Cycle Syncing: Fact or Fiction?


When my roommate and I moved in together at the beginning of the years, our moms were commenting on how small our dorm room was. During their conversation one of them made a remark about how since we would be living together and spending a lot of time together, it wouldn’t be long before our cycles synced. They were referring to the wide-spread belief that when girls spend a lot of time together, their menstruation cycles will sync up.

Ready for TMI? Eight weeks into the semester and our cycles (which were not previously on the same schedule) are now in sync. However, just because our cycles synced doesn’t prove that it happened for a reason, it could just be due to chance. As we’ve discussed in class time and time again, ‘correlation is not causation.’ So I decided to investigate if there was any scientific evidence to support the hypothesis.

This idea was first introduced by Martha McClintock, a psych student at Harvard in 1971. She published a paper in the journal Nature with the hypothesis “synchrony and suppression among a group of women living together in a college dormitory suggest that social interaction can have a strong effect on the menstrual cycle.”

In McClintock’s study, she observed 135 female residents of a college dorm. She focused on the start day (which she called the onset date) of each females cycle and compared it to among roommates and close friends. She asked the participants to recall their onset start dates at three times during two semesters. When she compared her data she found that friends started their cycles (on average) 6.4 days apart in October, and were within 4.6 days apart by April.  Her results found that there was an increase in the onset date of girls cycles when they spent a lot of time together.

The idea of menstrual cycle syncing is closely associated with McClintock’s research. On Wikipedia, it even lists that Menstrual Synchrony is also known as the McClintock effect. Her observational study set out to prove that cycle synchronization among close females happens, not find what made the synchronization happen.  In the analysis of her findings, McClintock questioned what mechanism made this cycle synchronization possible, which she is still studying today.

In looking at if correlation is casual, one of the main grounds for skepticism is if there is no demonstrable biological mechanism. Although in this case there is no biological mechanism that explains why this happens, there is a proven cause and effect between time and cycle onset.  Therefore the lack of a biological mechanism does not discount McClintock’s findings as a false positive.

After her study was published, other scientists tried to replicate her findings. Some studies validated her findings, while others failed. I was not surprised by this. Menstruation cycles are easily influenced by so many factors, including: stress, diet, exercise, sex, and change in routine. Therefore it would be extremely difficult to replicate these results in women undergoing huge amounts of stress and change while adjusting to college.  In my mind the fact that McClintock found girls cycles synced, despite all of the confounding variable that effect menstruation, makes her results even more legitimate.

However, the phenomenon of cycle syncing could just be a myth supported by chance. Unless there is biological mechanism found that proves women who spend a lot of time together will sync their menstrual cycles, there’s no way to know for sure.

3 thoughts on “Cycle Syncing: Fact or Fiction?

  1. Megan Fleming Post author

    Hi Asia,

    You asked why I believe cycle syncing is a thing. As I explained in my post, I decided to look into the idea after my cycle synced with my roommates. Given the research I did on the topic, the main variable in this causal relationship is the proximity and amount of time women spend together, which explains cycle syncing happens. However, McClintock is still researching this topic today because there are so many confounding variables, and the ever-present possibility of chance! Thanks for your comment and feedback on my article.

  2. Madison Canter

    I believe that menstrual cycle syncing is indeed a fact! My roommate and I are synced after living with each other for just 4 months. Also, in high school, a few of my school friends and I shared the same cycle, as well. Although there is limited research on what may cause this phenomenon, recent research suggests that it could be due to chemical signals given off by our skin, called pheromones. Animals use pheromones to send messages to others of their species, usually for reproduction and sex purposes. You can find out about the study in this article.

  3. Asia Grant

    I am a strong believer that female cycle syncing is a thing. Like you, I have experienced many times where my cycle and girls’ that I see on a regular basis start on the same schedule. My question to you now is, why do you think that? Your description about the previous study conducted by McClintock was very informative, but didn’t give much explanation on what could be the main components out of the factors you listed that could be affecting girls’ cycles.

    Maybe there depending on the relationship between the girls is something to consider. There is a possibility the closer you are with a person the more likely your cycles will sync up. I had a roommate my freshman year and we were not very close and our cycles were never on the same schedule, however, one of my closest friends here who I only get to see about twice a week has the same schedule as I do. Also, just another thing to think about, is one girl’s cycle more dominant than others–like is there one cycle that is more likely to be synced up to? If so, that may be a hormonal component that you might be interested in exploring. Thanks for the post!

Leave a Reply