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 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:
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.
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.
Read more about this topic: Menstrual Synchrony
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