Can We Finally Dispel the Myth That Women’s Periods Sync?

The theory that close friends have their periods at the same time has been around for over 40 years, but there has never been much evidence to back it up. The people behind the Clue period tracker recently checked their own data and found another nail in the coffin: zero evidence that closeness is causing people to bleed in sync.

Clue asked users if they thought they were syncing with someone, and ended up with 360 pairs of people who both tracked at least the last three consecutive cycles with Clue, weren’t on hormonal contraception, and lived in the same city (so no long distance relationship). The results, summarized in a Clue blog post , showed that only 79 couples had their menstrual periods closer over time, while 273 couples did the opposite. “This does not mean that the pairs are not syncing – it means that they have never been synced,” Maria Vlajic, Clue data scientist, told the Guardian .

Why this myth deceived us so easily

It may seem that we are synchronizing for two reasons, although in reality we are not. First, if you and your best friend are on hormonal contraception, then the timing depends entirely on the pills. If you both start taking your pills at about the same time, you will be in sync, but it has nothing to do with spending time together or sniffing each other’s pheromones.

Otherwise, you are probably just seeing natural variations. We all don’t have cycles of the same duration: I might have cycles that average 32 days in length, and you might have 28, perfect for a tutorial. So maybe ten days separates us when we first move together, and then six days. next month, and only two days later next month. We are not syncing; the numbers just add up. Wait a few more months and our cycles will diverge again. In fact, it is mathematically impossible for the cycles to be synchronized, and that women also differ from each other in the length of the cycle.

The 1971 article that started the synchronization myth fell short of scrutiny. For example, a 2006 study of women in university dormitories found no evidence that their cycles were synchronized, so the authors revised the 1971 paper and found serious errors in their analysis of the data. Other studies over the years have come to similar conclusions. In short, as another Clue scholar put it , synchronization appears to be “a methodological artifact of one study that has since become an urban myth.”


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