Why is male fertility declining?

Is the world in the midst of a male fertility crisis? Or are we overstating the problem? Here's everything you need to know:

How has male fertility declined?

Human sperm count has fallen by over 50 percent over the last 50 years, according to a 2022 study published in the journal Human Reproductive Update. Global birth rates are also at a record low, with more than 50 percent of the world's population living in countries "with a fertility rate below two children per woman," according to BBC. And low sperm count is not the only driving factor here — research has also indicated a decline in testosterone levels, as well as higher rates of testicular cancer and erectile dysfunction among men, per BBC.

"I think one of the fundamental functions of any species is reproduction," urologist Michael Eisenberg told CNN. "So I think if there is a signal that reproduction is in decline, I think that's a very important finding."

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Overall, infertility affects one out of every six people in the world, according to a new report from the World Health Organization.

Why is male fertility declining?

The exact cause of the decline remains unclear, but some theories have emerged.

Lower sperm counts among men could come as a result of epigenetic changes, or "alterations to the way genes work, caused by environmental or lifestyle factors," as defined by BBC. "There are signs that it could be cumulative across generations," Hagai Levine, a professor of epidemiology, told the outlet. "This [declining sperm count] is a marker of poor health of men, maybe even of mankind."

Speaking of, a general decline in health could also be a factor. "There is a strong link between a man's reproductive health and his overall health. So it could also speak to that too, that maybe we're not as healthy as we once were," Eisenberg, the urologist, told CNN. Further, "stress of the mother, maternal smoking, and especially exposure to manmade chemicals that are in plastic, such as phthalates," can also disrupt the development of the male reproductive system," Levine added, speaking with CNN.

Even climate change and the environment could play a role. A 2022 study found that rising temperatures negatively impact sperm quality. Indeed, "heat stress is considered to be the most influential cause of reproductive function in mammals," the study said. Additionally, BBC writes, "chemicals found in plastics, household medications, in the food chain and, in the air," can cause DNA fragmentation or "damage or breaks in the genetic material of the sperm."

So is this cause for concern?

Opinions on the matter are mixed.

As Rachel Gross points out for The New York Times, "no one knows what an 'optimal' sperm count is." A person needs 40 million sperm per milliliter of semen to be considered fertile, and a count above that "does not mean a man is more fertile."

Further, "the way that semen analysis is done has changed over the decades. It has improved. It has become more standardized, but not perfectly," Dr. Alexander Pastuczak, a surgeon and assistant professor at the University of Utah School of Medicine, told CNN. "Even if you were to take the same semen sample and run it and do a semen analysis on it in the 1960s and '70s versus today, you'd get two different answers."

Even so, the matter "should be studied," Levine, the epidemiology professor, argued to BBC. "We are facing a public health crisis — and we don't know if it's reversible." The decline in both sperm quality and count could be indicative of a larger problem, he said.

Dr. Scott Lundy, a urologist at the Cleveland Clinic, agrees. "While it's not a cause for panic, because the counts are by and large still normal, on average," he told CNN, "there is a risk that they could become abnormal in the future, and we have to recognize that and study that further."