It’s long been speculated that women’s periods can sync when they spend time together. Some women swear by it and an interaction of pheromones (chemicals that affect behaviour) is usually offered as an explanation.
The idea has been around since a researcher called Martha McClintock studied the cycles of 135 American students in 1971 and claimed that the onset of menstruation was more similar among roommates than random pairings of women.
A popular evolutionary explanation emerged that this phenomenon helps females avoid being monopolised by a dominant male, because the women are fertile simultaneously.
At a time when feminism was gaining traction, the idea that females would cooperate in the face of male domination was persuasive. But more than 30 years of investigation have passed since McClintock’s research and multiple studies since have failed to provide evidence for this phenomenon. Others have also pointed out methodological oversights in the original study.
Of course, period synchronisation is a tricky topic to examine. As women’s cycle lengths vary so much, we don’t know if pheromones can influence menstruation. Plus, any study should expect some women’s cycles to overlap by random chance.
The theory of menstrual synchrony is likely to stick around though. After all, the thought of sharing periods with someone may make you feel closer to them and less stigmatised – not to mention making three-for-two tampon offers all the more tempting.
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