For the latest study, which appeared in Human Reproduction, researchers drew on data collected from more than six hundred and fifty men who sought treatment for infertility at the Massachusetts General Hospital at some point between 2000 and 2017. The men, who ranged in age from eighteen to fifty-six, provided semen samples that were analyzed for sperm count, concentration, motility, and well-being—“whether the sperm had one head or two, one tail or two, that sort of thing,” Chavarro said. Blood tests measured the levels of hormones involved in the production of sperm. And each man filled out a questionnaire that asked, among other things, what kind of underwear he typically wore. Slightly more than half the men wore boxers; the rest wore briefs, boxer briefs, bikinis, or other tight-fitting underwear.