McClintock effect
Encyclopedia
Menstrual synchrony is a phenomenon reported in 1971 wherein the menstrual cycles of women who lived together (such as in home
Home
A home is a place of residence or refuge. When it refers to a building, it is usually a place in which an individual or a family can rest and store personal property. Most modern-day households contain sanitary facilities and a means of preparing food. Animals have their own homes as well, either...

s, prisons, convents, bordellos, dormitories, or barracks
Barracks
Barracks are specialised buildings for permanent military accommodation; the word may apply to separate housing blocks or to complete complexes. Their main object is to separate soldiers from the civilian population and reinforce discipline, training and esprit de corps. They were sometimes called...

) reportedly became synchronized over time. The existence of menstrual synchrony has not been definitively established, and studies investigating it have been controversial (see also Whitten effect
Whitten effect
The Whitten effect is a phenomenon observed by Wesley K. Whitten , whereby male mouse pheromone-laden urine synchronizes the estrus cycle "among unisexually grouped females."...

).

Research

Psychologist Martha McClintock
Martha McClintock
Martha McClintock is an American psychologist best known for her research on human pheromones and her theory of menstrual synchrony...

 was the first scientist to do a study on menstrual synchrony, reporting her findings in Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

in 1971.

A prospective study by McClintock in 1998 was claimed to support her earlier findings by showing that underarm (or axillary) materials collected from female donors, when wiped under the noses of other women, influenced cycle length. Odors taken on the day that donors ovulated (and the next two days) delayed ovulation and hence lengthened the total cycle of the recipients. These phase-advancing and phase-delaying effects may show human axillary compounds regulate biological rhythms.

Another study experimented with 20 lesbian
Lesbian
Lesbian is a term most widely used in the English language to describe sexual and romantic desire between females. The word may be used as a noun, to refer to women who identify themselves or who are characterized by others as having the primary attribute of female homosexuality, or as an...

 couples had results which showed that more than half of the couples tested had the same synchronization within a two day period of each other. Another study with 29 lesbian couples showed no evidence of synchrony, and tentative evidence towards divergence of menstrual cycles.

Criticism

If all women had an average-length menstrual cycle (of 28 days duration), the maximum time between two women's onsets would be 14 days and the minimum time between onsets would be zero days (synchronization). On average, the difference would be seven days, and (in small groups) half the time would be less (if one assumes there is no McClintock effect). McClintock observed a five day difference in her 1971 study and some have suggested this could have been a random occurrence.

Most studies of menstrual synchrony have been retrospective, introducing recall bias
Recall bias
In psychology, recall bias is a type of systematic bias which occurs when the way a survey respondent answers a question is affected not just by the correct answer, but also by the respondent's memory. This can affect the results of the survey. As a hypothetical example, suppose that a survey in...

into the data.

The interaction of theorized menstrual synchrony with differing cycle lengths has not been explained. Two women with cycle lengths that differed by two days might initially begin menstruating on the same day, but the next month would be two days apart, the month after that four days, and so on. No studies have claimed to show that the McClintock effect causes women with historical cycles of different lengths to synchronize.

Methodological errors have also been proposed. A critical review of the evidence for menstrual synchrony gave this example:
This type of error is more likely in smaller sample sizes, like those used in studies of menstrual synchrony.

H. Clyde Wilson of the University of Missouri analyzed the research and data collection methods McClintock and others used in their studies. He found significant errors in the researchers' mathematical calculations and data collection as well as an error in how the researchers defined synchrony. Wilson's clinical research and his critical reviews of existing research, including the suggestion that pheromones can trigger synchrony in humans, demonstrated that when the studies are corrected for such errors, the evidence for menstrual synchrony disappears.

External links

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