For the second time in five years, scientists are warning about declining human sperm counts. (I wrote about this issue in “Declining sperm counts: Nature’s answer to overpopulation?” early last year.)

Besides confirming the results of an important 2017 study, the authors now note an acceleration in the decline of sperm counts. In other words, whatever is causing that decline is getting worse. The rate of decline has doubled since 2000.

It’s important to remember that when the fertility rate declines below replacement—currently 2.1 births per woman in so-called developed countries—populations shrink. This may not be a bad thing at first since overpopulation and overconsumption are huge barriers to building sustainable societies. But there comes a point when if fertility rates don’t level off and then rise to replacement, extinction become a possibility.

That is apparently where we are heading as a global society. A phenomenon as complex as fertility cannot be explained by one or even a few factors. There is, for example, what is called the “demographic transition,” a theory which posits that the size of households declines as societies industrialize. This could result from many factors such as the empowerment of women (to control their own fertility); improvements in public health and nutrition that reduce mortality among infants and children (making parents less likely to have many children because some are likely to die); the rising cost of raising and educating children; and cultural factors that lead parents to want to have more time for themselves.

But some scientists have been pointing to other factors associated with industrialization, namely, the widespread dispersion of toxic chemicals in the environment that can adversely affect fertility; the increasing use of pharmaceuticals; the ubiquitous presence of plastics in the environment and human bodies; smoking; poor diet; and obesity (which itself may be a product of endocrine disruption caused by environmental toxins).

The shocking conclusion of the 2017 study was that if the rate of decline in sperm counts observed then continued, those counts would reach zero by 2045. This latest study concludes that we as a society may be moving even more quickly toward that destination.

Given the monumental increases in world population since the beginning of the industrial revolution, it is hard to imagine a collapse in human population that might be almost as swift. And, yet that is what population biology and this latest sperm count study suggests. If the suspects listed above are major reasons for the decline, it’s hard to see how the monied interests behind them would allow much to be done.

Photo: Human sperm stained for semen quality testing. By Bobjgalindo – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=5654847