Period synchronization: myth or reality?

Many women believe that the menstrual cycles of friends, roommates or work colleagues who spend a lot of time together synchronize over time. But is this really a scientifically proven phenomenon or is it just a myth based on coincidence and selective perception?

We shed light on the background to period synchronization and what science has to say about it.

The origin of the concept: the McClintock study

The belief in period synchronization goes back to a study published in 1971 by Martha McClintock, an American psychology student at the time. She observed that the menstrual cycles of some of her roommates had synchronized over time and wanted to get to the bottom of this phenomenon.

In her study, McClintock examined the menstrual behavior of 130 women who lived together in a student dormitory. She found that women who had close social relationships developed similar menstrual cycles over time, leading to the popularization of the term “period synchronization” or “menstrual synchrony”.

In a second study in the 1990s, McClintock wanted to find out what could cause this so-called period synchronization. Her hypothesis was that pheromones were behind it all. For the study, 29 participants had to wear cotton pads under their armpits. The sweat extract was then dissolved in alcohol from the pads and wiped under the noses of the other participants.

McClintock and her team also used urine and blood values to check which phase of the cycle the women were in.

Even after this study, the budding psychologist saw her assumption confirmed, as the result was that women who had the pre-ovulation extract under their noses had a shorter cycle than those who had the post-ovulation extract under their noses.

Controversial nature of the Clintock study

Although Martha McClintock's studies are relatively well-known, they should still be viewed with caution. Firstly, a study with 29 participants is quite small and not particularly representative and secondly, some researchers came to different conclusions in subsequent studies.

One of the best-known studies that cast doubt on McClintock's findings was published in 1992 by H. Weller and L. Weller. They examined the menstrual cycles of women living in a kibbutz, a communal settlement in Israel. The results showed that there was no significant synchronization of the cycles. They argued that the synchronization observed by McClintock was more likely due to statistical coincidence and misinterpretation.

In another large-scale study from 2006, which was published in the journal “Human Nature”, the researchers also found no evidence of significant period synchronization. The test subjects were 186 Chinese women who lived together in pairs. In their analysis, the researchers came to the conclusion that the women's cycles often diverged over time instead of converging.

Coincidence instead of synchronization

Many scientists currently assume that period synchronization is merely a coincidence. Menstrual cycles are on average between 21 and 35 days long. It would therefore come as no surprise that women's periods sometimes start at the same time, especially if they have similar cycle lengths. However, as most women's cycle lengths vary slightly, synchronization can quickly be reversed.

Selective perception also plays a role in the phenomenon of period synchronization. This is because women are more likely to remember the cycles in which their period coincided with that of their girlfriend or roommate. This can quickly give the impression that the cycles are regularly synchronized, although this is not the case.

Conclusion: A myth or reality?

Many women are convinced of period synchronization, but there is no convincing scientific evidence. While the McClintock study popularized the phenomenon, there have been no other studies to substantiate the claims made. The influence of pheromones on the human menstrual cycle has also not yet been sufficiently researched.


Overall, it seems that period synchronization is more of a myth that is deeply rooted in cultural perception, but not a biological phenomenon.