Other researchers published further critiques, including Beverly Strassmann, an anthropology professor at the University of Michigan. In a 1999 paper in the journal Human Reproduction, Strassmann pointed out a fundamental flaw in the period-syncing logic:¶¶Given a cycle length of 28 days (not the rule-but an example), the maximum that two women can be out of phase is 14 days. On average, the onsets will be 7 days apart. Fully half the time they should be even closer. Given that menstruation often lasts 5 days, it is not surprising that friends commonly experience overlapping menses, which is taken as personal confirmation of menstrual synchrony.¶¶In other words, as Strassmann explained to me in an interview, women have their periods at the same time as other women an awful lot, so it’s common to mistake menstrual overlap for menstrual synchrony: “A quarter of the time, [two] women’s menses should be overlapping, based on random chance,” she said.