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...