Men aren’t producing as many sperm as they were decades ago. It’s a trend observed around the globe. The pace of decline is accelerating.
Those are the headline findings of a new analysis of sperm count studies published Tuesday in the journal Human Reproduction Update. It’s the largest report to examine the issue.
In a news release, the researchers behind the analysis framed their findings as a “looming crisis” and a “canary in a coal mine” that could “threaten mankind’s survival.”
Shanna Swan, an author of the new analysis, said in an interview that the new research should sound alarms about men’s overall health and reproductive fitness.
“There is a decline in reproductive function,” said Swan, a professor of environmental medicine and public health at the Icahn School of Medicine at Mount Sinai in New York City. “This is not an isolated phenomenon.”
Sperm count is an imperfect measure of fertility, and some outside researchers said they have reservations about the new analysis. But even those critics say the research raises crucial questions about men’s reproductive health — a topic some view as having been neglected by science and ripe for more thorough exploration.
“We still, after decades, don’t know a whole lot about the normal sperm concentrations in men around the world, and to date this represents the best effort of taking all the data available out there and trying to put it together,” said Dr. Bradley Anawalt, a reproductive endocrinologist and a chief of medicine at UW Medical Center in Seattle.
But Anawalt said the inherent limitations of this type of analysis — which combined results from more than 200 sperm count studies — could steer toward misleading conclusions. More research is needed to better understand whether sperm concentrations are declining so dramatically and what might be behind the issue.
“I wouldn’t want people to think we’re in dire jeopardy of imploding as a species,” Anawalt said. “One still has to ask the question: Is this potentially a smoke signal?”
The researchers behind the study first made waves in 2017, when they published a paper that showed falling sperm counts in North America, Europe and Australia — places where data was readily available.
That paper got significant media attention and sparked scientific debate, including criticism from a Harvard research group over its narrowed geographic focus and the language it used to describe the areas that were studied. The paper was used as fodder for wild speculation about men’s health, as well as unfounded and racist theories by some white supremacist and alt-right groups, according to the Harvard researchers.
“We were challenged by some critics who felt that we were only talking about white men, and that was not our intention,” Swan said. “In areas where labs were less available and resources were scarcer, there were fewer studies.”
Swan said that more high-quality sperm count studies have been produced since the 2017 paper and that the research group was able now to fill the geographic gaps.
To assess sperm counts worldwide, researchers evaluated hundreds of scientific articles, ultimately combining the data and findings of 223 previous articles about sperm concentration. The researchers evaluated the estimates, which included data on semen samples from 1973 to 2018. The authors tried to control for factors like age and abstinence time.
The new data, which incorporated studies from around the world, “followed the same trend” as the 2017 study, Swan said. “To our surprise, the pace had accelerated. The decline had gotten steeper.”
Despite the negative trend, the average sperm count for men in 2018 remained higher than levels the World Health Organization considers normal.
Swan said previous research of small groups of men has linked sperm count reductions to pesticides and chemicals that disrupt the endocrine system.
“I don’t think there’s any doubt these chemicals affect semen quality. Where there is doubt is how you parse out how much change is due to lifestyle factors and how much is due to chemicals,” Swan said.
Outside researchers said that the new analysis was thorough and careful but that filtering and combining so many different studies by separate research groups could inevitably cause biases.
“You’re combining all sorts of methodologies. You’re going to introduce bias,” Anawalt said. “We have to take this with a bit of a grain of salt.”
Trends in how previous research was conducted or promoted over decades could skew the overall view of the issue, researchers said.
It’s possible that medical and scientific journals are more likely to publish results that show declines, said Dr. John Amory, a professor of medicine at the University of Washington who wasn’t involved in the analysis. It’s also possible that the type of men involved in sperm count studies is different from those who weren’t.
Sperm can be difficult to count and characterize accurately, meaning numbers could vary from study to study and over time, depending on how sperm are counted.
“You’re talking about millions of cells, and they’re moving,” Amory said. “There’s other things in ejaculate that aren’t sperm — round cells and debris.”
Moreover, sperm count is only one factor in determining fertility. Motility — how effectively sperm can swim — and morphology — sperm size and shape — are also important indicators for male fertility, Amory said.
The authors of the new analysis acknowledged those limitations and took pains to limit their effects on the outcome. They used only studies that counted sperm according to World Health Organization guidelines or used the same techniques, Swan said.
Other studies have shown declines in other sperm parameters and an increase of men seeking treatment for fertility issues, said Dr. Ryan Smith, an associate professor of urology at the University of Virginia, who wasn’t involved in the new analysis.
“I think the consistency of what’s being found in the research today is definitely concerning,” Smith said. “We can’t say anything conclusive at present, but I think, as clinicians and researchers, we need to direct research support and advocacy toward this.”
Male fertility can be a strong indicator of general health.
Men with infertility issues are at increased risk of other diseases, said Amory, who believes those issues can be harbingers of health problems or alert patients to diseases they didn’t realize were affecting their lives.
Obesity, opioid use and other health factors can drive infertility. Some prescription medications can have negative impacts on fertility.
It’s possible that environmental factors and pollution could be affecting sperm counts broadly.
“Identifying individual culprits is challenging,” Smith said.