Oh my! We had no idea that we needed a PhD in statistics to read about this topic. There have been about thirty years of research about the possibility that women who spend a lot of time together can synchronize their menstrual cycles. We invite you to delve into the annals of research on estrous synchrony or menstrual synchrony yourself and read about the controversy surrounding this topic. It is exciting stuff!

In 1971, Martha McClintock, a psychologist, published a paper that first reported this phenomenon. Since then, many scientists and statisticians have investigated the possibility and haven’t found any link. There is no physiological explanation as to why this happens. But, based on statistical analysis-in other words, simply by sheer coincidence-some overlap of periods can be expected in any group of friends.