For the past few months, I’ve noticed the start date of my period arriving closer and closer to that of my best friend. This month, we were only two days apart.

At some point, we’ve all likely heard that women who spend a significant amount of time together will notice a “syncing up,” or alignment of, the times in which they get their periods. Maybe you’ve even experienced this a few times yourself. For those who swear by “syncing up” with their mothers, daughters, close friends, or coworkers, the evidence may be surprising: the notion of women’s periods “syncing up” is probably a myth.

Ironically, this recurring misconception actually stems from science. A 1971 study, which analyzed the cycles of female college students, did find that the start dates of women’s periods came closer together when women were close friends or roommates. However, the experiment received heavy criticism for having flaws within the experimental design, and the fact that the results haven’t been replicated consistently.

A recent, more reliable study has emerged, where the period-tracking app “Clue” teamed up with Oxford University. Researchers wanted to track whether women who spend a lot of time together experience a syncing of periods. Out of the 360 pairs of women they served, 271 experienced a divergence of periods, while only 79 experienced a convergence of periods. Looking at the data from this study and others with similar results, it can be concluded that period’s tend to “sync up” as much as they “sync out.”

If period syncing isn’t a real occurrence, then why does the myth persist? Likely because of two reasons: pure chance and psychology.

The average cycle lasts around 28 days, and between two women who each have 5 to 7 day periods, there are 10 to 14 days per cycle where at least one of them is on their period. Because this span takes up a fair proportion of the 28-day cycle, women’s periods lining up every once in a while is highly plausible.

In addition, confirmation bias, or the the tendency to interpret evidence as confirmation of one’s existing beliefs, might be at play. Let’s say in one year, two close women have ten diverging periods and two converging periods. If they want their periods to align as evidence of their close-knit friendship, they may zero-in on the two instances where their periods lined up, and disregard the other ten. These women may share their experience of “syncing up” and leave out their experience of “syncing out,” thereby allowing the myth to live on.

The truth is that my friend and I aren’t actually syncing up. Maybe we’re forgetting about the ten months where our periods became further apart.

PhotoHindustan Times via Shutterstock

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