question: Why/How do women sometimes synchonize their period with women around them?
comment: They/we don’t. It’s a common myth and simply doesn’t happen. As a 40 year old woman who has had many female roommates over the years, I can tell you from personal experience.
passages: Menstrual synchrony
  
  Menstrual synchrony, also called the McClintock effect, is an alleged process whereby women who begin living together in close proximity experience their menstrual cycle onsets (the onset of menstruation or menses) becoming more synchronized together in time than when previously living apart. "For example, the distribution of onsets of seven female lifeguards was scattered at the beginning of the summer, but after 3 months spent together, the onset of all seven cycles fell within a 4-day period."
   Yang and Schank conducted the largest study to date with 186 Chinese college women. Ninety three of the women lived in 13 dorm rooms, 5 to 8 women per room. The other ninety three women lived in 16 dorm rooms, 4 to 8 women per room for a total of 29 rooms. The women were given notebooks to record the onset of each of their cycles and they collected data for over a year for most of the women.
   Section::::Scientific details.:Studies.:1980s.
   In 1997, Weller and Weller published one of the first studies to investigate when menstrual synchrony occurs in complete families. Their study was conducted in Bedouin villages in northern Israel. Twenty seven families, which had from two to seven sisters 13 years or older and collected data on menstrual cycle onsets over a three-month period. Using the methods of, they reported menstrual synchrony occurred for the first two months, but not for the third month for roommate sisters, close friend roommates, and for families as a whole.
   Section::::Scientific details.:Studies.:1990s.
   Quadagno et al. conducted the second replication of McClintock's study. There were 85 women living in dormitories, sorority houses, and apartments who attended a large midwestern university in the United States. Their study used methods similar to McClintock's study except in addition to two women living together, there were also groups of three and four women living together. They reported that the women synchronized their menstrual cycles and concluded that pheromones may have played a role in synchronization.
   When McClintock published her study on menstrual synchrony, she speculated that pheromones may cause menstrual synchrony. In a study on Norway rats, McClintock proposed and tested a coupled oscillator hypothesis (see section on rats below). The coupled-oscillator hypothesis proposed estrous cycles in rats were cause by two, estrous phase dependent pheromones that mutually modulated the length of cycles in a group and thereby causing synchrony.
   Wilson, Kiefhabe, and Gravel conducted two studies with college women. The first study consisted of 132 women who were members of a sorority or roommates of members at the University of Missouri. The women were 18 to 22 years of age and the study aimed to replicate McClintock's original study. However, instead of asking women to recall when their last and next to last menstrual onsets occurred, one of the researchers visited the sorority daily to record the occurrence of menstrual onsets and to collect other biographical data. The second study consisted of 24 women living in a cooperative house near the University of Missouri. The women were 18 to 31 years of age. One of the researchers visited the house three times a week recording menstrual onset and collecting more extensive biographical and psychological test data than in the first study. They found no menstrual synchrony in either study. They considered the possibility that women with irregular cycles may reduce the likelihood of detecting synchrony, so they re-analyzed their data after they removed women with irregular cycles, but again there was no statistically significant effect of synchrony. They concluded that "It is clear no meaningful process of selection or exclusion of pairs can produce a significant level of menstrual synchrony in our samples... Therefore, whether or not menstrual synchrony occurs among women who spend time together must remain a hypothesis requiring further investigation" (p. 358).
   Section::::Scientific details.:Hypothesized mechanisms of synchronization.
  
