Any woman can relate, and any man within eyeshot has probably witnessed the following scenario: A woman suffering from mind-numbing cramps, hobbling over to the closest female coworker’s desk for pharmaceutical relief, and bonding over their synced cycles. If women spend enough time around female friends or co-workers, they’re bound to get on the same schedule… right? Greatist examines the science behind the sync.

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Sync Up or Sex Up — The Need-to-Know

In 1971, a study speculated that regular contact with other women could affect female’s menstrual cycles. Researchers found social interaction was key to matched cycles, and close friends and women who were in contact for a substantial portion of every day (say, hanging out or working together) were more likely to sync up than women who merely lived in proximity to each other (e.g., on the same floor of a dorm) The impact of social interaction factors on menstrual synchrony in the workplace. Weller, A. and Weller, L. Department of Psychology, Bar-Ilan University, Ramat-Gan, Israel. Psychoneuroendocrinology, 1995;20(1):21-31 Menstrual synchrony: only in roommates who are close friends? Weller, L., Weller, A., Avinir, O. Department of Sociology, Bar-Ilan University, Israel. Physiology & Behavior, 1995 Nov;58(5):883-9 . Follow-up studies suggested pheromones were responsible and that “menstrual synchrony” (the technical term for syncing) might be just the tip of the iceberg: It’s possible periods and ovulation could be socially regulated throughout a woman’s lifespan, from puberty to pregnancy and everywhere in between Whither menstrual synchrony? McClintock, MK. Department of Psychology, University of Chicago, Illinois. Annual Review of Sex Research, 1998;9:77-95 Positive relationship between menstrual synchrony and ability to smell 5alpha-androst-16-en-3alpha-ol. Morofushi, M., Shinohara, K., Funabashi, T., et al. Department of Physiology, Yokohama City University School of Medicine, Japan. Chemical Senses, 2000 Aug;25(4):407-11 .

But while many researchers uphold the syncing theory, boy has there been a backlash. The pro-syncing crowd has been criticized for poor research methods, and the existence of natural synchrony hasn’t been confirmed in non-human primate studies A critical review of menstrual synchrony research. Wilson, HC. Department of Anthropology, University of Missouri- Columbia. Psychoneuroendocrinology, 1992 Nov;17(6):565-91 Menstrual-cycle variability and measurement: further cause for doubt. Schank, JC. Department of Psychology, University of California. Psychoneuroendocrinology, 2000 Nov;25(8):837-47 Is 3alpha-androstenol pheromone related to menstrual synchrony? Jahanfar, S., Awang, CH, Rahman, RA, et al. Royal College of Medicine, University of Kuala Lumpur, Malaysia. The Journal of Family Planning and Reproductive Health Care, 2007 Apr;33(2):116-8 Proximity and estrous synchrony in Mahale chimpanzees. Matsumoto-Oda, A., Kasuya, E. Department of Welfare and Culture, Okinawa University, Japan. American Journal of Primatology, 2005 Jun;66(2):159-66 Measurement of cycle variability: reexamining the case for ovarian-cycle synchrony in primates. Schank, JC. Department of Pscyhology, University of California. Behavioural Processes, 2001 Dec 3;56(3):131-146 .

In fact, synced cycles might not make sense evolutionarily. 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 You mate, I mate: macaque females synchronize sex not cycles. Furtbauer, I., Mundry, R., Heistermann, M., et al. Primate Social Evolution Group, Courant Research Centre Evolution of Social Behavior, Germany. PLoS One, 2011;6(10):e26144 . Instead of having matched cycles, women might unknowingly sync up phases of “sexual receptivity” and the lack thereof — meaning one woman is ready to get pregnant while another is just starting to menstruate, giving fertile women a more diverse choice of mates Menstrual-cycle synchrony: problems and new directions for research. Schank, JC. Department of Psychology, University of California. Journal of Comparative Psychology, 2001 Mar;115(1):3-15 . (Of course, these are all very biological arguments, operating on the presumption that every woman’s goal in life is to get pregnant.)

Menstrual Mystery — The Answer/Debate

Unfortunately, there isn’t a definitive answer to the science of syncing, says Frederick Naftolin, Professor of Obstetrics and Gynecology and Director of Reproductive Biology Research at NYU. Synchronization is possible — in non-human animals, anyway. Manually matching up cycles (typically with the hormone progesterone) is key to many agricultural areas that rely on artificial insemination of cows, goats, and buffalos, as synchrony allows for simultaneous impregnation of a herd Advances in modeling of the bovine estrous cycle: Synchronization with PGF2α. Stotzel, C., Plontzke, J., Heuwieser, W., et al. Computational Systems Biology Group, Numerical Analysis and Modeling, Zuse Institute, Berlin. Theriogenology, 2012 Oct 15;78(7):1415-28 . But evidence for naturally-occurring human synchrony hasn’t been confirmed via adequately controlled, randomized trials.

Still, some researchers strongly maintain menstrual synchrony is real Menstrual synchrony and cycle variability: a reply to Schank. Weller, L. and Weller, A. Department of Sociology, Bar-Ilan University, Israel. Psychoneuroendocrinology, 2002 May;27(4):519-26 Menstrual synchrony can be assessed, inherent cycle variability notwithstanding: commentary on Schank. Weller, A. and Weller, L. Department of Psychology, Bar-Ilan University, Israel. Journal of Comparative Psychology, 2002 Sep;116(3):316-8 . Anecdotal evidence certainly seems to support the idea of syncing, says Martha Thomas, Associate Residency Program Director in the OB/GYN department at York Hospital/Wellspan Health. And it’s hard to say how factors like stress, sexual partners, and birth control play into the syncing game — if synchrony does exist, it’s possible these factors override it, making matched cycles appear less common than they might actually be The effect of job stress and job interdependency on menstrual cycle length, regularity and synchrony. Matteo, S. Department of Psychology, Stanford University. Psychoneuroendocrinology, 1987;12(6):467-76 .

But this is mostly speculation, stresses Thomas. Most contemporary research holds that it’s unlikely women will actually sync up month after month after month, especially because the length of individuals’ cycles can vary so dramatically Avoiding synchrony as a strategy of female mate choice. Schank, JC. Department of Psychology, University of California. Nonlinear Dynamics, Psychology, and Life Sciences, 2004 Apr;8(2):147-76 . What seems most likely is that women don’t actually synchronize, but rather have the occasional synced period when their cycles’ lengths randomly overlap.

Surprised? Us too. But we’ll still share the Midol, whether or not we’re suffering too.

Thanks to Dr. Frederick Naftolin and Dr. L. Martha Ann Thomas for their help with this article.

Do your experiences contradict the science on this one? Let us know in comments below, or get in touch with the author on Twitter @LauraNewc.