When women live together, do their menstrual cycles tend to synchronize? It’s been a long time since I first heard that claim. I didn’t believe it, for a number of reasons. I had never observed it myself, I saw no plausible mechanism to explain how it could happen, I thought the statistics to prove it would be problematic and complicated, and I suspected that confirmation bias and selective memory might have persuaded people that a spurious correlation existed. How often do women say “Oh, look! We’re having our periods at the same time”? How often do they say “Oh, look! We’re having our periods at different times”? Now that many years have passed since my first encounter, I thought it would be fun to revisit the claim and see whether science has supported it or rejected it.
A perusal of PubMed and other Internet sources left me confused and amused.
Synchrony Is Difficult to Define
Consider that the normal menstrual cycle can vary from 21 to 35 days and can last 2 to 7 days. Consider that some women are regular and consistent, while others have variable patterns, even “regularly irregular” patterns. Consider that anovulatory cycles and other conditions often lead to menstrual irregularities that fall outside the normal range. Consider that strenuous exercise and other life events can affect menstruation. Put all that together, and you can see that often cycles will overlap simply by chance, and that it is difficult to define synchrony.
If two women have regular 28 day cycles and 7 day periods, the maximum number of days they could not overlap is 14. On average, their periods will be 7 days apart, and half the time they will be closer.
How could a 21 day cycle ever “synchronize” with a 35 day cycle? For example if you compare a woman with a regular 35 day cycle who starts on January 1 to a woman with a 21 day cycle who starts two weeks later on January 15, their next periods will coincide almost perfectly (Feb 4-10 and Feb 5-11) but they will diverge after that. Would it count if the last day of one woman’s period overlapped with the first day of another woman’s? What if half the periods coincide and half don’t? The whole thing is problematic.
What Does the Literature Say?
It all started with Martha McClintock. In a paper published in Nature in 1971 she found that “social interaction” in a college dormitory setting could have a strong effect on the menstrual cycle. A follow-up study in 1998 tended to support the hypothesis that pheromones were involved: smelling armpit secretions of other women could either lengthen or shorten cycles depending on what part of her cycle the donor was in.
I’ll summarize rather than trying to cover everything published on the subject. A Scientific American article did a good job of reviewing the literature as of 2007. Suffice it to say that about half the published papers support the synchronization hypothesis and half don’t; and the half that do have been harshly criticized for their poor design and poor statistical analyses. So we haven’t reached a consensus, but it’s looking more likely that synchronization is a myth.
A study in a nursing journal assumes that synchronization occurs and addresses the subjective meaning of the experience to
assist nurses to understand the holistic aspects of this everyday experience of women and to design effective strategies and techniques to help women gain knowledge about their cycle functions, promote healthy attitudes toward menstruation as a process, and acknowledge and honor this natural, healthy aspect of their menstrual cycle.
I will be kind to those nurses and apply Thumper’s rule. (In Bambi, he said “If you can’t say something nice, don’t say nothing at all.”)
What About Non-Humans?
It was originally thought that other primates and other mammals demonstrated menstrual synchrony due to pheromones, but recent studies have shown that it doesn’t occur in chimpanzees, hamsters, mandrills, or golden tamarins.
Do Human Pheromones Exist?
The existence of human pheromones is controversial. Pheromones are chemical signals released by one individual that affect the behavior of another individual of the same species. They can be useful for alarms, food trails, sex attractants, and other purposes. They have been well documented in insects and even in plants. In one memorable demonstration, a drop of oleic acid was all it took to persuade other ants that a live ant was dead; they dragged it back to the trash heap over and over, ignoring the fact that it kept protesting its trips to the ant graveyard by vigorously kicking, and that it kept “coming back from the dead.”
In humans, pheromones have been postulated and even sold as sexual attractants. But there is little or no peer-reviewed evidence to suggest that any pheromone influences human behavior. No human pheromones have been identified, and the vomeronasal organ that detects pheromones in other mammals is rudimentary and nonfunctional in humans.
