In her book, Dr. Swan suggests that sperm counts have plummeted largely due to the rise of endocrine disruptors, a class of hormone-mimicking chemicals found in everything from shampoo to TV-dinner packaging. (She also cites lifestyle factors like obesity, alcohol, and smoking.) Dr. Swan has shown in previous studies that exposure to these chemicals in utero can alter male and female sexual development.
Dr. Richardson and her co-authors suggested an alternative explanation: Perhaps sperm levels naturally rise and fall over time and within populations. The question has not been explored by reproductive researchers and cannot be answered easily, as global sperm counts before 1970 are largely unknown.
There are other possible explanations, as well. Sperm-counting is a tricky business and notoriously prone to human error, Dr. Pacey said. (“I say it from the point of view of someone who spent 30 years counting sperm and knows how difficult it is,” he added.) In a 2013 review article, he noted that as methodologies for counting had improved and been standardized since the 1980s, sperm counts had appeared to fall. In other words, it may simply be that earlier scientists were overcounting sperm.
Dr. Swan and Dr. Levine agreed that exploring these alternative hypotheses was important, so that threats to reproductive health could be established and prevented. “We showed evidence for decline, and raised alarm,” Dr. Levine wrote in an email. “We need to study the causes, including the unlikely possibility of non-pathological decline.”
There was one point that every author agreed on: Men’s reproductive health matters. And until now, it has been surprisingly neglected.
Male infertility contributes to at least half of all cases of infertility worldwide. Yet historically, women have shouldered most of the blame for the inability to conceive. And with the rise of reproductive technologies like in vitro fertilization, women’s bodies are the ones that have been meticulously measured and tracked by reproductive medicine.
As a result, science still lacks basic knowledge when it comes to sperm, said Rene Almeling, a sociologist of medicine and author of “GUYnecology: The Missing Science of Men’s Reproductive Health.” For instance, just this year, researchers reported for the first time that sperm swim in a corkscrew motion, rather than undulating like eels.