It is a myth that many women swear is true, and that most men have never heard of: Women who live together tend to have synchronized menstrual periods. The phenomenon was first presented to the scientific community more than 30 years ago, and has since remained a matter of much debate. There is controversy over whether or not synchronous menstruation is in fact true, and if it is, what is the cause.
Evidence for the Health Claim
Various studies have been conducted to examine menstrual cycle patterns of roommates, close friends, coworkers, and family members. The first scientific observation was carried out by Martha McClintock in 1971, who interviewed 137 members of her all-women’s dorm at Wellesley College several times over the course of a school year to obtain onset dates for each woman’s period. After comparing data for roommates, close friends who spent a lot of time together, and living groups, McClintock found that the difference between onset dates decreased over the course of the year for the women who spent the most time together. Subsequent studies have produced similar results.
There is limited research on what may cause menstrual synchrony, but the most common hypothesis has to do with pheromones–airborne, odorless, chemical signals produced by the skin, which animals use to share information about sex and reproduction.
Recent human studies involved placing cotton pads under the arms of women who were in the follicular phase of the menstrual cycle (the first two weeks before ovulation occurs), to collect their sweat and pheromones. The cotton pads were then wiped on the upper lips (below the nose) of another group of women, who thought they were controls in the experiment, three times a week for at least five months. By the end of the study most of the “recipients” were menstruating at the same time as their “donors,” possibly because of a change in the release of a hormone from the pituitary gland that triggers ovulation.
A follow-up study conducted by McClintock over 25 years after her initial research found that when women were exposed to sweat obtained from other women in the follicular phase, the recipients’ cycles became shorter. When they were exposed to compounds from the ovulatory (middle) phase, the recipients’ cycles became longer.
Not all women are affected by one another’s menstrual cycles. Some pheromone researchers have found that women vary in their sensitivity to certain chemicals. Furthermore, the specific compounds that may affect menstrual synchrony have not yet been identified.
There is much speculation over possible biological advantages of women menstruating together. Some researchers hypothesize that in a tribal environment, people had to cooperate to survive, and synchronized menstrual cycles may have provided some survival advantage among a community of women.
Others suggest that women give off a pheromonic signal during the part of the menstrual cycle when they are most fertile, which indicates to men that it is a good time to have sex. In prehistoric times, men took multiple wives, and if the wives were on different cycles a man may pick up a pheromonic signal from one woman but choose the wrong one to have sex with, thus decreasing the likelihood of reproducing. If all of the women were fertile at the same time, he could not go wrong!
Evidence Against the Health Claim
Critics of menstrual synchrony raise a variety of research deficiencies in studies supporting the theory: flawed statistical methods, small sample sizes, sampling biases and uncontrolled confounding factors.
They also point out that since the length of the menstrual cycle varies from woman to woman, two women with cycles of different lengths will not ever actually synchronize. In one study analyzing the length of the menstrual cycle, researchers argue that if two women have 28–day cycles, the maximum amount of time that they both cannot be having their periods is 14 days. If this is true, on average, the onset of these two women’s periods will be seven days apart, and half the time they should be closer. Because women normally have their period for about five days, an overlap is likely. So
statistically, overlapping menstrual cycles can occur, regardless of whether or not women actually have an effect on one another’s periods.
It is interesting to note that many of the arguments against menstrual synchrony come from men, while research supporting the phenomenon is often conducted by women. There is evidence from both human and animal studies supporting the health claim. However, there are also a number of fair criticisms of this research. So no one can claim to have the last word on this one! Period.