Secretary Kennedy Comes to Teen Sperm Journal Club
Please stop making me read academic papers about this
Last Thursday, US Health and Human Services Secretary Robert F. Kennedy, Jr. claimed that teenage boys’ sperm count was 50% of the sperm count of 65-year-old men. I immediately cocked an eyebrow, since clinical studies are tightly regulated by ethics oversight panels called Institutional Review Boards (IRBs). Studies involving minor children are even more tightly regulated, since they cannot legally consent to participation. I found it implausible that any IRB would approve a study in which sample collection is dependent on either invasive or even surgical methods or facilitating minor children masturbating into a cup for no reason besides “let’s see what their sperm counts are.” Curiosity about sperm counts is insufficient scientific justification for clinical studies with significant potential to harm participants’ physical or mental health. That’s beside the general level of creepiness that Kennedy’s seeming obsession with child fertility evokes.
I did a literature search on PubMed, a search engine for academic papers and preprints operated by the National Center for Biotechnology Information (NCBI), which is among the many NIH centers that Kennedy oversees. My suspicion was that there was likely insufficient data to make population-level estimates about teen boys’ sperm counts. Querying “teenage sperm count” resulted in quite a few hits, but after looking through them, they were mostly irrelevant. As I anticipated, very few studies quantify sperm counts in healthy adolescents.
Most of the papers indexed under these search terms were examining the impact of various medical conditions on fertility. There were a number of studies on spermatocele (a cyst in the epididymis) or varicocele (a hemorrhoid in the scrotum), usually investigating the effect of treatment. Studies of teen boys with known testicular disorders aren’t representative of all teen boys as a larger demographic group. There were also some studies involving teenagers receiving puberty blockers or hormone therapy in the context of gender transition. These are not representative of all teen boys either.
The vast majority of the studies were actually studies done in adult men. These overwhelmingly show that while sperm counts have declined over time, they also decline with age. Even studies in “young men” that involved 18 and 19-year-old participants showed that sperm counts drop substantially with age. In this study, for example, there is a clear age-related decline in semen volume, sperm count, and motility.
Even though there’s not enough data to assess teenage sperm counts, it seems that no matter what, they decline in men over age 65. So it seems unlikely that teens would deviate from this pattern of sperm counts starting off high and dwindling in older adult life.
As usual, some of what Kennedy says is vaguely true (sperm counts are declining overall compared to years past and sperm counts are age-dependent), but his specific claims that there is an epidemic of less fecund teenaged boys in America compared to the virile old baby-making codgers in his demographic is unsupported. That’s not even getting into his claims about testosterone (not true), early-onset puberty in girls (exaggerated by about 5.75 years), and the preposterous notion that “our parents are not having children.” If only his parents weren’t.
I never expect Kennedy to meaningfully engage his critics regarding his scientific and medical claims. His modus operandi has historically been to cherry-pick, misinterpret, or outright make up data to support his outrageous statements on everything from vaccines and adjuvants to illegally importing bird flu-ridden ostriches to the sanctuary of Center for Medicare and Medicaid Services (CMS) Administrator Mehmet Oz’s Florida estate.
So I was surprised when Kennedy showed up in my hellsite replies responding to a demand to “support his crackpottery with research” by posting an actual peer-reviewed paper from a legitimate journal: “Temporal trends in sperm count: a systematic review and meta-regression analysis” by Levine and colleagues, published in Human Reproduction Update in 2017.
Good scientists must always be open to new evidence emerging, so I read Levine et al. I love a vigorous academic paper discussion! I am always up for an evidence-based scientific debate, which Kennedy is usually not, so I am not going to miss this opportunity to engage the MAHA man himself. Get ready, teen sperm count enthusiasts: time to discuss a scientific paper!
Gold Standard Semen
Kennedy shared this paper in direct response to the demand that he produce “research published in a reputable, peer-reviewed academic journal” falsifying the hypothesis that “the sperm count and testosterone of an adolescent is 50% that of a 65-year-old is something that Bobby pulled out of his ass.” To evaluate this, I will examine whether Levine et al quantifies sperm count in teenagers vs older men.
Human Reproduction Update is a very reputable journal. It is published by the European Society of Human Reproduction and Embryology and has an Impact Factor of 16.1, which is very good. Impact Factor is calculated based on citations over a two year period, so it can be skewed by a couple prominent papers and doesn’t always reflect what is scientifically impactful as much as what is popular. It’s a superficial and imperfect way of assessing journal quality, but it does indicate that Human Reproduction Update publishes papers that get cited a lot. According to their journal policies, Associate Editors (academics serving as editors) send all manuscripts for external peer review by qualified, unconflicted experts. Kennedy did indeed share a paper from a reputable, peer-reviewed academic journal.
Levine et al is a systematic review, which means that the authors did a literature search to identify relevant studies on sperm count data and included or excluded them based on meeting specific criteria according to a reporting standard called PRISMA. These criteria were established in advance of conducting the study, which is best practice for systematic reviews. Then they analyzed data from the papers that met their inclusion criteria using regression analyses.
