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Men’s sperm have been decreasing in number and getting worse at swimming for some time now—and, at least in the United States and Europe, new research says it’s getting worse. A pair of new studies unveiled this week at the Scientific Congress of the American Society for Reproductive Medicine (ASRM) in Denver suggest that American and European men’s sperm count and sperm motility—that is, the “swimming” ability of sperm cells—have declined in the past decade, which follows a similar, broader trend observed by many scientists over the past few decades.

One study presented at the ASRM summit, conducted jointly by a fertility center in New Jersey and a fertility center in Spain, found that the percentage of nearly 120,000 male infertility patients whose total motile sperm count (TMSC) numbered more than 15 million (sperm counts below which are considered low, according to the Mayo Clinic) decreased from 85 percent in the 2002–05 period to 79 percent in the period of 2014–17. The percentage of patients whose TMSC clocked in between zero and 5 million, meanwhile, increased from 9 percent to more than 11.5 percent.

The other study, conducted by researchers at Mount Sinai’s Icahn School of Medicine in collaboration with the California Cryobank and Reproductive Medicine Associates of New York, compared more than 124,000 samples from 2,600 sperm donors between the ages of 19 and 38 in Los Angeles, Palo Alto, Houston, Boston, Indianapolis, and New York City. The researchers found that total sperm count, sperm concentration, and TMSC all decreased over time from 2007 to 2017—except in New York City, where all three parameters held steady. (In Boston, too, the researchers note, sperm count held steady, while concentration and TMSC declined.) “Given that donors have higher than average sperm counts, these trends would likely be magnified in the general population,” writes the lead study author, Sydney Chang, a fellow at Reproductive Medicine Associates of New York.

Taken together, the two studies suggest that men who are already experiencing issues with infertility are experiencing further decreases in viable sperm—but fertile men are experiencing decreases in viable sperm, too.

Peter Schlegel, the president-elect of ASRM and a physician at New York’s Weill Cornell Medical Center, shrugged off this odd New York City data point with a pretty good one-liner: “New Yorkers tend to be physically active and our water system provides some of the cleanest and highest-quality water in the U.S. We also have the best pizza.” Good pizza’s effect on sperm motility has yet to be researched, however.

As for the rest of the data, the researchers found no single or definitive underlying cause: Ashley Tiegs, the lead author of the fertility-clinic study, wrote that the results of her study “may reflect a selection bias, in that more infertile men are presenting for treatment each year, or adverse effects of environmental factors,” while Chang, the lead author of the other study, suggested the trend may have to do with “chemical exposures or increasingly sedentary lifestyles.”

Scientists have been quietly observing—and debating the cause of—declines in sperm count for decades. Throughout the 1970s and ’80s, a number of studies suggested sperm counts in men were on the decline, but these had been “largely ignored or dismissed on the grounds that they have been subject to selection bias and/or the inclusion of data from men with fertility or testicular problems,” the author Richard M. Sharpe wrote in the Journal of Endocrinology. But then the British Medical Journal published its landmark, comprehensive 1992 study that indicated “a genuine decline in semen quality over the past 50 years,” and scientists began hypothesizing about potential causes.

In the late 1980s, some researchers suggested environmental pollution was contributing to men’s decreased sperm production, or possibly lifestyle factors like stress, smoking, and drinking. Sharpe suggested there might be an endocrine cause. The journal Clinical Chemistry hypothesized a few years later, in 1995, that perhaps “exposure to environmental estrogenic agents during fetal and childhood development”—such as diethylstilbestrol, or DES, a synthetic estrogen medication popularly prescribed to pregnant women in the 1960s to prevent miscarriages—was at least partly to blame. Still, over the following decades, other researchers continued to question whether a decline in global sperm counts was simply a myth. Perhaps the methodologies of studies that showed a decline were flawed, or perhaps the data claiming to prove a “global” decline in sperm count overrepresented men in the United States or the West, or perhaps studies that showed no decline were being ignored in favor of those that lent themselves to a more dramatic narrative.

As new data emerge, however, many scientists now believe sperm counts have indeed fallen—and continue to fall. As a story in GQ noted earlier this year, a widely cited study published in 2017 by researchers from the Hebrew University and Mount Sinai’s medical school found that among nearly 43,000 men from North America, Europe, New Zealand, and Australia, sperm counts per milliliter of semen had declined more than 50 percent from 1973 to 2011. And not only that, “but total sperm counts were down by almost 60 percent: We are producing less semen, and that semen has fewer sperm cells in it,” wrote the GQ contributor Daniel Noah Halpern. When Halpern asked several scientists why, they presented a united front: It was the unprecedented amount of chemicals now routinely entering the human body. “There has been a chemical revolution going on starting from the beginning of the 19th century, maybe even a bit before,” one biologist told Halpern, “and upwards and exploding after the Second World War, when hundreds of new chemicals came onto the market within a very short time frame.”

Halpern went on to explain that many chemical compounds that are used to make plastic hard (like Bisphenol A, or BPA) or soft (like phthalates) can mimic estrogen in the bloodstream—so men with lots of phthalates in their system are likely to produce less testosterone and fewer sperm (though the U.S. Food and Drug Administration stated earlier this year, somewhat controversially, that its research continues to support its claim that the authorized amounts and uses of BPA are safe for consumers). Plus, chemicals like BPA and phthalates can alter the way genes express themselves, making some of the conditions these chemicals cause inheritable. “Your father passes along his low sperm count to you, and your sperm count goes even lower after you’re exposed to endocrine disruptors,” Halpern wrote. “That’s part of the reason there’s been no leveling off even after 40 years of declining sperm counts—the baseline keeps dropping.”

Sharpe, however, now a professor at the University of Edinburgh’s Medical Research Council Center for Reproductive Health, isn’t totally convinced by the BPA-and-phthalates theory. While there’s a much more cohesive consensus throughout the field of reproductive medicine these days than there may have been 10 or 20 years ago that sperm counts are indeed falling, he says, “the controversy and lack of agreement continue regarding what has caused the fall and when in life has the effect been induced.” Though many consider environmental chemicals to be the primary cause of declining sperm counts, Sharpe says he’s “increasingly skeptical” of that hypothesis: “I would favor that it results from our huge dietary and lifestyle changes, both by pregnant women and by young men.”

Studies like the new ones presented by ASRM, in other words, increasingly serve as bolstering evidence to what many scientists already believe. As scientists reach a consensus that something is happening to men’s sperm in the Western world, the next phase will be to figure out exactly what, and why.

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