Math Disproves The Myth That Women’s Periods Sync Up, Contrary To Popular Belief

One of the most widely circulated myths about menstruation has been proven false by math. It turns out women don’t sync their periods when they’re in close proximity — it’s all just a lie. In case you haven’t heard it before, many people believe that women who are close, spend a lot of time together, or live together, somehow, by magic or by the moon, start to get their period on the same days.

The idea was first floated by psychologist Martha McClintock in 1971. She published her evidence in Nature after studying a bunch of women in a college dorm, but most scientists have always thought it was bullshit and that her evidence wasn’t really solid. There have been numerous studies debunking or investigating the claim since the ’70s.

You might have experienced it yourself, living with female roommates or finding that your group of female friends all tend to get their period around the same time. But think about it, Sherlock — it’s just chance. Take two women with 28-day cycles, Simon Oxenham wrote in New Scientist, “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.” He added, “If we assume menstruation lasts five days, it’s hardly surprising that in a group of close friends, there will be some overlap.”

The myth really should be debunked, so start spreading the word. There is some weird fascination with the idea that periods sync, even among women who to seem to think it’s powerful or cool that there’s something “unexplainable” about our periods. But it’s also just impossible that the moon is connecting us or that our uteruses know what’s going on. The break down of probability is much more likely.

Also, the syncing myth plays into this idea that women’s behavior (along with their bodies) is out of their control. Think about it: it would be pretty fucking mystical for menstrual cycles to organize themselves across hallways and brunch tables. Wendy Wood at the University of Southern California has looked at 58 studies on menstruation and their findings — about synching, about how women choose mates or our voices chance while menstruating — and found them all to be full of shit. She told New Scientist, “The challenge [in debunking menstruation myths] has been the oversimplified notion that if you study women’s menstrual cycles, you learn something directly important about their social judgments.” It’s much more complicated than that.

So while it may be comforting or interesting to blame some mating behaviors on your period or think you and your bestie are connected by your cycles (which is weird, come on), it’s not good for how women are perceived in the world. These theories simplify women and their experiences, because science always wins.