Explained: Why girls’ periods sync up when they live together


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Explained: Why girls’ periods sync up when they live together

Like clockwork

It’s a phenomenon most all-female flatmates have heard of – and one most people I know seem to hold as true – but do your periods really sync up with the people you live with? Is it magical or based on science?

Alexandra Alvergne, an Associate Professor in Biocultural Anthropology at the University of Oxford, recently tried to review the evidence surrounding the phenomenon. “It’s a popular belief”, she told the BBC. “As humans we always like exciting stories. We want to explain what we observe by something that is meaningful. And the idea that what we observe is due to chance or randomness is just not as interesting.”

Who knows why eh

According to Alvergne the idea of ‘syncing’ started with research findings published in a scientific journal back in 1971, which studied the menstrual cycles of 135 American college girls. The study found that syncing was common among friends and roommates compared to random pairings of women – and it concluded that this was because the women, who spent a lot of time together, had the chance for their pheromones to affect each other.

The theory goes that syncing isn’t just a weird magical time for close friends, but in fact an evolutionary strategy designed to help girls co-operate with each other (which is a nice thought, really). Speaking in evolutionary terms syncing up meant that everyone was fertile at the same time, and no one man would be able to reproduce with all of them (again, nice).

Alvergne explains: “He can’t manipulate all the females at the same time so that’s why it was believed to be a form of co-operation between females. From a feminist point of view, the idea that females would co-operate in the face of male domination is attractive.”

Other academics however put the idea of syncing down to chance and chance alone. But this, Alvergne rightly points out, is a lot less interesting than the feminist evolutionary theory. She calls it the “boring model”.

Anecdotally at least it seems to hold true that syncing does exist. Every girl I asked said they had experienced it at least once. Lucy told me: “I synced up with two of my housemates. They were already synced because they had lived with each other for a year already.

“I was two weeks out when I first moved in, but by February we all came on around the same time. I genuinely don’t know how it happened, it still baffles me.”