You’ve probably all heard about how close friends’ menstrual cycles sync up when they live together, but is there any truth to it or is it simply an urban legend? And can the synchronicity occur with people you only interact with online? There’s plenty of anecdotal data, but let’s see what science has to say about it.
Studies on menstrual synchrony have had mixed results. The first study of the phenomenon was published in 1971; after psychologist Martha McClintock noticed that many close friends in her all-girls dorm had their periods at the same time, she polled 135 dorm dwellers about when their periods started at three different times during the school year. Her study found that in October close friends started their periods 6.4 days apart on average, while in April, after they’d been living together longer, the difference was down to 4.6 days. However, a 1992 re-examination of her data by H. Clyde Wilson of the University of Missouri”“Columbia found that the initial data had been misinterpreted and the difference thus became statistically insignificant. Other studies have been inconsistent, with one grouping apparently demonstrating synchrony and another not, with no discernible explanation as to why.
Part of the problem may be with how the studies are performed. Simply looking at a snapshop of period start dates doesn’t necessarily tell you much about whether their cycles were affected, or simply following their usual but different patterns. As an example, let’s look at four hypothetical students as seen in the chart below. When they move in together at the start of the school year, A & C have 28 day cycles and B & D have 31 day cycles, with B & C starting their first cycle of the school year on August 15 and A & D starting on September 1. If you simply take a snapshot of their cycles in January, students A & B appear to have synchronized, as have C & D. However, D has actually gone through one cycle fewer than C. By May, the apparent synchronous pairs are once again A & D and B & C, though in both pairs one student has had more cycles than the other. All of the students have retained their usual patterns, but picking and choosing what data you look at can give you the impression that there’s something else at work.
There have been several studies that show that ovulation can be affected by exposure to female pheromones. In a separate study from the one mentioned above, McClintock was able to alter some women’s cycles by exposing them to underarm secretions from other women who were about to ovulate. Other studies seem to point to similar results with the same secretions or with exposure to secretions from breastfeeding women. While this is interesting and merits further study, researchers still don’t know why some women respond and not others, and why women who do seem to synchronize don’t stay in synchronous cycles over a long stretch of time. Also, syncing ovulation doesn’t necessarily cause periods to sync up; due to varying menstrual cycle lengths, it can actually push period start dates farther apart.
While it hasn’t been specifically studied, I can say with some certainty that periods absolutely cannot sync up over the internet. I know, it’s fun to joke about it and it’s therapeutic to know others are going through PMS at the same time as you, but it’s mostly due to coincidence, sample size, and observational bias. If you complain about cramps on tumblr and five friends chime in that they’re about to start their period too and several other say they just started, it can seem like there’s a mechanism at work affecting all of you. But if you complain the next month, some of the other PMSers may have already started, some who had already started last month may just be PMSing, some may not be having their period at all, and others may chime in who were unaffected last month. If you have, say, 180 followers and (based on some highly unscientific polling of my friends) about 2/3 have a monthly period and many others have periods every three months or so due to their birth control, on any given day of the month about five of them should be starting their cycle, and a few dozen more will have already started or will with a few days. That’s not syncing, it’s math. You don’t notice the other 100 or so who aren’t menstruating, so it seems like a bigger deal. This applies to living in dorms as well. When you have several hundred women living together, of course some are going to start their periods at the same time. You notice it more amongst your friends because you’re more likely to talk to them about it than someone you aren’t close with. Sorry!