In 2007 Susan Brown encountered the repelling power of period blood. While studying what menstrual fluid might reveal about a woman’s health, she wanted data from a cross section of subjects beyond the student volunteers at the University of Hawaii at Hilo, where she worked as an evolutionary psychologist. Brown’s team members set up a booth near the entrance of a Walmart in downtown Hilo and hung a sign that said, “Menstrual Cycle Research.” Then they waited. All afternoon women and men would spot the sign, then gingerly skirt past without making eye contact.