“Menstrual synchrony” is a popular idea that has its roots in a heavily circulated study about the timing of cycles among groups of college roommates. It made its way into just about every women’s magazine, dorm, and girls’ slumber party. Oh my god, I got my period too! Men became convinced that their teenage daughters and wives were on the same cycle and used this as an excuse to take extended fishing trips. Girls and women took comfort in thinking that even if they got caught without a tampon or pad, certainly a roommate would have a spare.
This perfect hormonal harmony isn’t real. Better and more recent studies do not find that women living together synchronise their cycle phases, and part of the misunderstanding may be due to the variability of cycle length. But first, let’s look at why this phenomenon seems implausible in the first place. For starters, it doesn’t simply mean women get their periods at the same time; menstrual synchrony means all phases of women’s cycles line up, including any effects on sexual and mating behaviours.
If females living together had synchronised cycles, they’d be at peak fertility at the same time. Consider the implications for ancestral women; those without steady or satisfying partners would have been forced to compete for the same men at the exact same time. But more importantly, what would be the benefit of synchronizing in the first place? The underlying physiology would be complex and metabolically (and reproductively) costly. A woman would first have to detect the cycle phase in her close friends and family and then adjust her own hormone cycle, perhaps shrinking her follicular phase (giving an egg insufficient time to mature) or rushing her luteal phase (perhaps giving her uterus less time to receive a fertilized egg or rejecting an implanted egg that had made it to the womb).
If all women cycled together, this would make it easier for outsiders to detect fertility— whereas it makes sense for women to keep signs of fertility on the “dark side of the moon.” In short, a good evolutionary rationale for why human females would have this complex strategy with its costly underlying physiology is just not there. (Perhaps there is a rationale for some other species in which ovulation is more clearly on display. Synchrony can allow for greater female choice of the fathers of their offspring because the pushiest males cannot dominate all of the fertile females at once).
the farther away women’s cycles are initially from one another, the more they will appear to sync up
The reason that it’s so easy to think that menstrual synchrony exists in humans is that “normal” cycles among a group of women can easily overlap — and appear to converge. For instance, let’s imagine four females sharing an apartment. Roommate A has a 28-day cycle; Roommate B has a 32-day cycle; Roommate C has a 25-day cycle; Roommate D never keeps track and also never remembers when the rent is due, even though it’s the same time every month. Roommate A ovulates on or about Day 14, B about Day 16, C about Day 12 or 13, D can’t tell you. All of them have regularly occurring periods, but some menstruate for two to three days and others closer to a week. A’s third and final day of menstruation happens to be Day 1 for B, and it’s also the day before C gets her period. Meanwhile, D is convinced she’s getting her period any day now.
Do you see where this is going? It’s inevitable that phases of the cycle will overlap at some point. Moreover, the farther away women’s cycles are initially from one another, the more they will appear to sync up. There’s only one direction to go: closer — just due to chance alone. (This phenomenon is called regression towards the mean, which is well-known to statisticians and can explain a variety of illusory phenomena, such as when the economy bottoms out and then is apparently improved by some new policy or leader).
There is simply no solid evidence that women somehow synchronise their hormonal activity when they’re living in close proximity, and there’s no good reason why we would have evolved to do so. If you ever hear a husband, father or brother complain about all the women in the household acting “hormonal” at the same time (and hogging the bathroom), you can safely say that the only time women truly cycle together is when they’re on a group bike ride (or by random chance).