  Section::::Scientific details.:Hypothesized mechanisms of synchronization.:Lunar synchronization.
   Graham and McGrew were the first researchers to attempt to replicate McClintock's study. There were 79 women living in halls of residence or apartments on the campus of a college in Scotland. The women were 17 to 21 years old at the time of the study and the procedures followed were similar to those used in McClintock's study. She partially replicated McClintock's study reporting that close friends but not neighbors synchronized their cycles. Unlike in McClintock's study, close friends did not synchronize in groups. They considered a pheromone mechanism a possible explanation of synchrony, but noted that if pheromones were the cause, neighbors should have synchronized as well. They concluded that the mechanism of synchrony remains unknown, but emotional attachment may play a role.
   Both Wilson and Arden and Dye pointed out that menstrual synchrony can occur by chance when there is menstrual cycle variability. Yang and Schank argued that when there is cycle variability (i.e., either women have irregular cycles, have cycles of different frequencies, or both), most women will have the opportunity to experience synchrony even though it is a result of cycle variability and not a result of a mechanism such as the exchange of pheromones. For example, consider two women A and B. Suppose A has menstrual cycles that are 28 days long and B has cycles that are 30 days long. Suppose further that when A and B become close friends, B has a cycle onset 14 days before A's next onset. The next time both of them have menstrual cycle onsets, B will have a cycle onset 12 days before A. B will continue to gain two days on A until their onsets coincide, then their cycles will begin to diverge again. The cycles of A and B will repeatedly converge and diverge creating the appearance of synchrony during convergence. This is a mathematical property of cycles of different frequencies and not due to the interaction of A and B. If, in addition, the duration of menstruation is considered (typically 3 to 5 days with a range of 2 to 7 days), then the experience of synchrony may last a number of months.
   Researchers pointed out several flaws in their study. Whitten's main critiques was with their using only their first cycles as a control for the subsequent conditions. He argued that this eliminate all within-subject variance . Control conditions should have been run between each experimental condition and just at the beginning of the study. He was also skeptical about whether the coupled-oscillator model from rat research could be applied to humans.
  
  Section::::Scientific details.:Perception and awareness of synchrony.
   Arden and Dye investigated women's awareness and perception of menstrual synchrony. Their study consisted of 122 women (students and staff) at Leeds University. A four-page questionnaire was sent to each participant. After providing personal details, they were given a description of menstrual synchrony: "Menstrual synchrony occurs when two or more women, who spend time with each other, have their periods at approximately the same time" (p. 257) After reading the description they were asked whether they were aware of menstrual synchrony and whether they had experienced it. They were then asked details about their experience of synchrony such as how many times they experienced and how long it lasted.
   Trevathan, Burleson, and Gregory also conducted a study with 29 lesbian couples (22 to 48 years of age), but they incorporated the methodological critique of Wilson into the design of their study. In particular, Wilson emphasized the importance of using actual menstrual cycle lengths with their inherent variability. The lesbian couples were drawn from a larger sample of women who had kept daily records of their menstrual cycles for three months and who had participated in a previous study. They found no evidence of synchrony. They discussed several factors that could have prevented synchrony in their study but they strongly suggested that menstrual synchrony may not be a real phenomenon because of the methodological issues Wilson raised and because menstrual synchrony appears to lack adaptive significance.
   McClintock's study consisted of 135 female college students who were 17 to 22 years old at the time of the study. They were all residents of a single dormitory, which had four main corridors. The women were asked when their last and second to last menstrual period had started three times during the academic year (which ranged from September to April). They also were asked who (other women in the dormitory) they associated with most and how often each week they associated with males. From these data, McClintock placed women into pairs of close friends and roommates and she also placed them into groups of friends ranging in size from 5 to 10 women. She reported statistically significant synchrony for both her pairwise sorting of women and her group sorting of women. That is, whether women were placed into pairs of close friends and roommates or whether they were placed into larger groups of friends, she reported that they synchronized their menstrual cycles. She also reported that the more often women associated with males, the shorter their menstrual cycles were. She speculated that this may be a pheromone effect paralleling the Whitten effect in mice but that it could not explain menstrual synchrony among women. Finally, she speculated that there could be a pheromone mechanism of menstrual synchrony similar to the Lee-Boot effect in mice.
   "Timema" males, in sexual species of "Timema", show a consistent pattern of courting behavior. The male climbs onto the back of the female and, after a short display of vibrating and waving, they proceed to mate. (Rejection by the female is possible but uncommon.) The male then rides on the female's back for up to five days, a behavior often referred to as "guarding" the female.
  
