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The idea that women living or working in proximity to one another synchronize their menstrual cycles is particularly persistent, and has now become accepted as conventional wisdom. However, it's a relatively new idea — it has its origins in a survey conducted on a population of college students at an all-female college published in Nature in 1971 — and was based on a small population monitored only for about eight menstrual cycles. It found that there was increased synchronization between roommates and close friends: There were then, and are now, many biologically-informed explanations for such a phenomenon, referred to in the scientific community as socially mediated synchrony, discussed by University of Oxford anthropologist Alexandra Alvergne: The method by which humans could signal each other's cycle to nearby females, similarly, is not without broad biologic grounding, as laboratory experiments suggest that humans can unknowingly react to the pheromones of others in a way that can affect menstruation. The presence or absence of socially mediated synchrony in humans has been investigated, with varying degrees of scientific and quantitative rigor, in a variety of social contexts including roommates, lesbian couples, co-workers, close-friends, and various combinations of these social relationships, ever since. While some studies do demonstrate the presence of menstrual syncing, these small scale studies have been called into question. The literature provides far from conclusive evidence for the existence of menstrual synchrony at all, and increasingly researchers are finding methodological and theoretical flaws in the work with suggesting its existence. This was discussed in a 2006 study titled Women Do Not Synchronize Their Menstrual Cycles which, true to its title, found no evidence for synchrony: Another 2006 study attempted to replicate the results of the first 1971 study with less problematic statistical approaches, finding: From a mathematical perspective, most critiques of studies purporting to find positive results focus on the demonstrable likelihood that perceived synchrony happens by chance or by viewing data through a biased lens. A 1992 review argued that the studies demonstrating a menstrual syncing have used methods that increase the probability of finding menstrual synchrony in a sample. This study used a mathematical proof to demonstrate that the trends demonstrated in these studies are likely based on chance alone: A 1997 study utilized a comparably large amount of data gathered from women of the Dogon hunter-gatherer society in Mali, finding no evidence of synchrony. This population was selected because evolutionary explanations for synchrony would, many biologists argue, best be tested in a natural fertility population in which couples do not attempt to control their fertility in a parity-dependent [based on how many pregnancies a woman has had] fashion, as these were the conditions under which human reproductivity evolved: While later studies have also failed to empirically demonstrate it exists, the issue was brought back into the limelight when the period-tracking App Clue teamed up with Oxford anthropologist Alex Alvergne (cited earlier). On 9 March 2017, they described a pilot study on their website (which was not published as a peer-reviewed study) based on data collected in the app: Though not as statistically involved an analysis as some of the published studies, this report does make use of a large dataset and finds, again, no evidence to support synchrony in menstrual cycles. In a piece for The Conversation on the topic, written before the Clue post, Alvergne said: An additional aspect may be attributed, according to the author of the 1997 study, to human bias: We rate this claim as unproven due to an increasing lack of scientific evidence demonstrating that menstrual synchrony exists at all, but at the same time, we allow that the topic is complicated by myriad confounding variables.
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