It was not until the late 1980s that scientists really began to grapple with the larger question of why menstruation happens at all. As an undergraduate, evolutionary biologist Beverly I. Strassmann wrote a paper on how concealing ovulation could entice more paternal partners. (Because a woman’s fertile window is more or less invisible, it encourages what researchers call pair-bonding: human males invest in fewer sexual relationships and protect and care for the resulting offspring as a way to ensure their paternity.) Strassmann, now a professor of anthropology at the University of Michigan, wanted to explore human attitudes toward menstruation by collecting data in a community where women spend five nights of their period sleeping in huts that are separate from the rest of the tribe.¶¶In 1986 Strassmann moved to Mali to conduct field research on the Dogon, an ethnic group of millet farmers that hew to their traditions. Dogon people who continue to practice their indigenous religion believe that a menstruating woman’s presence would desecrate the religious objects in the family compounds. Researchers had not previously considered that these religious beliefs were rooted in any kind of reproductive agenda. But, as Strassmann explains, she hypothesized that this was “a cultural pattern embedded in religion that did directly serve reproduction.” Although research on modern indigenous communities can offer only clues about how humans lived thousands of years ago, Strassmann hoped to show that long-standing cultural taboos around menstruation had developed to support our larger evolutionary goals.¶¶During her initial fieldwork, Strassmann studied the community’s use of menstrual huts for almost three years, collecting urine samples from 93 women to test hormone levels and prove that their use of the huts correlated with actual menstruation patterns. She also observed how quickly most of the women got pregnant again after their visits to the huts. Although the practice was ostensibly about keeping menstruation sequestered, the huts themselves were located in full view of a shade shelter used by men in the community. So the huts made a woman’s fertility status clear to her husband and his family whether she liked it or not. (As noted earlier, women enter their “fertile window” after their period.)¶¶Other religious practices around menstruation, such as the Orthodox Jewish purification ritual of sending menstruating women to mikvah baths, can also be traced to men’s need to track female fertility and schedule sexual activity accordingly. And although the advent of the pill means that many women can now control their reproductive life in ways that render the purpose of such practices moot, the taboos still persist, Roberts says. “We still think of menstruation as something that women have to keep hidden and separate.”¶¶PERIOD EVOLUTION¶Although Strassmann’s work was primarily about understanding the biological underpinnings of menstrual taboos, her data also revealed important characteristics about the process of menstruation itself. Perhaps her most oft-cited finding was published in 1997 in Current Anthropology: across human history, menstruation has been a rather infrequent event. That is because women tend to get pregnant earlier, have more babies and spend more time breastfeeding in communities where birth control is unavailable or difficult to access than they do in communities with high rates of birth-control usage. “We think of periods as happening 12 times a year, but if you’re pregnant and then nursing for extended time frames, that’s a stretch of two or three years for each child when you’re not menstruating,” Strassmann explains. Her data showed that in the 1980s the average Dogon woman menstruated only around 100 times in her life, compared with the average American woman’s experience of as many as 400 periods in her lifetime. And Dogon women’s experience is closer to what all women would have experienced throughout history before the development of the pill.¶¶This historical infrequency of menstruation helps to explain why humans evolved to do something as potentially disadvantageous as releasing blood—losing iron, protein and other nutrients and probably attracting predators in the process. It could also help explain why periods and the week before their onset can be so unpleasant for many women. Michael Gillings, a professor of molecular evolution at Macquarie University in Australia, became interested in women’s experiences of premenstrual symptoms (PMS) when premenstrual dysphoric disorder (PMDD) was added to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders in 2013.¶¶PMDD is defined as severe irritability, depression or anxiety in the week or two prior to menstruation, with symptoms easing two or three days after menstruation begins. But Gillings, along with many feminist scholars, balked at the characterization of mood swings as disordered. “Up to 80 percent of women report these symptoms; that makes PMS normal, not a psychological disorder,” he says. “So we have to ask, Was there, at some point in history, an advantage to having these symptoms?” In 2014 he published a paper in the journal Evolutionary Applications arguing that PMS offered a selective advantage because it caused tension between pair-bonds and therefore might help women dissolve relationships with infertile men. “It is difficult to prove a hypothesis like this,” he acknowledges. And the media response characterized him as insensitive to the suffering of women. “I was burned in effigy on five continents,” he says. Some researchers counter Gillings’s claim that PMS is a product of evolution—and contend that its roots are more cultural than biological because it manifests differently around the world. Roberts sees the concept mostly as one influenced by the menstrual taboo and a way to dismiss women’s emotions.