The authors did a defined PubMed search and got 7518 hits, screened these for sperm data based on the title and the abstract, then screened again based on the full text. They excluded studies that selected participants by infertility, semen quality, disease, or genital abnormalities, studies that used automated counting methods, studies that used samples not obtained through masturbation, and animal studies. They found 244 studies that met inclusion criteria.
Figure 2 shows a decline in both sperm concentration and total sperm counts over the decades, as measured by a simple linear regression analysis of data reported in each study meeting inclusion criteria. This reflects many other studies also showing similar declines. However, it is important to note that this analysis is not stratified by age. The authors separated groups by the country where the study was done (“Western”=North America, Europe, New Zealand, and Australia, “Other”=everywhere else) and by whether or not subjects had previously impregnated someone. There’s a lot of variability, but the trend is clear: sperm counts are going down.
Then they did their own meta-regression analysis of the data itself, in which they adjusted for confounding variables and the trend continues. A meta-regression is a way of saying they extracted and combined the data from these different studies and ran their own regression analysis on it. This showed the same thing for all the groups, except for “Fertile Other” men: sperm counts are plummeting! In 2010, sperm counts were more than 50% lower than they were in the 1970s in the “Western” studies. Bad news for white ethnonationalists—sperm counts in demonstrably fertile men from Asia, Africa, and South America are going up!
Now, you may notice that age was a confounding variable used to adjust the statistical model in this meta-analysis. However, the authors didn’t stratify these data by age and perform a subgroup analysis. They didn’t analyze data comparing teens to older people at all. It’s not included in the supplementary data. Therefore, there is no way to compare sperm counts between age groups in any time period in this study.
There are also some pretty significant limitations to this paper. The authors tried to standardize how sperm counts were quantified across papers and only included studies where sperm were manually counted using a hemacytometer. However, there are multiple ways to count cells on a hemacytometer. We don’t count sperm in my lab, but we use a hemacytometer all the time and we count cells slightly differently than the method described in the WHO lab manual for sperm counting cited by the authors as the standard. This can lead to variability between studies. And studies were included that “likely” used a hemacytometer, so that would increase variability further. Considering this would affect the count data used for their meta-regression analysis, this could impact the magnitude of the observed decline pretty significantly.
Another big issue is that Levine et al does not connect declining sperm counts to impaired fertility. Sperm counts may be dropping, but is that the reason why birth rates are declining? Human reproduction is highly complex and influenced by genetic, hormonal, physiological, medical, behavioral, social, and cultural variables. Probably it’s more complicated than just sperm counts going down.
It’s also worth noting that Shanna Swan, the senior author of Levine et al, wrote a book called Count Down based on these findings. Uh oh—she thinks her findings indicate that humanity is headed for extinction!
Sure, the sperm count zero future that’s just 20 years away sounds pretty bad for humanity’s prospects, but how likely is that future? Based on his statements last Thursday, I would guess that Kennedy thinks it is pretty likely. However, we don’t know the cause of the sperm count declines, so we cannot use them to accurately project what will happen. We can’t assume they will continue to decline at the same rate. And we can’t assume that this has anything to do with teenagers now.
The bottom line is that Levine et al shows that sperm counts are lower for most men now than in decades past, but it does not in any way examine counts in teens or older men at any time. It does not address testosterone in any way. It does not support the claim that teens have 50% of the sperm count or testosterone levels of men over the age of 65. If Kennedy wants to show this, he should repeat the meta-analysis for subgroups stratified by age (though I think it’s highly unlikely there will be enough data to assess minor child sperm counts, given the inclusion criteria that samples must be collected by masturbation). If he is unable to manage the statistical analysis, surely there are scientists working at CDC or funded by NIH that could do it, if he hasn’t illegally fired them or stripped their grants on the basis of ideological incompatibility with MAHA principles. The hypothesis that Kennedy pulled this out of his ass stands unfalsified.
Declaration of Conflicts of Interest
I am a virologist and have no expertise whatsoever in adolescent fertility. I have no professional expertise in sperm outside of viruses that infect sperm cells, replicate in the testes, or can be transmitted in semen. There are actually a lot of viruses that can do this (gonads are unique immune environments and viruses can establish a niche there). But my detailed knowledge of viral ball infections stops short of teenagers.
Kennedy frequently discusses his critics’ conflicts of interest so I’ll note that I have no grants to investigate anything about teenagers, nor any funding or ongoing projects to look at fertility. I do study the impact of sex hormones on viral pathogenesis (check out my rock star student’s preprint on sex bias in MERS-CoV disease), but have not studied this in the context of fertility or relative to age. I currently have a grant from the Canadian Institutes of Health Research (CIHR) to study the role of the host (including host sex as a variable) in mpox and am an author on a forthcoming preprint looking at mpox virus infection in mouse penis and testes. I am in the process of preparing a CIHR grant application to examine sex bias in host responses across several emerging viruses, but I’m not proposing studies that involve collecting or analyzing sperm count data (in mice).
