The theory that close friends have their periods at the same time is over 40 years old now, but there has never been much evidence to support it. The people behind the period tracker Clue checked their own data recently and came up with another nail in the coffin: Zero evidence that closeness makes people bleed in sync.
Photo by Monica Galentino.
Clue asked users if they thought they had synced with someone, and ended up with 360 pairs of people who both had tracked at least three recent consecutive cycles on Clue, were not on hormonal birth control, and lived in the same city (so, no long distance relationships). The results, summarised in a Clue blog post, found that only 79 of the pairs saw their periods get closer together over time, versus 273 who saw the opposite. "This doesn't mean that pairs go out of sync -- it means they were never in sync in the first place," Clue's data scientist Marija Vlajic told the Guardian.
Why This Myth Fooled Us So Easily
There are two reasons we might seem to sync up when we really don't. First, if you and your bestie are taking hormonal birth control, then the timing is totally up to the pills. If you both begin a pill pack at about the same time, then you're synced -- but that has nothing to do with spending time together or sniffing each other's pheromones.
Otherwise, you're probably just seeing natural variation. We don't all have cycles that are the same length: I might have cycles that average 32 days and you might be a textbook-perfect 28. So maybe we're 10 days apart when we first move in together, then six days apart the next month, and only two days apart the month after that. We're not syncing; it's just the way the numbers work out. Wait a few more months and our cycles will drift apart again. In fact, it's mathematically impossible for cycles to sync up and for women to also differ from each other in cycle length.
The 1971 paper that started the myth of syncing hasn't stood up well to criticism. For example, a 2006 study of women in a university dorm found no evidence that their cycles synced up, so the authors revisited the 1971 paper and found serious errors with its data analysis. Other studies over the years have come to similar conclusions. In short, as another Clue scientist put it, syncing seems to be "a methodological artefact from one study that has since turned into an urban myth".