Menstrual synchrony, also called the McClintock effect, or the Wellesley effect, is a hypothetical 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.
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.
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, have been performed. Menstrual synchrony was not found in some studies such as those done on a natural-fertility population, and college students living together like the original McClintock study
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). 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
thumb|upright|Two women dancing. Rock engraving from the Upper Yule River, [[Pilbara, Western Australia.]]
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.
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...".
Scientific details
The phenomenon of menstrual synchrony is the closeness in time of the menstrual cycle onsets of two or more women. 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
thumb|right|325px|Group synchrony scores over time with 99% [[confidence intervals from McClintock's study (an approximation of Fig 1). they refined their approach to measuring to better handle the problem of cycle variability. Specifically, they concluded that several menstrual cycles should be measured from each woman and that the longest average cycle length in a pair or group of women should be the basis for calculating the expected cycle onset difference. In analyzing whether menstrual synchrony occurs among Dogon women, she was aware of Wilson's and Law hypothesized that menstrual synchrony is caused by menstrual cycles synchronizing with lunar phases. However, neither of them agree on what phase of the lunar cycle menstrual cycles synchronize with. Cutler hypothesizes the synchronization with the full moon More recently, Strassmann investigated menstrual synchrony among Dogon village women. The women were outdoors most nights and did not have electrical lighting. She hypothesized that Dogon women would be ideal for detecting a lunar influence on menstrual cycles, but she found no relationship. They investigated whether a coupled-oscillator mechanism first reported for Norway rats (see section below on rats) could also exist in humans. The coupled-oscillator hypothesis in humans proposed that human females release and receive pheromones that regulate the length of their menstrual cycles. This was assumed to occur without consciously detecting any odor. The study was conducted by collecting compounds from axillae (underarms) of donor women at prescribed phases during their menstrual cycles (i.e., the follicular phase, ovulatory phase, and luteal phase), and applying the compounds daily under the noses of recipient women. In order to collect the axillary compounds, the donor women wore cotton pads under their arms for at least 8 hours, and then the pads were cut into smaller squares, frozen to preserve the scent, and readied for distribution to the recipients. The recipients were split into two groups, and were exposed to the compounds via application of the thawed axillary pad under their noses daily. Whitten's main critiques was with their using only their first cycles as a control for the subsequent conditions. He argued that this eliminate all within-subject variance. Control conditions should have been run between each experimental condition and not just at the beginning of the study. He was also skeptical about whether the coupled-oscillator model from rat research Their study consisted of 122 women (students and staff) at Leeds University. A four-page questionnaire was sent to each participant. After providing personal details, they were given a description of menstrual synchrony: "Menstrual synchrony occurs when two or more women, who spend time with each other, have their periods at approximately the same time" (p. 257). pointed out that menstrual synchrony can occur by chance when there is menstrual cycle variability. Yang and Schank), then the experience of synchrony may last a number of months. —is 7 days. Considering that the mean duration of menses is 5 days The basic principle is that the more females are fertile at any one time, the harder it is for any single male to monopolize access to them, impregnating all simultaneously at the expense of rival males. In the case of nonhuman primates, once the number of co-cycling females rises above a critical threshold, a harem-holder may be unable to prevent other males from invading and mating with his females. A dominant male can maintain his monopoly only if his females stagger their fertile periods, so that he can impregnate them one at a time (see figure a, right). Suppose a group of female baboons need between them just one dominant male, desirable in view of his high-quality genes. Then, logically, they should avoid synchronizing their cycles. By the same token, if males during the course of human evolution became valued by females for additional purposes – hunting and bringing home food, for example – then females should resist being controlled by dominant male harem-holders. If males are useful partners to have and keep around, then ideally each female should have at least one for herself. Under those circumstances, according to this argument, the logical strategy would be for females to synchronize as tightly as they can (see figure b, right).
One implication is that there may be a link between the degree of synchrony in a population (whether seasonal, lunar or both), and the degree of reproductive egalitarianism among males. Foley and Fitzgerald objected to the idea that synchrony could have been a factor in human evolution on the grounds that for hominins with inter-birth intervals of 3–5 years, achieving synchrony was unrealistic. Infant mortality would disrupt synchrony since it would be too costly for a mother who had miscarried or lost her baby to wait until everyone else had weaned their babies and resumed cycling before having sex and getting pregnant herself. On the other hand, while conceding that it would be impossible to get clockwork synchrony throughout an inter-birth interval, Power et al. argued that once we take account of birth seasonality – enhancing the effects of menstrual synchrony by clumping fertile cycles within a relatively brief time-window – it emerges that reproductive synchrony can be effective as a female strategy to undermine primate-style sexual monopolization by dominant males. The controversy remains unresolved.
Adopting a compromise position, one school of Darwinian thought sets out from the fact that the mean length of the human menstrual cycle is 29.3 days, which is strikingly close to the 29.5 day periodicity of the moon. It is suggested that the human female may once have had adaptive reasons for evolving such a cycle length – implying some theoretical potential for synchrony to a lunar clock – but did so in an African setting under prehistoric conditions which today no longer exist. Not all archaeologists accept that lunar periodicity was ever relevant to human evolution. On the other hand, according to Curtis Marean (head of excavations at the important Middle Stone Age site of Pinnacle Point, South Africa), anatomically modern humans around 165,000 years ago – when inland regions of the continent were dry, arid and uninhabitable – became restricted to small populations clustered around coastal refugia, reliant on marine resources including shellfish whose safe harvesting at spring low tides presupposed careful tracking of lunar phase.