Although I study vaccines and viral pathogens, I don’t have any funding from or investment in pharmaceutical companies and vaccine manufacturers, so I’m not shilling for Big Pharma. I do get a small stipend for serving as the co-Editor-in-Chief of the journal Vaccine. Vaccine is a peer-reviewed journal covering all aspects of vaccine research. Our entire team of scientific editors and our editorial board are all experts who recognize that the evidence shows vaccination has saved millions of lives globally. Expertise is not a conflict, although I suspect Kennedy would disagree. However, since Kennedy’s recent claim does not overtly blame vaccines for ruining teen fertility, I don’t see how my editing a journal about vaccinology is relevant when he hasn’t demonstrated that teen fertility is declining relative to old guys’ in the first place.
Personally, I have no teen sperm count conflicts of interest either. I have two adult stepsons in their twenties, and, while I have discussed sexual health with them in a parenting context (wear a condom, get your HPV vaccines, here’s how to avoid infections and unwanted pregnancy, get tested when you are sexually active, respect your partners, always get consent, etc), I have never had so much as a passing thought about their sperm counts during any phase of their lives (at least until Kennedy brought it up). I love them very much and care deeply about their health and well-being, but they are adults and I respect their privacy and autonomy. Their fertility is their business, not mine. My husband is almost 65, and I confirmed with him that there have been no comparative father-son sperm count discussions at any point. I have no nuts in the sack of teen sperm count science, professionally or personally.
Kennedy does, however, have some conflicts. The anti-vax organization he founded, Children’s Health Defense (CHD), raised millions during his leadership, mostly through fearmongering-inspired donations, litigation, sales of subscriptions, books, DVDs, and merchandise. During his tenure as chairman from 2015-2023, their blog The Defender published nearly 80 hand-wringing articles claiming that sperm counts (as well as penis size) are cratering due to phthalates, plastics, and other endocrine disruptors, Pfizer COVID-19 vaccines, SARS-CoV-2 spike protein, oat plant growth regulators, 5G wireless signals, household cleaners, glyphosate and GMOs, prior NIH Director Francis Collins, the aluminum salt adjuvants in the HPV vaccine, and the mumps component of the MMR vaccine. Many of these articles were written by Kennedy himself. CHD continues to publish extensively on the topic, adding articles implicating obesity, pesticides, Toxoplasma gondii, polyunsaturated fats, food stabilizers, wildfire smoke, and herbicides in GMO poultry feed.
Kennedy has also been party to numerous lawsuits on male fertility as an attorney, witness, and consultant. According to his own financial disclosures, he made nearly $20 million through his work as an attorney. Kennedy has made nearly $2.5 million by referring cases to the law firm Wisner Baum, which has pursued extensive litigation against Bayer and Merck that include claims about declines in fertility regarding GMOs and vaccines, respectively. There have already been $11 billion in payouts from the Bayer/Monsanto lawsuits, with 61,000 cases still pending. Kennedy served as co-counsel on some of the resolved cases, indicating that he has a significant financial interest in connecting fertility declines to GMOs and vaccines. Although Kennedy has pledged to divest his future contingency fees from the Merck Gardasil lawsuits to his son Conor (who is an attorney at Wisner Baum and thus also profiting from this litigation), to the best of my knowledge there is no evidence that he has done so. CHD actively bankrolls litigation in this space, as well, allowing Kennedy yet another conduit for profiting from this organization besides the nearly $1 million in salary he drew between 2021-2024.
So if you are wondering why Kennedy has spent countless hours fabricating quackerous sperm count claims about children, it’s because he has millions of good reasons in his bank account to convince people that vaccines and biotechnology are going to turn planet Earth into a Children of Men-style hellscape of hopeless impending human extinction. Also, I assume it’s because he’s as weird and creepy with kids as he is with adult women, as one must be to spend this much time crafting false pro-natalist narratives about teen semen.
I suggest Kennedy focus future research efforts on actually engaging with the evidence base before making absurd, easily disproven claims that he is unable to support with papers he doesn’t appear to have read or understood. I will happily engage in future discussions about how to interpret scientific papers if asked and I suspect many of my colleagues will too, including people more qualified to comment on fertility and sperm counts than I am. His track record suggests that he will not do so, as he has strong financial and ideological disincentives from adhering to evidence-based scientific methods.
But speaking of Kennedy’s ideology, if he is really worried about declining fertility, he’d do well to table his efforts to cancel the MMR vaccine. Around 30% of post-pubescent males who get mumps orchitis have longer-term fertility problems. The MMR vaccine prevents this. Other vaccine-preventable infections that impact testicular function, semen quality, and fertility include SARS-CoV-2, influenza virus, hepatitis B virus, Ebola virus, and HPV. To safeguard America’s reproductive future, Kennedy should champion vaccines.
thanks for providing insight into the actual literature, and for clearly indicating why there are no studies of teenage sperm counts in normal individuals.
Your recitation of Kennedy's copious conflicts of interest was alone worth the price of admission. This is a kakistocracy chock full of the worst possible people. Hell, cabinet meetings alone have three rapists sitting at the table, including Mr. Sperm Count Kennedy.