Women's Periods Don't Actually Sync Up With Their Friends

There’s a commonly accepted idea that women who spend a lot of time with each other will eventually have synchronized periods. Although many women swear by it, new research suggests it’s unlikely to be true, banishing this piece of anecdotal evidence to the resting place of myth, misinformation, and stigma still surrounding menstrual cycles.

The new research was carried out by Dr Alexandra Alvergne from the University of Oxford and the period-tracking app Clue. Together, they used data from the application and questioned 1,500 users about their lifestyles. They asked them whether they thought their cycles had been syncing with someone, their relationship with that person (friends, siblings, parent, child, partners, roommates, colleagues), if they live together, and whether they’re on hormonal birth control.

They analyzed three consecutive cycles in 360 pairs of women who were close to each other and found that 273 pairs actually experienced their menstrual cycles becoming more out of sync over the course of the study. Out of the people studied, just 79 experienced the gap between cycle start dates shorten.

“In other words, according to these results, cycles are actually more likely to diverge (get out of sync) over time,” Clue said on its website.

The modern incarnation of this idea popped up with an influential study published in the journal Nature during the early 1970s. The research suggested that menstrual cycle start dates of women living and studying together at college in the US did sync up. Others built on this idea, suggesting that perhaps it evolved as a method for women to negate the power of dominant males. In theory, it is easier for a dominant male to maintain their "sexual monopolization" on a group of women if they have staggered periods, as he can impregnate them one at a time.

The idea of menstrual synchrony has stayed strong in the minds of people, despite plenty of recent scientific studies throwing doubt on the theory and potential mechanisms that explain it.

Speaking about this to BBC News last year, Dr Alvergne explained why this theory has stuck around for so long: "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.”

“One [explanation of menstrual synchronicity] was the model with the exciting hypothesis also known as the evolved strategy… And the other model was the boring model, where the patterns are explained by chance," she added. "Maybe actually, what we observe is nothing more than randomness."