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It’s not uncommon for Health and Human Services Secretary Robert F. Kennedy Jr. to mention sperm counts when he makes a public appearance.

In recent television interviews, political speeches and congressional hearings, Kennedy has repeatedly claimed that teenage boys today have half the sperm that men in their 60s do — a stat that’s not exactly accurate. Kennedy has cited the talking point as evidence of a broader health crisis in the U.S.

“We have fertility rates that are just spiraling. A teenager today, an American teenager, has less testosterone than a 68-year-old man. Sperm counts are down 50%,” he told Fox News’ Jesse Watters in April, adding: “It’s an existential problem.”

Contrary to Kennedy’s claims, sperm counts decline with age, so young men have much higher counts than older men. And data about sperm counts in teen boys largely does not exist.

Some researchers contend that men’s overall sperm counts are lower than they were generations ago, based mostly on two papers published in the last decade. Others say there’s no convincing evidence of the trend. And many agree that even if sperm counts are declining, it does not amount to a full-blown fertility crisis.

“This is a very contentious issue in our field, and for every paper that you find that suggests a decline and raises an alarm for this issue, there’s another paper that says that the numbers aren’t changing, and that there’s no cause for concern,” said Dr. Scott Lundy, a reproductive urologist at the Cleveland Clinic.

Andrew Nixon, an HHS spokesperson, said Kennedy is “sounding the alarm on a public health issue others are too timid, or too politically cautious, to confront.”

The secretary’s warning feeds on a burgeoning narrative that men today face a fundamental threat to their fertility. Similar claims have been spread by various wellness influencers, tech startups and young men on social media. Young men concerned about a decline in virility have opted to freeze their sperm, abstain from sex or undergo testosterone replacement therapy. A 2022 study found that “semen retention” was the most popular men’s health subject on TikTok and Instagram.

Meanwhile, adherents of the “pro-natalist” movement have argued that more families should be having children to compensate for a decline in fertility and birth rates in the U.S. The most prominent figure among them, Elon Musk, has cited the declining birth rate as an omen of humanity’s collapse.

Researchers who study male fertility say the reality is far more complicated and little cause for panic. Fertility and birth rates in the U.S. are declining, in part, because people are choosing to have fewer children or delaying having kids until later in life. Though some men do struggle to have kids, in many cases the issue can be corrected through medical interventions or lifestyle changes.

In 1993, scientist Louis Guillette shocked Congress when he testified at a hearing that “every man sitting in this room today is half the man his grandfather was.”

Guillette was referring to a generational decline in sperm count. A year before his testimony, a review of papers published from 1938 to 1991 determined that the average sperm count had fallen around 50%.

But many researchers have since found flaws in the review — among them, that it included relatively little data from the first few decades of the analysis, the men in the studies were evaluated using different methods and the data analysis did not account for the fact that many men’s sperm counts fall within a lower range.

“The paper was widely, wildly cited,” but “the statistics were not solid,” said Dolores Lamb, who researches male infertility at Children’s Mercy Kansas City.

In a follow-up review of studies published from 1992 to 2013, eight studies showed a decline in semen quality, 21 showed no change or an increase, and six showed ambiguous or conflicting results. Based on that, Lamb said, “the preponderance of the data suggests that there was no decline.”

In 2021, reproductive epidemiologist Shanna Swan reignited the debate with her book “Count Down,” which warned of falling sperm counts “imperiling the future of the human race.”

A paper Swan and her co-authors published in 2017 determined that from 1973 to 2011, sperm counts declined by 52% in North America, Europe, Australia and New Zealand. A follow-up analysis in 2022 showed a similar trend worldwide. In an interview with The Guardian, Swan said her work implied that the median sperm count could reach zero by 2045.

The research was picked up by men’s rights groups, which pointed to it as evidence that men were losing their masculinity. It even inspired a viral publicity stunt to raise awareness about a possible future where people couldn’t reproduce: A crowd gathered to watch sperm cells race under a microscope.

HHS’ Nixon said the 2017 and 2022 papers support Kennedy’s claims about declining reproductive health.

“A growing body of peer-reviewed research shows significant declines in sperm counts over the past decades, and pretending this isn’t a serious trend is irresponsible,” he said. “The data is real, the stakes are high and ignoring it doesn’t make it go away.”

Lamb said the analyses from Swan and her co-authors had a major weakness in their methodology. They assumed that laboratories in different parts of the world were collecting and testing semen in the same way, she said, when in fact the methods likely varied.

Swan stood by her team’s results, telling NBC News in an email that they accounted for differences in methodologies across studies, as well as the challenges of getting accurate sperm counts.

Lundy, of the Cleveland Clinic, said measuring sperm counts can be hard to do consistently. The count itself can go up and down depending on the frequency of ejaculation, time of year, or whether someone is injured or has a fever.

His analysis last year found a subtle decline in sperm count among men in the U.S. from 1970 to 2018, but one that likely wouldn’t impact fertility in real life.

“What it has done is showed that there’s no cause for widespread panic for the typical U.S. male,” Lundy said.

Researchers who believe sperm counts are declining said it might be influenced by two factors: obesity and environmental chemicals.

“We know that obesity is one of the strongest predictors of serum testosterone, and also to a lesser extent, of sperm counts,” said Jorge Chavarro, a professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health. In particular, he said, obesity can decrease the secretion of key hormones in the brain that regulate reproduction in both men and women.

A 2023 study also found an association between exposure to pesticides and significantly lower sperm concentrations.

Pesticides “can imitate or interfere with naturally occurring hormones, and those hormones are necessary for the production of healthy sperm,” said Melissa Perry, the study’s author and dean of the College of Public Health at George Mason University.

Kennedy has blamed both factors for falling sperm counts in the U.S., but some researchers say it’s too soon to draw a link to national or worldwide trends.

Vaping, cigarette smoking and binge drinking can also decrease sperm counts. (Research on marijuana use is mixed, with one study suggesting it can increase sperm counts and another finding the opposite.) Testosterone replacement therapy — a treatment that has exploded in popularity among young men looking to feel more energized or to increase their sex drive — can also shut off sperm production entirely.

“Men on testosterone are almost uniformly azoospermic and totally infertile, and sometimes that is only partially reversible if they’ve been on high-dose testosterone for many years,” Lundy said.

Kennedy himself told Newsmax in 2023 that he takes testosterone replacement as part of an “anti-aging protocol.” Most doctors say the treatment should be reserved for people with a medical condition and is not meant to counteract the normal aging process or increase vitality in young men.

While sperm count can influence fertility, it’s not the only factor. The shape and movement of sperm can also have an effect, since slow or misshapen sperm can have trouble reaching or fertilizing an egg. Swollen veins in the scrotum called varicoceles can play a role, too.

“If you lined up 100 men who are having fertility problems, about 35% or 36% would have varicoceles,” said Dr. Stanton Honig, a urology professor at Yale School of Medicine. “That’s one of the most treatable, reversible causes of male factor infertility.”

Honig said doctors tend to get concerned when sperm counts fall below 15 million sperm per milliliter of semen, or less than 31% of sperm being mobile. But even then, a suboptimal sperm count doesn’t necessarily mean an inability to reproduce.

“You have to get to pretty low sperm concentration levels before you start seeing an impact on a couple’s ability to become pregnant,” Chavarro said.

Even men with high sperm counts may struggle to have kids. Up to half of male infertility cases have an unknown cause, according to a 2007 study.

Lundy said the issue deserves more attention to better understand men’s health — not because of any fears about humanity dying out.

“This is not the end of our species as we know it,” he said.