Other Explanations
Not knowing if it occurs hasn’t stopped people from speculating about why it occurs or about why it doesn’t. The evolutionary reasoners have chimed in with just-so stories both about why it would and wouldn’t offer a survival benefit. For instance, if everyone ovulated at the same time, it would be harder for a woman to get a man. Or being banished to the menstrual hut at the same time would allow women to bond and collaborate on social enterprises. I was particularly amused by one chain of reasoning:
Synchronization also doesn’t make sense because during their mentrual cycles, women often experience lack of energy. These spells of low energy among women in a tribe would be detrimental to a hunter-gatherer lifestyle. Furthermore, if all of the women get pregnant at the same time, they won’t be able to help each other. Given our anthropological history, evolution should select that women do not cycle together. (errors in the original)
And this comment on the Scientific American article left me rolling on the floor:
Astrology explains this for better than modern science. Normally close friends have similar sun, moon positions in their horoscopes. The moon and mars determine the menstrual cycles. Astrological harmony makes them friends than living together which in turn is the cause of synchronization. Even if they are living far in different continents they will be in sync. In India any normal astrologer can check a woman’s horoscope and can tell the date of her menstruation time ( especially used when they have to attend auspicious ceremonies) and plan the events. (errors in the original)
Conclusion
Do women’s menstrual cycles tend to synchronize when they live together? As so often in medicine, science doesn’t provide a clear answer. Even if the phenomenon occurs, and even if human pheromones cause it, so what? It’s a matter of curiosity, not of clinical importance. When there is a reason to manipulate ovulation or menstrual timing, pharmaceutical hormones work very well. The paucity of evidence for human pheromones suggests that if they do exist, their effects must be too small in magnitude to be very important.
- Yes, But. The Annotated Atlantic. - November 7th, 2009 [November 7th, 2009]
- Health Insurance Benefit Costs by Region - November 8th, 2009 [November 8th, 2009]
- For an Operator, Please Press... - November 8th, 2009 [November 8th, 2009]
- Pollyanna With a Pen: Maine Governor Signs 18 New Health Care Bills into Law - November 8th, 2009 [November 8th, 2009]
- AMA Sounds the Alarm, Medicare Making Yet Another Attempt to Cut Reimbursement - November 8th, 2009 [November 8th, 2009]
- Mass Governor Asks Blue Cross to Keep Higher Employer Contribution - November 8th, 2009 [November 8th, 2009]
- Lifespan and Care New England Plan Monopoly (Again) - November 8th, 2009 [November 8th, 2009]
- Dirigo Health: Con Artists, Liars, and Thieves? - November 8th, 2009 [November 8th, 2009]
- New Orleans: Health Challenges - November 8th, 2009 [November 8th, 2009]
- August a Flurry of Activity - November 8th, 2009 [November 8th, 2009]
- Maine's Dirigo Health Savings One-Third of Original Estimate - November 8th, 2009 [November 8th, 2009]
- “Methodolatry”: My new favorite term for one of the shortcomings of evidence-based medicine - November 8th, 2009 [November 8th, 2009]
- Suzanne Somers’ Knockout: Dangerous misinformation about cancer (part 1) - November 8th, 2009 [November 8th, 2009]
- A science-based blog about GMO - November 8th, 2009 [November 8th, 2009]
- A Not-So-Split Decision - November 8th, 2009 [November 8th, 2009]
- Military Medicine in Iraq - November 8th, 2009 [November 8th, 2009]
- The effective wordsmithing of Amy Wallace - November 8th, 2009 [November 8th, 2009]
- A Science Lesson from a Homeopath and Behavioral Optometrist - November 8th, 2009 [November 8th, 2009]
- Join CFI in opposing funding mandates for quackery in health care reform - November 8th, 2009 [November 8th, 2009]
- Mainstreaming Science-Based Medicine: A Novel Approach - November 8th, 2009 [November 8th, 2009]
- Those who live in glass houses… - November 8th, 2009 [November 8th, 2009]
- J.B. Handley of the anti-vaccine group Generation Rescue: Misogynistic attacks on journalists who champion science - November 8th, 2009 [November 8th, 2009]
- When homeopaths attack medicine and physics - November 8th, 2009 [November 8th, 2009]
- The cancer screening kerfuffle erupts again: “Rethinking” screening for breast and prostate cancer - November 8th, 2009 [November 8th, 2009]
- All Medicines Are Poison! - November 8th, 2009 [November 8th, 2009]
- When Loud Wins: Will Your Tax Dollars Pay For Prayer? - November 8th, 2009 [November 8th, 2009]
- It’s All in Your Head - November 8th, 2009 [November 8th, 2009]
- The Skeptical O.B. joins the Science-Based Medicine crew - November 8th, 2009 [November 8th, 2009]
