Menstrual synchrony

Menstrual synchrony is a phenomenon reported in 1971 wherein the menstrual cycles of women who lived together (such as in homes, prisons, convents, bordellos, dormitories, or barracks) 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).^{[citation needed]}

Research

Psychologist Martha McClintock was the first scientist to do a study on menstrual synchrony, reporting her findings in Nature in 1971.^[1]

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.^[2] 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.^[3]

Another study experimented with 20 lesbian 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.^[4] Another study with 29 lesbian couples showed no evidence of synchrony, and tentative evidence towards divergence of menstrual cycles.^[5]

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.^[6]

Most studies of menstrual synchrony have been retrospective, introducing recall bias into the data.^[7]

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.^[7]

Methodological errors have also been proposed. A critical review of the evidence for menstrual synchrony gave this example:^[7]

Suppose a study starts on October 1. Subject A, with a 28-day cycle, has an onset on September 27, another on October 25, and a third on November 22. Subject B, with a 30-day cycle, has an onset on October 5 and another on November 4. A naive investigator could report that these subjects were 20 days apart at the outset (October 25 vs October 5) and 18 days apart at their second onset (November 4 vs November 22). Therefore, the two are synchronizing. In fact, the two subjects were eight days apart to start with (September 27 vs October 5) and are diverging.

This type of error is more likely in smaller sample sizes, like those used in studies of menstrual synchrony.^[7]

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,^[8] demonstrated that when the studies are corrected for such errors, the evidence for menstrual synchrony disappears.^[9]

References

1. ^ McClintock MK (1971). "Menstrual synchrony and suppression". Nature 229 (5282): 244–5. doi:10.1038/229244a0. PMID 4994256. 
2. ^ Stern K, McClintock MK (1998). "Regulation of ovulation by human pheromones". Nature 392 (6672): 177–9. doi:10.1038/32408. PMID 9515961. 
3. ^ Weller, A (1998). "Communication through body odour". Nature 392 (6672): 126–27. doi:10.1038/32283. 
4. ^ Weller A. Weller L. 1992. menstrual synchrony in female couples. Psychoneuroendocrinology. 17(2-3):171;177
5. ^ Trevathana, W.R., Burlesonc, M.H. and Gregory, W.L. 1993. No evidence for menstrual synchrony in lesbian couples. Psychoneuroendocrinology. 18(5-6):425;435 [1]
6. ^ Yang, Zhengwei; Jeffrey C. Schank (2006). "Women Do Not Synchronize Their Menstrual Cycles". Human Nature 17 (4): 434–447. doi:10.1007/s12110-006-1005-z. http://transactionpub.metapress.com/openurl.asp?genre=article&issn=1045-6767&volume=17&issue=4&spage=434. Retrieved 2007-06-25. 
7. ^ ^{a b c d} Adams, Cecil (2002-12-20). "Does menstrual synchrony really exist?". The Straight Dope. The Chicago Reader. http://www.straightdope.com/columns/021220.html. Retrieved 2007-01-10. 
8. ^ Wilson, H.C. (1987). Female axillary secretions influence women's menstrual cycles: A critique. Hormones and Behavior, 21, 536-546.
9. ^ Wilson HC (1992). "A critical review of menstrual synchrony research". Psychoneuroendocrinology 17 (6): 565–91. doi:10.1016/0306-4530(92)90016-Z. PMID 1287678. 

External links

• The story of menstrual synchrony and suppression
• The Claim: Menstrual Cycles Can Synchronize Over Time - New York Times, February 5, 2008
• Dr. Harriet Hall, Menstrual Synchrony: Do Girls Who Go Together Flow Together? Science-Based Medicine, September 6, 2011