"Period-syncing" is a myth

No matter how inaccurate the myth of period syncing may be, the idea that women’s bodies can fall into collective rhythms carries a certain mysterious, otherworldly appeal and, lending the myth more inertia, gives women a way to feel connection, empathy, and collective empowerment with other women.

Period syncing—or, more formally, “menstrual synchrony”—was introduced into the popular consciousness in 1971 by a researcher named Martha McClintock. Her study on the menstruation patterns of students at a women’s college, published in the journal Nature, tracked the period start dates of 135 women who lived together in a dormitory over a time frame of about six months. It claimed to find “a significant increase in synchronization (that is, a decrease in the difference between onset dates)” among roommates and among groups of women who independently identified one another as a “close friend.” At the beginning of the study, these friends averaged about six and a half days’ difference between period start dates. By the end, they averaged a little less than five. (McClintock conducted this research while she was still an undergraduate at Wellesley College.)..

In a 1999 paper in the journal Human Reproduction, Strassmann pointed out a fundamental flaw in the period-syncing logic:

Given a cycle length of 28 days (not the rule—but an example), the maximum that two women can be out of phase is 14 days. On average, the onsets will be 7 days apart. Fully half the time they should be even closer. Given that menstruation often lasts 5 days, it is not surprising that friends commonly experience overlapping menses, which is taken as personal confirmation of menstrual synchrony.

Strassmann considers the idea of period syncing to have been “debunked,” but of course much of the general population remains convinced that it’s a real thing, thanks to what she calls “an appealing narrative that overrides the science.”

Discussed at length in The Atlantic.