstruated... Each one made a loop of the other one's menstrual blood, after which they put the string loops around their necks.
The !Kung (or Ju|'hoansi) hunter-gatherers of the Kalahari "believe ... that if a woman sees traces of menstrual blood on another woman's leg or even is told that another woman has started her period, she will begin menstruating as well". Among the Yurok people of northwestern California, according to one ethnographic study, "all of a household's fertile women who were not pregnant menstruated at the same time...".
 ;templatestyles src="Template:Blockquote/styles.css" / ;She said that she had been instructed in the menstrual laws by her maternal aunts and grandmother, who in their times were well-known, conservative Yurok women... the young woman said that in old-time village life all of a household's fertile women who were not pregnant menstruated at the same time, a time dictated by the moon; that these women practised bathing rituals together at this time... If a woman got out of synchronization with the moon and with the other women of the household, she could 'get back in by sitting in the moonlight and talking to the moon, asking it to balance her'.
Scientific details.
The phenomenon of menstrual synchrony is the closeness in time of the menstrual cycle onsets of two or more women. The phenomenon is not synchronization in the strict sense of concordance of menstrual cycle onsets but the term "menstrual synchrony" is still used perhaps misleadingly. As an undergraduate, Martha McClintock published the first study on menstrual synchrony; her report detailed the menstrual synchrony of undergraduate women living in a dormitory in Wellesley College. Since then, there have been attempts to replicate her findings and to determine the conditions under which synchrony occurs, if it exists. Her work was followed up by studies reporting menstrual synchrony and by other studies that failed to find synchrony.
Thus, a number of studies were published from the 1980s to the mid 2000s, which attempted to replicate menstrual synchrony in college women, determine the conditions under which menstrual synchrony occurred, and to address methodological issues that were raised as these studies were published. The rest of this section discusses these studies in chronological order, briefly presenting their findings and main conclusions grouped by decade followed by general methodological issues in menstrual synchrony research.
Studies.
1970s.
McClintock's study consisted of 135 female college students who were 17 to 22 years old at the time of the study. They were all residents of a single dormitory, which had four main corridors. The women were asked when their last and second to last menstrual period had started three times during the academic year (which ranged from September to April). They also were asked who (other women in the dormitory) they associated with most and how often each week they associated with males. From these data, McClintock placed women into pairs of close friends and roommates and she also placed them into groups of friends ranging in size from 5 to 10 women. She reported statistically significant synchrony for both her pairwise sorting of women and her group sorting of women. That is, whether women were placed into pairs of close friends and roommates or whether they were placed into larger groups of friends, she reported that they synchronized their menstrual cycles. She also reported that the more often women associated with males, the shorter their menstrual cycles were. She speculated that this may be a pheromone effect paralleling the Whitten effect in mice but that it could not explain menstrual synchrony among women. Finally, she speculated that there could be a pheromone mechanism of menstrual synchrony similar to the Lee-Boot effect in mice.
1980s.
Graham and McGrew were the first researchers to attempt to replicate McClintock's study. There were 79 women living in halls of residence or apartments on the campus of a college in Scotland. The women were 17 to 21 years old at the time of the study and the procedures followed were similar to those used in McClintock's study. She partially replicated McClintock's study reporting that close friends but not neighbors synchronized their cycles. Unlike in McClintock's study, close friends did not synchronize in groups. They considered a pheromone mechanism a possible explanation of synchrony, but noted that if pheromones were the cause, neighbors should have synchronized as well. They concluded that the mechanism of synchrony remains unknown, but emotional attachment may play a role.
Quadagno et al. conducted the second replication of McClintock's study. There were 85 women living in dormitories, sorority houses, and apartments who attended a large midwestern university in the United States. Their study used methods similar to McClintock's study except in addition to two women living together, there were also groups of three and four women living together. They reported that the women synchronized their menstrual cycles and concluded that pheromones may have played a role in synchronization.
Jarett's study was the third to attempt to replicate McClintock's original study using college roommates. There were 144 women who attended two colleges. The women were 17 to 22 years old and the procedures followed were similar to McClintock's study except only pairs of roommates were used. She reported that the women did not synchronize. Jarett concluded that whether menstrual synchrony occurs in a group of women may depend on the variability of their menstrual cycles. She conjectured that the reason the women in her study did not synchronize their menstrual cycles was because they happened to have longer and more irregular menstrual cycles than in McClintock's original study.
1990s.
Wilson, Kiefhabe, and Gravel conducted two studies with college women. The first study consisted of 132 women who were members of a sorority or roommates of members at the University of Missouri. The women were 18 to 22 years of age and the study aimed to replicate McClintock's original study. However, instead of asking women to recall when their last and next to last menstrual onsets occurred, one of the researchers visited the sorority daily to record the occurrence of menstrual onsets and to collect other biographical data. The second study consisted of 24 women living in a cooperative house near the University of Missouri. The women were 18 to 31 years of age. One of the researchers visited the house three times a week recording menstrual onset and collecting more extensive biographical and psychological test data than in the first study. They found no menstrual synchrony in either study. They considered the possibility that women with irregular cycles may reduce the likelihood of detecting synchrony, so they re-analyzed their data after they removed women with irregular cycles, but again there was no statistically significant effect of synchrony. They concluded that "It is clear no meaningful process of selection or exclusion of pairs can produce a significant level of menstrual synchrony in our samples... Therefore, whether or not menstrual synchrony occurs among women who spend time together must remain a hypothesis requiring further investigation" (p. 358).
Weller and Weller conducted a study with 20 lesbian couples. They hypothesized that contact within couples should be maximal and contact with men minimal compared to previous studies, which should maximize the likelihood of detecting synchrony. The women ranged in age between 19 and 34 years of age. This was the first study that did not explicitly use college women, but instead the women were recruited at a bar by a research assistant who was a proprietor of a bar. Unlike previous studies, they only asked the women for the date of their last menstrual onset. They then assumed that all the women had menstrual cycles that were exactly 28 days long. Based on this assumption and one menstrual onset for each woman