Q: Why do females who live together often experience a menstrual cycle at a similar time of month?
Answer: Debate continues as to why this seems to occur amongst some females living together, yet not with others. Wikipedia has this to offer on the topic...
Hypothesized mechanisms of synchronization
Lunar synchronization
Cutler[52] and Law[53] hypothesized that menstrual synchrony is caused by menstrual cycles synchronizing with lunar phases. However, neither of them agree on what phase of the lunar cycle menstrual cycles synchronize with. Cutler hypothesizes the synchronization with the full moon[52] and Law with the new moon.[53] Neither offer hypotheses regarding how lunar phases cause menstrual synchrony and neither are consistent with previous studies that found no relationship between menstrual cycles and lunar cycles.[54][55] More recently, Strassmann investigated menstrual synchrony among Dogon village women. The women were outdoors most nights and did not have electrical lighting. She hypothesized that Dogon women would be ideal for detecting a lunar influence on menstrual cycles, but she found no relationship.[6]
Social affiliation
Jarett hypothesized that women who were more affiliative and concerned with how others viewed them would synchronize more. In her study, however, women with low affiliation scores were associated with greater synchrony. She found that women with a need for social recognition and approval from others were associated with synchrony, which is partially consistent with her hypothesis. Nevertheless, the group of women she studied did not synchronize their menstrual cycles.[45]
Coupled oscillators
When McClintock published her study on menstrual synchrony, she speculated that pheromones may cause menstrual synchrony.[3] 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.[56]
This idea was extended to humans in a study by Stern and McClintock.[57] 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.[57]
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. Stern and McClintock concluded that these findings "proved the existence of human pheromones" as well as illustrated manipulation of the human menstrual cycle.[57]
Researchers pointed out several flaws in their study.[58][59][60] 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[61] could be applied to humans.[59]
Perception and awareness of synchrony
Arden and Dye investigated women's awareness and perception of menstrual synchrony.[23] 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)[23] 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.[23]
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)[23]