Although period syncing has been a topic of discussion among friends for a long time, it was not until social psychologist Martha McClintok investigated the phenomenon in 1971 that the scientific community began to take an interest in it. McClintok studied 135 women living in the same dorm and had them track their periods throughout the school year. She found that their menstrual cycles tended to sync over time, theorizing that it occurs through sharing pheromones.

Pheromones are chemical signals secreted by animals, insects, and some plants that can be detected by their same species. In humans, they are believed to play a role in attraction, among other things. According to one theory, pheromones make periods sync as an adaptive mechanism that has evolved over time. If our cycles are the same, there is a higher possibility of getting pregnant at the same time, leading to benefits such as increased social support, shared childcare responsibilities in the community, and a population boost.

However, since McClintok published her findings, scientists have conducted several studies on roommates, couples, animals, and friends, with mixed and often conflicting results. Many of these studies fail to show any kind of syncing at all, including a Chinese study from 2006 that collected period data from 186 students living together in a dorm and research published by Oxford in 2017 that showed periods were more likely to step out of sync.

To make matters even more complicated, there is still no concrete evidence that humans even have pheromones in the first place. While it is likely that human sex pheromones exist because we are animals, the scientific community remains divided on this issue. At present, there is no proof that we secrete chemicals that alter the behavior of other people. So, what is the explanation for what 70% of people experience, and what was McClintok talking about in 1971?