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Biology 103
2003 Third Paper
On Serendip

Menstrual Synchrony

Julia Wise

Generations of women have noticed it: you and your sister, or your roommate, or lover, or mom, get your periods at the same time. It doesn't always happen, but it catches the attention when it does. Female rats living in the same air space ovulate at the same time. Menstruation in monkeys synchronizes with the full moon (7). So is it all in our heads, or is the same pattern present in humans?

The clearest argument against the existence of menstrual synchrony is that since the length of the menstrual cycle varies from person to person (2), two women with different cycle lengths will never synchronize. They may menstruate at the same time, but the next month they will be a little different, the next month more different, and so on. By this argument, synchrony is simply a myth.

I cannot believe this argument, since it assumes that menstruation can be graphed and analyzed like a sine wave. Human bodies rarely adhere to perfectly timed schedules. Many women have irregular periods, and the regularity of the menstrual cycle changes at different stages of life (3). So if a woman with a cycle of 25 days and another with cycle of 28 days live together, they might both shift to a cycle of 26 or 27 days. In this way, synchronization would still be quite possible.

So if this phenomenon does exist, what explanation can there be for it? One theory is that lunar cycles may have some connection to the pattern. At first this makes some sense, since both cycles happen about thirteen times each year. A study on the Dogon people of Mali found that although they had no electricity and spent most nights outdoors, thus being as likely as anyone to be affected by the light of the moon, menstrual cycles among the Dogon did not match up to any lunar phase. Another theory
was that synchronization might be due to the same factor that causes pendulum clocks near each other to tick at the same time. This was later shown to be purely mechanical, though, with the swinging of the heaviest pendulum merely rocking the shelf a little and throwing off the beat of other clocks (1).

The most likely theory is some kind of hormone change. Women's menstrual cycles respond to contact with men, becoming shorter and more regular. So rather than a mechanical synchronization, like pendulum clocks, the cause is more likely chemical. In 1971, Martha McClintock published a study about the 135 women in her dorm at Wellesley College. She found that the synchronization of menstruation between roommates and close friends did increase after the women began living together. McClintock's explanation was pheromones. She co-authored a followup experiment, exposing women to chemical compounds from the armpits of other women. She concluded that this did alter menstruation (4).

There are a number of problems dealing with the statistics of this and other experiments, though. The data can be interpreted to either support or negate McClintock's conclusion, depending on how it is analyzed (4).

Why synchronize? One possible benefit of simultaneous ovulation for the whole population is simultaneous birthing. When female rats give birth at the same time, their pups are significantly healthier and more likely to survive. Certain times of the year may be better for births, as when lambs are born in spring rather than fall. So synchrony may have developed because it is helpful in raising healthy young (6).

The research I have done has increased rather than answering my list of questions. Is the math used in these studies wrong? Most articles I found denouncing the theory were written by men; most supporting it (or even denouncing it but wishing it were true) were by women. So is this just something women want to believe because it would be cool and bring us closer together? Also, according to McClintock, some women responded strongly to other women's pheromones, while others did not respond at all (6). Does this mean that it is not strictly group behavior but leader/follower behavior,
with some women's cycles setting the trend for the others? If so, does this chemical leadership correlate to any kind of social behaviors, like alpha females among wolves? My conclusion can only be that despite all those sex-ed videos from seventh grade, menstruation is still awfully confusing.


1) "Blood, Bread, and 'Menstrual Mind'?", dealing with Judy Grahn's book on menstruation
2) "Convergence and Divergence of Menstrual Rythms", analyzing the math used on Martha McClintock's study
3) "Menstrual Cycle Length as Function of Age"
4) "Do the Menstrual Cycles of Women Living Together Tend to Synchronize?"
6) "Menstrual Synchrony", an interview with Martha McClintock
7) "Converging Menstrual Cycles"

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