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nights — she kept track of everyone who visited the hut. Among the Dogon, she found a woman on average has her first period at the age of sixteen and gives birth eight or nine times. From menarche, the onset of menstruation, to the age of twenty, she averages seven periods a year. Over the next decade and a half, from the age of twenty to the age of thirty-four, she spends so much time either pregnant or breast-feeding (which, among the Dogon, suppresses ovulation for an average of twenty months) that she averages only slightly more than one period per year. Then, from the age of thirty-five until menopause, at around fifty, as her fertility rapidly declines, she averages four menses a year. All told, Dogon women menstruate about a hundred times in their lives. (Those who survive early childhood typically live into their seventh or eighth decade.) By contrast, the average for contemporary Western women is somewhere between three hundred and fifty and four hundred times.

Strassmann’s office is in the basement of a converted stable next to the Natural History Museum on the University of Michigan campus. Behind her desk is a row of battered filing cabinets, and as she was talking, she turned and pulled out a series of yellowed charts. Each page listed, on the left, the first names and identification numbers of the Sangui women. Across the top was a time line, broken into thirty-day blocks. Every menses of every woman was marked with an X. In the village, Strassmann explained, there were two women who were sterile, and, because they couldn’t get pregnant, they were regulars at the menstrual hut. She flipped through the pages until she found them. “Look, she had twenty-nine menses over two years, and the other had twenty-three.” Next to each of their names was a solid line of x’s. “Here’s a woman approaching menopause,” Strassmann went on, running her finger down the page. “She’s cycling but is a little bit erratic. Here’s another woman of prime childbearing age. Two periods. Then pregnant. I never saw her again at the menstrual hut. This woman here didn’t go to the menstrual hut for twenty months after giving birth, because she was breast-feeding. Two periods. Got pregnant. Then she miscarried, had a few periods, then got pregnant again. This woman had three menses in the study period.” There weren’t a lot of x’s on Strassmann’s sheets. Most of the boxes were blank. She flipped back through her sheets to the two anomalous women who were menstruating every month. “If this were a menstrual chart of undergraduates here at the University of Michigan, all the rows would be like this.”

Strassmann does not claim that her statistics apply to every preindustrial society. But she believes — and other anthropological work backs her up — that the number of lifetime menses isn’t greatly affected by differences in diet or climate or method of subsistence (foraging versus agriculture, say). The more significant factors, Strassmann says, are things like the prevalence of wet-nursing or sterility. But overall she believes that the basic pattern of late menarche, many pregnancies, and long menstrual-free stretches caused by intensive breast-feeding was virtually universal up until the “demographic transition” of a hundred years ago from high to low fertility. In other words, what we think of as normal — frequent menses — is in evolutionary terms abnormal. “It’s a pity that gynecologists think that women have to menstruate every month,” Strassmann went on. “They just don’t understand the real biology of menstruation.”

To Strassmann and others in the field of evolutionary medicine, this shift from a hundred to four hundred lifetime menses is enormously significant. It means that women’s bodies are being subjected to changes and stresses that they were not necessarily designed by evolution to handle. In a brilliant and provocative book, Is Menstruation Obsolete?, Drs. Elsimar Coutinho and Sheldon S. Segal, two of the world’s most prominent contraceptive researchers, argue that this recent move to what they call “incessant ovulation” has become