I remember hearing in college that women who live together develop "menstrual synchrony" — their menstrual cycles somehow change to match those of other women around them.
There's debate if menstrual synchrony is real or not. When I told my wife I was thinking of writing about the myth of menstrual synchrony, she told me I was crazy. "There is no myth," she said. "It's clearly true and that's all there is to it." So, I may not tell her about this posting.
What's Behind Menstrual Synchrony?
The first description of menstrual synchrony was published in 1971. A graduate student at Wellesley College noticed menstrual synchrony in her dormitory. So, she studied 135 female fellow students and found that women who were closest tended to cycle together. It was nothing short of fascinating.
It implied that some mysterious communication system "bonded" the hormonal systems of women. The idea that women can biologically bond in this way may explain why menstrual synchrony is such an appealing idea.
Researchers suggested that airborne chemical signals, called pheromones, could align the menstrual cycles of women who are close. In other words, the smell of one person might play an important role in the biologic functions of another.
Chemical Attraction Among Other Species
Experts don't agree on whether pheromones and menstrual synchrony even exist in humans, and if these chemical signals control menstrual synchrony.
However, it's been documented that moths, for example, communicate through chemical signals. I'm told that the male moth can sense the presence of a distant fertile female and will drop what he's doing to mate. Sea urchins use "chemical attractants" as well. They release substances into the water that tell others it's time to reproduce. The insect and animal kingdoms are teeming with similar stories.
What about primates other than humans? Glad you asked! A 2011 study reviewed the evidence of menstrual synchrony for non-human primates and found no convincing evidence of menstrual synchrony. And the authors presented data on decades of menstrual cycles from mandrills (a relative of the baboon found in the tropical jungles of western-central Africa) that also demonstrated no evidence of menstrual synchrony.
Some studies have supported the idea that women tend to cycle together. There's even been research "proving" that pheromones exist in humans (though not necessarily involved in menstrual synchrony). For example, a study from the late 1990s found that the scent of another woman's perspiration could speed up or slow down ovulation and change the length of menstrual cycles. The effect varied depending on when during the menstrual cycle the sweat was collected.
And is there any convincing evidence that pheromones are important in human interactions?
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Do Humans Respond to Pheromones?
Several studies — mostly small and uncontrolled — suggest that the smells of certain chemicals can influence human sexual behavior.
For example, a 2002 study published in Physiology and Behavior tested the effect of a pheromone added to the usual perfume of 36 single women. None of the women knew whether she had the "sexual attractant" or placebo pheromone in her perfume. Both were odorless. After 14 weeks, 74% of the women who had the pheromone reported an increase in sexual behaviors (including frequency of dating, kissing, sexual intercourse and sleeping next to their partners). Only 23% of those who had the placebo reported this type of increase. We don't know, however, how much the partners' responses to pheromones affected these findings.
Then again, the Web is full of articles selling pheromones to attract mates, improve sex lives, and "blind" others to physical attractiveness. Yes, there's actually a study that showed photographs of women to men who were asked to rate their attractiveness. The men exposed to the supposed pheromone gave women, particularly those who were "most plain," higher ratings.
But there are literally hundreds of pheromones. The exact chemical make-up and action of all of them are still a mystery. So we can't yet single out the one that influences sexual behavior or interest.
If pheromones do exist, how do humans sense them? Animals that clearly respond to pheromones have a specialized nerve cell in the nose, called the "vomeronasal organ." It is separate from odor detection. Some researchers believe that tiny crevices in the human nose act as vomeronasal organs. They might be able to sense pheromones and send signals to the brain that are relayed to the complex hormonal system that regulates menstruation and, perhaps, other biologic processes involved in reproductive behavior.
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Is Menstrual Synchrony a Myth?
Many well-known media sources accept the idea of menstrual synchrony controlled by pheromones:
Washington Post, 1998: "...research settles a 40-year debate about whether humans produce and can respond to "pheromones."
CNN, 1998: "Study finds proof that humans react to pheromones"
BBC, 1998: "Pheromones are odourless chemical messengers secreted by animals and humans and are picked up by the nose at a subconscious level. They are mainly known for the role they play in sexual attraction, but much is unknown about what they do and how they work."
But what does the scientific evidence say? While many studies have found evidence of menstrual synchrony, other, more recent ones have not. They claim that:
- Women typically have menstrual cycles of different lengths. To synchronize, they would not only have to alter timing but also the length of cycles.
- It could easily appear over a short period of time that women are synchronized due to the temporary overlapping of cycles due to chance.
In fact, that's just the explanation given in several articles published in the journal Human Nature. In one of the largest studies, there was no evidence of menstrual synchrony in 186 women living in dormitories in China for more than a year. Another recent study in Poland studied 99 women living in a dorm over five months and also found no evidence of menstrual synchrony.
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The Bottom Line
Does it matter if menstrual synchrony is real or not? Maybe. If there's a smell that can alter menstruation by slowing down or speeding up ovulation, it could be used to treat infertility or to prevent pregnancy. And if that smell makes women synchronize their periods, the field of human pheromones would be confirmed. This could open up a vast and potentially useful area of biological research. Many people are already looking into it. As for the question of menstrual synchrony, there is still no clear answer. But don't try telling that to my wife.
McClintock, M. "Menstrual synchrony and suppression." Nature. 1971; 229, 244-245.
Stern K, McClintock MK. "Regulation of ovulation by human pheromones". Nature. 1998; 392(6672): 177–9.
McCoy, NL, Pitino, L. "Pheromonal influences on sociosexual behavior in young women." 2002; 75, 367–375.
Setchell JM, Kendal J, and Tyniec P. "Do non-human primates synchronise their menstrual cycles? A test in mandrills." Psychoneuroendocrinology. 2011; 36(1):51-59.
Yang, Z, Schank, JC. "Women do not synchronize their menstrual cycles." 2006; 17(4):433-447.
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Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.