185 studies reveal men's sperm count has plunged worldwide over the last 40 years

The swimmers are going missing - and no one knows why.

According to an analysis of 185 academic studies, sperm counts around the world have been declining steadily over the last four decades, such that today's counts are roughly half of what they were 40 years ago, the BBC reports.

Between 1973 and 2011, sperm counts declined 59.3% "with no evidence of a 'leveling off' in recent years," investigators from multiple international universities wrote in their report.

These declines imply that a growing proportion of men have sperm counts below the thresholds for reduced fertility or full-fledged infertility, the researchers noted. This could mean a few things in a broader sense.

Demographic experts have observed for close to two decades that developed countries are increasingly seeing falling fertility rates. (In Japan, the population is actually decreasing.) Gender equality has encouraged more women to enter the workforce, despite a laggard set of policies at many employers that don't accommodate both work and family life.

The latest findings add to that story of falling fertility by including biological reasons that couples aren't having nearly as many children as they used to.

Dr. Hagai Levine, the lead researcher on the study, said the findings left him feeling "very worried." He pointed to environmental factors such as the exposure to pesticides and chemicals, lifestyle habits like smoking and poor diet, and psychological factors like stress as potential causes for the steep decline in sperm count.

"Eventually we may have a problem, and with reproduction in general," he told the BBC. "And it may be the extinction of the human species."

Population forecasts for at least the next century haven't made such dire predictions. The United Nations, for example, still projects the population will grow to more than 9 billion over the next several decades due to high fertility rates found in other countries, such as those in sub-Saharan Africa.

Without a clear set of risk factors for declining sperm count, Levine advised men to maintain a healthy lifestyle devoid of smoking, junk food, or sedentary behavior. He also called on government agencies to better regulate the manufacture of man-made chemicals.