  Several species of "Timema" are parthenogenetic: that is, females can reproduce asexually, producing viable eggs without male participation.
   Schank conducted another experiment to test whether female rats could synchronize their cycles. He found that female rats did not synchronize their cycles and he argued that in the original McClintock study, the random control group was more asynchronous than expected by chance. When the experimental group was compared to the control group in McClintock's 1978 study, the experimental group was more synchronous than the control group but only because the control group was too asynchronous and not because the experimental group had synchronized their cycles. In a follow-up study, Schank again found no effect of estrous synchrony in rats.
   Ziomkiewicz conducted a study with 99 Polish college women living in two dormitories. Thirty six of the women lived in 18 double rooms and sixty three lived in 21 triple rooms. Women recorded their menstrual cycle onsets on menstrual calendars provided to them and 181 days' worth of menstrual cycle data were collected. The mean menstrual cycle length was 30.5 days (SD = 4.56).
   Martha McClintock's 1971 paper, published in "Nature", says that menstrual cycle synchronization happens when the menstrual cycle onsets of two or more women become closer together in time than they were several months earlier..
  
  After the initial studies, several papers were published reporting methodological flaws in studies reporting menstrual synchrony including McClintock's study. In addition, other studies were published that failed to find synchrony. The proposed mechanisms have also received scientific criticism. A 2013 review concluded that menstrual synchrony likely does not exist.
  
  Section::::Overview.
  
  Section::::Overview.:Original study by Martha McClintock.
   The phenomenon of menstrual synchrony is the closeness in time of the menstrual cycle onsets of two or more women. The phenomenon is not synchronization in the strict sense of concordance of menstrual cycle onsets but the term "menstrual synchrony" is still used perhaps misleadingly. As an undergraduate, Martha McClintock published the first study on menstrual synchrony; her report detailed the menstrual synchrony of undergraduate women living in a dormitory in Wellesley College. Since then, there have been attempts to replicate her findings and to determine the conditions under which synchrony occurs, if it exists. Her work was followed up by studies reporting menstrual synchrony and by other studies that failed to find synchrony.
   They found that 84% of the women were aware of the phenomenon of menstrual synchrony and 70% reported the personal experience of synchrony. The experience of synchrony occurred most commonly with close friends followed by roommates. There was considerable variation in the reported time spent together before synchrony occurred ranging from zero to four weeks to 12 months or more. The most common time was 12 months or more. The duration of menstrual synchrony also was highly variable with responses ranging from one to two months to 12 months or more. They conclude that "Whether or not future research concludes that menstrual synchrony is an objective phenomenon, subjective experiences, which are apparently widespread, need to be given careful consideration." (p. 265)
   Jarett's study was the third to attempt to replicate McClintock's original study using college roommates. There were 144 women who attended two colleges. The women were 17 to 22 years old and the procedures followed were similar to McClintock's study except only pairs of roommates were used. She reported that the women did not synchronize. Jarett concluded that whether menstrual synchrony occurs in a group of women may depend on the variability of their menstrual cycles. She conjectured that the reason the women in her study did not synchronize their menstrual cycles was because they happened to have longer and more irregular menstrual cycles than in McClintock's original study.
   This idea was extended to humans in a study by Stern and McClintock. They investigated whether a coupled-oscillator mechanism first reported for Norway rats (see section below on rats) could also exist in humans. The coupled-oscillator hypothesis in humans proposed that human females release and receive pheromones that regulate the length of their menstrual cycles. This was assumed to occur without consciously detecting any odor. The study was conducted by collecting compounds from axillae (underarms) of donor women at prescribed phases during their menstrual cycles (i.e., the follicular phase, ovulatory phase, and luteal phase), 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.
   Thus, a number of studies were published from the 1980s to the mid 2000s, which attempted to replicate menstrual synchrony in college women, determine the conditions under which menstrual synchrony occurred, and to address methodological issues that were raised as these studies were published. The rest of this section discusses these studies in chronological order, briefly presenting their findings and main conclusions grouped by decade followed by general methodological issues in menstrual synchrony research.
  
  Section::::Scientific details.:Studies.
  
  Section::::Scientific details.:Studies.:1970s.
   Section::::Method.
  
presuppositions: Women sometimes synchronize their period with women around them. Women synchronize their period with other women.
corrections: This is a common myth -women do not synchronize their period with the women around them. Women do not synchronize their period with other women, it is an urban legend.
raw_presuppositions: Women sometimes synchronize their period with women around them. Women synchronize their period with other women.
raw_corrections: This is a common myth -women do not synchronize their period with the women around them. Women do not synchronize their period with other women, it is an urban legend.