The menstrual synchrony theory asserts that the menstrual cycles of women who live together (such as in homes, prisons, convents, bordellos, dormitories, or barracks) can become synchronized over time. The existence of menstrual synchrony has not been definitively established, and studies investigating it have been controversial.
Research on human menstrual synchrony, also call the McClintock Effect, or the Wellesley Effect  is related to the larger question of whether or not humans have or perceive pheromones and utilize chemosignaling.  
Another study by McClintock and Stern (1998) investigated the possible existence of pheromone release/perception by human females as the possible mechanism for manipulation of the human menstrual cycle, the idea being that humans produced compounds that regulate neuroendocrine mechanisms in others, without the other party being aware of this event or consciously detecting any odor. The study was conducted by collecting compounds from axillae (underarms) of donor women at prescribed phases during their menstrual cycles, and applying the compounds daily under the noses of recipient women. In order to collect the axillary compounds, the donor women wore cotton pads under their arms for at least 8 hours, and then the pads were cut into smaller squares, frozen to preserve the scent, and readied for distribution to the recipients. The recipients were split into two groups, and were exposed to the compounds via application of the thawed axillary pad under their noses daily.
The researchers concluded that odorless compounds collected from women during the late follicular phase of their menstrual cycles triggered hormonal events that shortened the menstrual cycles of the recipient women, and that odorless compounds collected from women during the time of ovulation triggered a hormonal event in the recipient women that lengthened their menstrual cycles. McClintock and Stern asserted that these findings "proved the existence of human pheromones" as well as illustrated manipulation of the human menstrual cycle.
A different study by Weller and Weller experimented with 20 lesbian couples in which results showed that more than half of the couples tested had the same synchronization within a two-day timespan of each other. However, Trevathana et al. conducted a study of 29 lesbian couples that showed no evidence of synchrony, and tentative evidence towards divergence of menstrual cycles.
Human pheromones and chemosignals have also been studied in different contexts with regard to manipulation of the human menstrual cycle. Jacobs et al. examined the effects of natural compounds taken from breastfeeding women and their infants on the cycle length and cycle variance of recipient women. Axillary and breast secretions were taken from women who were exclusively breastfeeding and had not yet resumed menstruating. Recipient women met a set of selection criteria and were blind to the study aims and conditions. During the study, recipient women were exposed daily to the collected compounds, and the resulting effects on their menstrual cycles were tabulated. Exposure to compounds from breast feeding women and their infants increased variance in average cycle length among the women, and disrupted the cycles’ regulation. It also enhanced individual differences in cycle length and maintained the type of cycle length that a particular individual had at the time of exposure.
McClintock also conducted a 1978 study of menstrual synchrony in rats (Rattus norvegicus). This study found that the menstrual cycles of female rats living in groups of five were more regular than those of rats housed singly. Social interaction, and more importantly a shared air supply that allowed for olfactory communication but prohibited auditory cues, enhanced the regularity of the rats’ cycles and coordinated their estrous phases after two or three cycles, resulting in synchronization. It is not known whether this synchrony and enhancement is the result of signals from a "leader" within the female group or if it is due to the interaction of several equal signals from each group member.
This observation of menstrual synchrony in R. norvegicus is not the same as the Whitten Effect because it was the result of the continuous interactions of ongoing cycles within a female group, rather than the result of an exposure to a single external stimulus such as male odor, which in the Whitten Effect releases all exposed females simultaneously from an acyclic condition.
Adaptive value/function 
The phenomena of menstrual synchrony could serve several purposes depending on the mating and birthing cycles of a particular species. It could perhaps set the context for group mating (such as in lions) or serve to facilitate male sexual performance because of the increased presence of receptive females (rats), and could also help to ensure pregnancy.
||This article's Criticism or Controversy section may compromise the article's neutral point of view of the subject. (May 2013)|
If all women had an average-length menstrual cycle of 28 days duration, the maximum time between two women's onsets would be 14 days and the minimum time between onsets would be zero days (synchronization). On average, the difference would be seven days, and (in small groups) half the time would be less (if one assumes there is no McClintock effect). McClintock observed a five-day difference in her 1971 study and some have suggested this could have been a random occurrence.