- The Tragic Death Toll of Homebirth - November 8th, 2009 [November 8th, 2009]
- What’s the right C-section rate? Higher than you think. - November 8th, 2009 [November 8th, 2009]
- Recombinant Human Antithrombin – Milking Nanny Goats for Big Bucks - December 13th, 2009 [December 13th, 2009]
- Does C-section increase the rate of neonatal death? - December 13th, 2009 [December 13th, 2009]
- Man in Coma 23 Years – Is He Really Conscious? - December 13th, 2009 [December 13th, 2009]
- Why Universal Hepatitis B Vaccination Isn’t Quite Universal - December 13th, 2009 [December 13th, 2009]
- Ontario naturopathic prescribing proposal is bad medicine - December 13th, 2009 [December 13th, 2009]
- Naturopaths and the anti-vaccine movement: Hijacking the law in service of pseudoscience - December 13th, 2009 [December 13th, 2009]
- The Institute for Science in Medicine enters the health care reform fray - December 13th, 2009 [December 13th, 2009]
- Neti pots – Ancient Ayurvedic Treatment Validated by Scientific Evidence - December 13th, 2009 [December 13th, 2009]
- Early Intervention for Autism - December 13th, 2009 [December 13th, 2009]
- A temporary reprieve from legislative madness - December 13th, 2009 [December 13th, 2009]
- A critique of the leading study of American homebirth - December 13th, 2009 [December 13th, 2009]
- Lose those holiday pounds - December 13th, 2009 [December 13th, 2009]
- Endocrine disruptors—the one true cause? - December 13th, 2009 [December 13th, 2009]
- Acupuncture for Chronic Prostatitis/Chronic Pelvic Pain Syndrome - December 13th, 2009 [December 13th, 2009]
- Evidence in Medicine: Experimental Studies - December 13th, 2009 [December 13th, 2009]
- Midwives and the assault on scientific evidence - December 13th, 2009 [December 13th, 2009]
- The Mammogram Post-Mortem - December 13th, 2009 [December 13th, 2009]
- An Influenza Recap: The End of the Second Wave - December 13th, 2009 [December 13th, 2009]
- The End of Chiropractic - December 13th, 2009 [December 13th, 2009]
- Cell phones and cancer again, or: Oh, no! My cell phone’s going to give me cancer! (revisited) - December 20th, 2009 [December 20th, 2009]
- Another wrinkle to the USPSTF mammogram guidelines kerfuffle: What about African-American women? - December 20th, 2009 [December 20th, 2009]
- Acupuncture, the P-Value Fallacy, and Honesty - December 20th, 2009 [December 20th, 2009]
- The One True Cause of All Disease - December 20th, 2009 [December 20th, 2009]
- Communicating with the Locked-In - December 20th, 2009 [December 20th, 2009]
- Are the benefits of breastfeeding oversold? - December 20th, 2009 [December 20th, 2009]
- Measles - December 20th, 2009 [December 20th, 2009]
- Radiation from medical imaging and cancer risk - December 21st, 2009 [December 21st, 2009]
- Multiple Sclerosis and Irrational Exuberance - December 21st, 2009 [December 21st, 2009]
- Medical Fun with Christmas Carols - December 22nd, 2009 [December 22nd, 2009]
- Lithium for ALS – Angioplasty for MS - December 23rd, 2009 [December 23rd, 2009]
- “Toxins”: the new evil humours - December 24th, 2009 [December 24th, 2009]
- 2009’s Top 5 Threats To Science In Medicine - December 24th, 2009 [December 24th, 2009]
- Buteyko Breathing Technique – Nothing to Hyperventilate About - December 26th, 2009 [December 26th, 2009]
- The Graston Technique – Inducing Microtrauma with Instruments - December 29th, 2009 [December 29th, 2009]
- The “pharma shill” gambit - December 29th, 2009 [December 29th, 2009]
- Ginkgo biloba – No Effect - December 30th, 2009 [December 30th, 2009]
- Oppose “Big Floss”; practice alternative dentistry - January 1st, 2010 [January 1st, 2010]
- Causation and Hill’s Criteria - January 3rd, 2010 [January 3rd, 2010]
- The life cycle of translational research - January 10th, 2010 [January 10th, 2010]
- The anti-vaccine movement strikes back against Dr. Paul Offit - January 10th, 2010 [January 10th, 2010]
- Osteoporosis Drugs: Good Medicine or Big Pharma Scam? - January 10th, 2010 [January 10th, 2010]
- Acupuncture for Hot Flashes - January 10th, 2010 [January 10th, 2010]
- The case for neonatal circumcision - January 10th, 2010 [January 10th, 2010]
- A victory for science-based medicine - January 10th, 2010 [January 10th, 2010]
- James Ray and testosterone replacement therapy (TRT) - January 10th, 2010 [January 10th, 2010]
- The Water Cure: Another Example of Self Deception and the “Lone Genius” - January 12th, 2010 [January 12th, 2010]
- Be careful what you wish for, Dr. Dossey, you just might get it - January 13th, 2010 [January 13th, 2010]
- You. You. Who are you calling a You You? - January 15th, 2010 [January 15th, 2010]
- The War on Salt - January 16th, 2010 [January 16th, 2010]
- Is breech vaginal delivery safe? - January 16th, 2010 [January 16th, 2010]