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Menstrual synchrony
From Wikipedia, the free encyclopedia
Women's string-figure depicting "menstrual blood of three women", illustrating the Yolngu people's tribal mythology of menstrual synchrony [1]
Menstrual synchrony, also called the McClintock effect,[2] is the alleged process whereby women who begin living together in close proximity experience their menstrual cycle onsets (the onset of menstruation or menses) becoming closer together in time than previously. "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."[3]
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.[3] Several mechanisms have been hypothesized to cause synchronization.[4]
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. A 2013 review of menstrual synchrony concluded that menstrual synchrony is an erroneous theory.[4]
Contents [hide]
1 Overview
1.1 Original study by Martha McClintock
1.2 Proposed causes
1.3 Efforts to replicate McClintock's results
1.4 Terminology
1.5 Status of the theory
1.6 Evolutionary perspective
1.7 Myth and tradition
1.8 Media attention
1.9 Awareness and subjective experiences
1.10 Non-human species
2 Scientific details
2.1 Studies
2.1.1 1970s
2.1.2 1980s
2.1.3 1990s
2.1.4 2000s
2.1.5 Methodological issues
2.1.5.1 Initial onset differences
2.2 Hypothesized mechanisms of synchronization
2.2.1 Lunar synchronization
2.2.2 Social affiliation
2.2.3 Coupled oscillators
2.3 Perception and awareness of synchrony
2.4 Adaptivity of menstrual synchrony
2.5 Menstrual synchrony in traditional myth and ritual
2.6 Other species
2.6.1 Rats
2.6.2 Hamsters
2.6.3 Chimpanzees
2.6.4 Golden lion tamarins
2.6.5 Mandrills
3 References and notes
4 External links
Overview[edit]
Original study by Martha McClintock[edit]
Martha McClintock published the first study on menstrual synchrony among women living together in dormitories.
Proposed causes[edit]
McClintock hypothesized that pheromones could cause menstrual cycle synchronization.[3][5] However, other mechanisms have been proposed, most prominently synchronization with lunar phases.[4]
Efforts to replicate McClintock's results[edit]
No scientific evidence supports the lunar hypothesis and doubt has been cast on pheromone mechanisms.[4][6]
After the initial studies reporting menstrual synchrony began to appear in the scientific literature, other researchers began reporting the failure to find menstrual synchrony in women.[7][8]
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.[6][9][10][11]
More recent studies, which took into account some of these methodological criticisms, failed to find menstrual synchrony.[6][12][13]
Terminology[edit]
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.[6]
Status of the theory[edit]
Indeed, in a recent 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).[4]
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.[12] It may also be because of the high probability of menstruation overlap that occurs by chance.[6]
Evolutionary perspective[edit]
Researchers are divided on whether menstrual synchrony is adaptive.[4][5][6] McClintock has suggested that menstrual synchrony may not be adaptive but rather epiphenomenonal, lacking any biological function.[5] 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.[14] Divergent climate regimes differentiating Neanderthal reproductive strategies from those of modern Homo sapiens have recently been analysed in these terms.[15]
Myth and tradition[edit]
Menstruation in synchrony with the moon is widely assumed in myths and traditions as a ritual ideal.[16][17]
Media attention[edit]
The question of whether women do in fact synchronize their menstrual cycles when they live together has also received attention in the popular media.[2][18][19][20][21][22]
Awareness and subjective experiences[edit]
In addition, most women report that they are aware of menstrual synchrony and nearly as many report personally experiencing the phenomenon.[23]
Non-human species[edit]
Menstrual or estrous synchrony has been reported in other species including Norway rats,[24] hamsters,[25] chimpanzees,[26] and Golden lion tamarins.[27] In non-human primates, the term may also refer to the degree of overlap of menstrual or estrous cycles, which is the overlap of estrous or menses of two or more females in a group due, for example, to seasonal breeding.[28]
However, as with early human studies on menstrual synchrony, non-human estrous synchrony studies also were criticized for methodological problems.[29][30][31]
Subsequent studies failed to find estrous synchrony in rats,[32] hamsters,[33] chimpanzees,[34][35] and Golden lion tamarins.[36]
Scientific details[edit]
The phenomenon of menstrual synchrony is the closeness in time of the menstrual cycle onsets of two or more women.[3] 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.[6] 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.[3] 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[37][38][39][40][41][42][43][44] and by other studies that failed to find synchrony.[6][7][8][12][13][45][46][47][48][49]
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.