The interaction of theorized menstrual synchrony with differing cycle lengths has not been explained. Two women with cycle lengths that differed by two days might initially begin menstruating on the same day, but the next month would be two days apart, the month after that four days, and so on. No studies have claimed to show that the McClintock effect causes women with historical cycles of different lengths to synchronize.
Methodological errors have also been proposed. A critical review of the evidence for menstrual synchrony gave this example:
Suppose a study starts on October 1. Subject A, with a 28-day cycle, has an onset on September 27, another on October 25, and a third on November 22. Subject B, with a 30-day cycle, has an onset on October 5 and another on November 4. A naive investigator could report that these subjects were 20 days apart at the outset (October 25 vs October 5) and 18 days apart at their second onset (November 4 vs November 22). Therefore, the two are synchronizing. In fact, the two subjects were eight days apart to start with (September 27 vs October 5) and are diverging.
This type of error is more likely in smaller sample sizes, like those used in studies of menstrual synchrony.
H. Clyde Wilson of the University of Missouri analyzed the research and data collection methods McClintock and others used in their studies. He found significant errors in the researchers' mathematical calculations and data collection as well as an error in how the researchers defined synchrony. Wilson's clinical research and his critical reviews of existing research, including the suggestion that pheromones can trigger synchrony in humans, demonstrated that when the studies are corrected for such errors, the evidence for menstrual synchrony disappears.
- Shmerling, Robert. "Menstrual Synchrony".
- Sapolsky, Robert (March 29, 2010). "1. Introduction to Human Behavioral Biology - YouTube". Stanford University Lectures. Retrieved 3 October 2012.
- Anna Gosline (December 7, 2007). "Do Women Who Live Together Menstruate Together?". Scientific American. Retrieved 2 January 2012.
- Harriet Hall (Sep 06, 2011). "Menstrual Synchrony: Do Girls Who Go Together Flow Together?". Science-Based Medicine. Retrieved 2 January 2012.
- McClintock MK (1971). "Menstrual synchrony and suppression". Nature 229 (5282): 244–5. doi:10.1038/229244a0. PMID 4994256.
- Stern K, McClintock MK (1998). "Regulation of ovulation by human pheromones". Nature 392 (6672): 177–9. doi:10.1038/32408. PMID 9515961.
- Weller A. Weller L. 1992. menstrual synchrony in female couples. Psychoneuroendocrinology. 17(2-3):171;177
- Trevathana, W.R., Burlesonc, M.H. and Gregory, W.L. 1993. No evidence for menstrual synchrony in lesbian couples. Psychoneuroendocrinology. 18(5-6):425;435 ]
- Jacobs, S; Spencer NA, Bullivant SB, Sellergren SA, Menella JA, McClintock MK (2004). "Effects of breastfeeding chemosignals on the human menstrual cycle". Human Reproduction 19 (2): 422–429. doi:10.1093/humrep/deh057. PMID 14747191.
- McClintock, MK (1978). "Estrous Synchrony and its Mediation by Airborne Chemical Communication (Rattus norvegicus)". Hormones and Behavior 10 (3): 264–276. doi:10.1016/0018-506X(78)90071-5.
- McClintock, MK (1981). "The Social Control of the Ovarian Cycle and the Function of Estrous Synchrony". American Zoologist 21: 243–256.
- Yang, Zhengwei; Jeffrey C. Schank (2006). "Women Do Not Synchronize Their Menstrual Cycles". Human Nature 17 (4): 434–447. doi:10.1007/s12110-006-1005-z. Retrieved 2007-06-25.
- Adams, Cecil (2002-12-20). "Does menstrual synchrony really exist?". The Straight Dope. The Chicago Reader. Retrieved 2007-01-10.
- Wilson, H.C. (1987). Female axillary secretions influence women's menstrual cycles: A critique. Hormones and Behavior, 21, 536-546.
- Wilson HC (1992). "A critical review of menstrual synchrony research". Psychoneuroendocrinology 17 (6): 565–91. doi:10.1016/0306-4530(92)90016-Z. PMID 1287678.
- The story of menstrual synchrony and suppression
- The Claim: Menstrual Cycles Can Synchronize Over Time - New York Times, February 5, 2008
- Dr. Harriet Hall, Menstrual Synchrony: Do Girls Who Go Together Flow Together? Science-Based Medicine, September 6, 2011
See also