If you’ve ever had a close group of girlfriends or lived with roommates, then you’ve probably complained together about cramps, cravings, irritability and a myriad of menstrual unpleasantries. It’s a classic bonding moment from girlhood when a group realizes that they’re “synced”, i.e. their cycles have matched up. In its own strange way, a synced cycle is a moment of shared sisterhood by bonding together over some mystical uterus magic. But is “syncing up” grounded in biological truth or is it just pseudoscience?
Menstrual synchrony is also referred to as the “McClintock effect,” named after Martha McClintock, who performed the first study on the phenomenon of syncing in 1971. McClintock studied women who lived together in a Wellesley dormitory and had frequent social interaction with one another. The study found that after a few months of living together, the “mean onset difference” (difference in time of the first day of menstruation) reduced by two days. McClintock proposed that pheromones were the culprit behind all the matching cycles. Pheromones are unconscious chemical signals that influence the body and behavior and can be released through sweat, odors, and other bodily fluids. This seems to make sense because the closer women were in direct proximity through living arrangements and through frequent interactions, the more exchange of these pheromones that influenced each other’s cycle.
McClintock also pointed out that menstruation is “socially regulated,” in that women who were merely living together did not sync up as much as women who were close friends and had social and emotional attachments. This could explain why you and your best friend might sync, but not necessarily the girl who lives in the room one door over.
Even more interesting is a study conducted by the Sonoma State Hospital in which sweat samples from “menstrual pacesetters” (women who “make other women conform to their cycle”) was directly applied to other women. Four of the five who received the swabs were shown to sync to the cycle of the original donor within a few months. These results seem to confirm that pheromones from other women induce a synced cycle, no emotional connection or proximity necessary. Whether it’s socially regulated or chemically induced or some form of both isn’t completely clear from all the experiments.
But it sounds like some pretty solid scientific evidence right? Well, not exactly. Several notable follow-up experiments were conducted in an effort to recreate McClintock’s findings. Experiments involved women attending a Chinese university to a remote village in Dogon. Some researchers chalk up syncing to nothing more than coincidence of overlapping cycles. After all, every woman has a unique menstrual cycle with different starting dates and durations. It is highly unlikely that multiple women will have matching cycles of the exact same amount of time month after month because of this high degree of variability. It could be that women often perceive a synced cycle as more than just coincidence. And although the study about the menstrual pacesetters and the sweat-induced pheromone syncing is quite interesting, it involved an incredibly small sample size and thus shouldn’t be taken at full face value.
Other evidence shows that no other species such as rats or chimpanzees show signs of menstrual syncing within communities with several females members. One argument supports that syncing makes no evolutionary sense as, “Syncing up in a hunter-gatherer tribe, for example, would mean that for a week or so, no woman would be able to get pregnant — not ideal when you’re responsible for procreating the human race.”
The scientific community remains without a definitive verdict on the matter as of now. But despite some of the evidence in opposition to the existence of syncing, some women will swear by it, citing their best friends, roommates, and sisters as proof. Perhaps more than anything, the perception of syncing is a sort of way we seek solidarity and a common ground. After all, it is nice to have a stash of chocolates to share with a friend when the cramps get rough.
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