Many people think women’s menstrual cycles sync up with their friends’ when they spend a lot of time together, but a recent study found that cycles don’t actually link up, even if women live in the same place. In fact, it’s more common for women’s cycles to diverge over time.
As the Guardian reports, period-tracking and fertility app Clue recently partnered with the University of Oxford for what is believed to be the largest study of its kind. Researchers received 1,500 responses, though they narrowed it down to 360 pairs of women who live together. The women’s period data was analyzed for three cycles in a row, and the researchers determined that 273 pairs actually had a bigger difference in the dates their periods started at the end of the study than at the beginning.
“It’s very unlikely that cycle syncing is a real phenomenon. Menstrual syncing amongst the sample we had did not exist,” Clue’s data scientist Marija Vlajic told the Guardian.
The Guardian notes that while the concept of synchronized periods has been around for centuries, it wasn’t actually documented in a scientific study until 1971, when a Harvard researcher tracked 135 female college students in the same dorm and found “a significant increase in synchronization of onset dates.” In 1999, a different study found that 80 percent of women believed in the phenomenon, while 70 percent said that they even enjoyed it, according to the Guardian.
Vlajic explained that she can see how the idea of syncing periods can make women feel a special connection to one another, even if it’s not actually the case. “Even though we do it every month, periods are personal and the thought of sharing with someone makes the idea powerful. That’s why we continue to look for patterns even when they don’t exist,” she said.