For something that around half the world’s population will experience — most of them every single month, for a good chunk of their lives — menstruation is dogged by more than its fair share of misconceptions. I’ve written before about a male friend who believed they were governed by the moon, and similar questions fielded by other women I know (including one who was asked once why she couldn’t just “hold it in” like pee).
The stealthiest misconception of all, though, may be the idea that if women live together for long enough, their periods will inevitably sync up. This one is so widespread, so seemingly logical — so “resilient to debunking,” as Simon Oxenham wrote in a New Scientist column this week — that most people aren’t aware that the science doesn’t back it up.
The idea of “menstrual synchrony” was first put forth by psychologist Martha McClintock, who published the evidence for her theory in a 1971 Nature study, and the scientific community has largely treated it with skepticism ever since. A 2007 Scientific American article reported that “nearly half of the papers published on the topic” — you can find a couple examples here and here — “find no evidence that close co-habitation draws menstrual cycles closer together.” What’s more, studies that do find an effect have been dogged by harsh criticisms of poor design and naïve statistical analyses.” (The authors of one 2006 paper in the journal Human Nature, for instance, wrote that when they re-crunched McClintock’s original numbers, “We found that group synchrony in that study was at the level of chance.”)
So why does the idea remain so pervasive? In part, Oxenham explained, it’s because it’s an easy target for confirmation bias — we bend the facts to fit the theory. Here’s how he solved it with math:
If we imagine two women with cycles of 28 days, the maximum amount of time they could be out of synch would be 14 days. On average, we would expect them to be only seven days apart, with a 50 per cent likelihood that they are even more closely aligned, just through chance alone. If we assume menstruation lasts five days, it’s hardly surprising that in a group of close friends, there will be some overlap.
But more broadly, Oxenham argued, there’s another layer to the longevity of period misconceptions — namely, that they provide an overblown but tantalizing explanation for certain aspects of women’s behavior. “The challenge has been the oversimplified notion that if you study women’s menstrual cycles, you learn something directly important about their social judgments,” social psychologist Wendy Wood, who has studied how menstruation affects women’s choice of sexual partner, told New Scientist. “It turns out to be much more complicated than that.” Our fascination with this particular myth, in other words, may reveal more about our nature than the myth itself.