Menstrual synchrony

Menstrual synchrony, also called the McClintock effect,[1] or the Wellesley effect,[2] 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.

[7] 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,[5] and the probability of encountering such overlaps by chance is high.

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.

[19] Divergent climate regimes differentiating Neanderthal reproductive strategies from those of modern Homo sapiens have recently been analysed in these terms.

[20] 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.

[26] When early Pleistocene hominids in Africa were attempting to survive by robbing big cats of their kills,[27][28][29] 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.

[38] 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...".

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.

There were 85 women living in dormitories, sorority houses, and apartments who attended a large midwestern university in the United States.

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 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...

The lesbian couples were drawn from a larger sample of women who had kept daily records of their menstrual cycles for three months and who had participated in a previous study.

Twenty seven families, which had from two to seven sisters 13 years or older and collected data on menstrual cycle onsets over a three-month period.

[46] Strassmann investigated whether menstrual synchrony occurred in a natural fertility population of Dogon village women.

The mean difference in cycle onset was calculated for the beginning, middle, and end of the study for the pairs and triples of women.

She hypothesized that Dogon women would be ideal for detecting a lunar influence on menstrual cycles, but she found no relationship.

[12] In order to work out why menstrual synchrony might have evolved, it is necessary to investigate why individuals who synchronized their cycles might have had increased survival and reproduction in the evolutionary past.

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).

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.

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.

[79] With gradual offshore platforms during spring low tides, substantial areas of the intertidal zone are revealed, and these are the most productive and safest shellfish collecting times… Foragers should schedule visits to coastal residential sites at times during the lunar month when spring tides are present and then move slightly inland during neaps to broaden the size of the exploitable terrestrial area.

The axillary region's scents have been shown to be capable of mediating these effects (Preti et al., 1987; Russell et al., 1980; Stern and McClintock, 1998), but their active ingredients have not yet been discovered.

There are connections from the olfactory bulbs to the hypothalamus, the brain region in charge of regulating the release of luteinizing hormone, in both systems.

In the current work, we looked at how menstrual synchrony and the sense of smell for the putative pheromones 3 androstenol and 5 androstenone related.

[82][83] Menstrual or estrous synchrony has been reported in other species including Norway rats,[81] hamsters,[82] chimpanzees,[84] and golden lion tamarins.

[86][87][88] Subsequent studies failed to find estrous synchrony in rats,[89] hamsters,[90] chimpanzees,[91][92] and golden lion tamarins.

[86] He found that female rats did not synchronize their cycles and he argued that in the original McClintock study,[81] the random control group was more asynchronous than expected by chance.

[87] In a follow-up experimental study motivated by this methodological critique, no evidence for estrous synchrony was found for female hamsters.

[88] Since then Matsumoto and colleagues have reported estrous asynchrony in groups of free-living chimpanzees in Mahale Mountains National Park, Tanzania.

They tested three hypotheses about the adaptiveness of estrous asynchrony: (1) females become asynchronous to increase copulation frequency and opportunities for giving birth; (2) paternity confusion to reduce infanticide; and (3) sperm competition.