Re: menstrual synchrony

From: Wade T. Smith (
Date: Thu 29 May 2003 - 01:30:18 GMT

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    On Wednesday, May 28, 2003, at 06:28 PM, Dace forwarded:

    > It is known that humans can transmit
    > information to each other that could not reasonably be considered
    > memetic.
    > For example, Russell, Switz, and Thompson (1980) showed that human
    > menstrual
    > cycles become synchronized through olfactory cues.

    Not the best of examples, I'm afraid-

    Apparent clustering of menstrual onsets doesn't necessarily mean anything. Assuming an average cycle of 28 days, the maximum time between two women's onsets is 14 days. Since the minimum is zero, the average difference--what you'd expect purely by chance--is seven days, and half the time would be less. (In 1971 McClintock said she'd observed a decline in the average difference from seven days to five.) What's more, women recording their onsets after the fact often misremember or are influenced by the recollections of their friends, skewing the data.

    Menstrual synchrony in any meaningful sense is impossible when the women have cycles of different lengths. (Cycle length varies considerably among women not using the pill.) Though a woman with a 27-day cycle might initially have her onset on the same day as a woman with a 29-day cycle, the next month she'd be two days earlier, the month after that four days, and so on. No one has shown that supposedly synchronized women have cycles of the same length--or that their cycles, if of different lengths at first, diverge less than they should over time.

    Methodological errors can easily bias a data set to show menstrual synchrony where none exists. To demonstrate one common problem: 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). Ergo, the two are synchronizing. In fact, the two subjects were eight days apart to start with (September 27 vs October 5) and are diverging. Of course you can set up the numbers to arrive at the opposite conclusion; the point is that given the small samples commonly used in studies of menstrual synchrony, it's easy to lead oneself astray. One skeptic
    (H.C. Wilson, 1992) has claimed that when you correct all the errors, including McClintock's, the evidence for menstrual synchrony evaporates.

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