Growing up I remember my mom complaining that we went through feminine products way to fast because we had the same cycle. I was always just a few days behind my mom and my friends that I spent a lot of time with were always within a few days of me as well. Last week I was talking to my friend who was complaining that her cycle started a full week late and her roommates came a full week early and started a day apart. Her big dilemma was that both girls are on birth control pills so their cycles should not be effected. I went into my research looking for how syncing can happen and then was planning on branching into how that worked with individuals on birth control. As I started my research I was surprised to find the controversy around whether cycles sync at all.

McClintock in Nature in 1998 published her findings that women’s cycles do sync. She did a study on rats and odors and found that the phases of the rats reproductive cycle varied based on the smells. She conducted many studies which are now being challenged. On the other hand, in 1992 Clyde Wilson re-analyzed McClintock’s experiment and found that there was an inflated difference between period startdate between the period start dates at the start of McClintock’s study.

The father-son duo of Leonard and Aron Weller at Bar-Ilan University in Israel conducted a mass study on roommates, athletes, lesbian couples,mothers, sisters, friends and even colleagues throughout the 1990’s and found that their data sometimes corresponded with that of McClintock and they found synchrony, but other times it did not. In another study in the workplace it was found that women that worked in close quarters for over a year synched up menstrual cycles. This study defines ‘synchrony’ as anywhere under 9 days distance in the start time of the cycle. A different study found the exact opposite through a study of women living in dorms. They found that the women did not sync up and through meta-analysis of other similar studies came to the conclusion that that the findings of synchrony are at the level of chance.

In regards to the synchrony of women’s menstrual cycles, through years of research not much has been found supporting it. The leading idea in forms of a mechanism for the potential synchrony is Pheromones, which like Andrew mentioned in class are the signals animals produce when they are fertile and attracting a member of the opposite sex. It is not known for sure whether human pheromones even exist, although the idea is plausible. Scientists Yang and Schank, and Ziomkewicz in 2006 reviewed 8 studies that had reported that pheromones effect menstrual cycles. They found that women’s cycles do not sync, the apparent synchrony is due to cycle variability. This study concluded that it seemed reasonable to conclude that there is no hard evidence that mammals cycles can sync.

I set out looking for why they sync up, and came out to realize there is no hard evidence either way on this matter. Scientists have been unable to reject the null hypothesis that nothing is going on, but there are studies that are finding contradictory results. As science progresses it may be able to explain if cycles really do sync up, for example if we are able to find a mechanism that is experimentally withstanding, but until then we rest on the null hypothesis. Women living in close proximity do not sync their periods, there is nothing going on but cycle variability. So next time it’s your roommates time of the month no need to fret.