¶¶Scientists are also divided over whether the act of bleeding itself serves an evolutionary purpose. “It’s never made sense to me that we have this free-flowing blood, while other animals reabsorb it,” Brown argues. Many evolutionary biologists now think that the essential feature of women’s cycles is not the bleeding but rather the ability of the uterus to thicken its lining in preparation for implantation and then dispose of the endometrium when it is not needed. “A healthy endometrium requires constant metabolic support, so it is less energy-intensive for the female body to tear down and rebuild it each cycle than it is to maintain it in a constant state of readiness for embryo implantation,” Strassmann explains. Human circulation happens to result in a particularly bloody endometrium. “Our physiology doesn’t permit reabsorption, so much of the blood gets discharged as menstruation,” she says. Bleeding may therefore be an insignificant by-product of evolution rather than an advantage.¶¶A WORLD WITHOUT PERIODS?¶If the act of shedding menstrual blood poses no clear health benefit or evolutionary advantage and if, historically, women have not even done it all that often, then why, in this postpill era, do women continue to do it all? The answer: some do not. In January the Royal College of Obstetricians and Gynecologists in London released new guidelines that approved skipping the placebo pills in birth control to reduce the frequency of periods or avoid them altogether.¶¶Although this formal acknowledgment is new, the practice is not. Medical menstrual suppression has long been embraced by clinicians, the media and women frustrated by the pain, mood swings or inconvenience of their menstrual cycle. The pharmaceutical industry also took notice: as the researcher who first measured and quantified the frequency of human menstruation, Strassmann has been asked to present her data to drug manufacturers, who have offered several versions of the pill and other forms of contraception that are formulated to let women skip their periods more often, if not avoid them altogether.¶¶¶Phone apps, such as Clue (shown), help women to keep track of their monthly cycles. Credit: Jamie Chung¶Skipping that monthly ordeal can mean avoiding debilitating pain, prolonged heavy bleeding, migraines and other symptoms that can dramatically impair a woman’s quality of life. The approximately 25 percent of reproductive-age women and girls who struggle with additional kinds of severe menstrual pain may be at increased risk for developing other chronic pain conditions. “We suspect the cyclical experience of monthly menstrual pain somehow alters how some women process all kinds of pain,” explains Laura Payne, who directs pain research for a pediatric program at the David Geffen School of Medicine at the University of California, Los Angeles.¶¶To many doctors faced with patients whose periods cause problems, “the pill is the closest thing we have to a panacea in women’s health,” says Jonathan Schaffir, an associate professor of obstetrics and gynecology at the Ohio State University. But is it? “The pill isn’t a treatment for these conditions,” Kissling says. “It’s a way of refusing to treat them.” It can take up to a decade or longer from disease onset for a woman to be diagnosed with endometriosis, for example, in part because doctors are so quick to prescribe the drug to teenagers reporting bad cramps without investigating to see if there is an underlying cause, says endocrinologist Jerilynn Prior of the University of British Columbia. And where one version of the pill may succeed in masking a woman’s symptoms, another may exacerbate them. “You can spend years jumping from one pill to another, not finding relief,” notes Kissling, who published a paper on how women end up “treating each other,” for better or worse, in online forums, where they share alternative medicine remedies and other tips out of frustration with their doctors’ limited repertoire.¶¶Strassmann and many others are skeptical about the health effects of medically induced menstrual suppression, which may expose women to hormone levels higher than what they would have experienced in the evolutionary past or even now, when regularly cycling on the pill. “It’s true a monthly menstrual period is not necessary,” she says. “But taking more progestin to skip your period is not living like our ancestors did 500 or 1,000 years ago.” Research shows taking the pill reduces the risk for endometrial and ovarian cancers but slightly raises the risk for breast cancer, stroke and blood clots.¶¶In 2017 Strassmann and her colleagues published a paper in Evolution, Medicine, & Public Health tracking how exposure to synthetic hormones varied depending on the type of birth-control pills used. “We know that American women experience more periods than the Dogon because they start menstruating earlier and have fewer children, and we know that having more periods is associated with a higher breast cancer risk,” she explains, noting that the relation is likely because of the additional hormone exposures accrued from those extra periods. “But we don’t really know how that risk squares with the hormone exposure women are also getting from long-term use of birth-control pills.” After analyzing data from 12 studies, as well as the information on birth-control package inserts, Strassmann’s team concluded that some types of the pill exposed women to a quadruple dose of progestin (a synthetic form of progesterone contained in the pill), relative to the progesterone their naturally cycling body would produce.¶¶Nobody knows for sure what that exposure to synthetic hormones will mean long term for women using the pill to suppress their cycles indefinitely. This knowledge gap speaks to broader concerns about our ignorance around menstruation. If Rock and Pincus had begun their work with a deeper understanding of menstruation’s evolution and purpose, how might that have affected the pill’s development? Would women today have more—and more targeted—options to manage their menstrual pain and associated disorders?¶¶In this latest iteration of our menstrual taboo, dispatching with the period instead of researching its complexity might have unforeseen health consequences, Prior says. “Our data on the pill come from generations of women who followed the schedule for 28-day cycles and didn’t stay on it for nearly as long as women do today,” Kissling says. “What we have now” with women using birth control for long-term suppression “is the largest uncontrolled medical experiment on women in history.”¶¶This article was originally published with the title “The Point of a Period” in Scientific American 320, 5, 32-40 (May 2019)