IS MALE FERTILITY DECLINING? WHAT THE LATEST SCIENCE REALLY SAYS

Introduction:
A recent statement by U.S. Health and Human Services Secretary Robert F. Kennedy Jr. has alarmed many. In a press conference, he claimed: “Today the average teenager in this country has 50% of the sperm count, 50% of the testosterone of a 65-year-old man. Our girls are hitting puberty 6 years early.” Such a dramatic assertion calls for careful scrutiny. Are young men’s reproductive parameters truly that diminished compared to older men? And what does science really say about long-term trends in sperm counts? This article takes a deep dive into the peer-reviewed evidence on male fertility and hormone levels to separate fact from fiction. We’ll examine the accuracy of RFK Jr.’s statement, review the latest research on sperm count trends over the decades, and reconcile conflicting findings from different studies. The goal is an evidence-based, nuanced understanding of male reproductive health trends – delivered in a professional yet approachable tone for both healthcare providers and the general public.

Claim Check: Do Teens Have Half the Sperm Count and Testosterone of 65-Year-Old Men?

Secretary Kennedy’s claim suggests that today’s teenagers are far worse off, reproductively speaking, than older men. Let’s break this into two parts – sperm count and testosterone – and compare adolescents to senior men in each:

• Sperm Count by Age: It is not true that an average teenager’s sperm count is half that of a 65-year-old man. In fact, if anything, the opposite is closer to reality. Sperm count and overall semen quality tend to decline with advancing age. Research shows that men over 50 are much more likely to have low sperm counts than young men. For example, a large study of 2,678 men in Chile found that men above age 50 had over 6 times greater odds of an abnormally low total sperm count compared to men aged 21–30. Similarly, men over 50 were about 2 times more likely to have low sperm concentration (sperm per milliliter) than 20-somethings. Aging is accompanied by decreased testicular function – after about age 40, semen volume, sperm motility, and normal morphology all gradually worsen. In contrast, by late adolescence (the teen years), most males have completed puberty and achieved normal adult sperm production capacity. A healthy 18- or 19-year-old man would typically have a higher sperm count and better sperm motility than a man in his 60s. In short, the suggestion that teens have half the sperm count of seniors is not supported by biology or clinical data. On the contrary, older men often experience declining sperm parameters with age.
• Testosterone by Age: The claim that teenagers have 50% of the testosterone of 65-year-olds is flat-out incorrect. Under normal physiology, young men have significantly higher testosterone levels than older men. Testosterone (T) surges during puberty and typically peaks in late adolescence and early adulthood, fueling the development of male muscle mass, strength, body hair, and libido. After about age 30, men’s testosterone levels begin to gradually decline – roughly by 1% per year on average. By age 65, many men have substantially lower T than they did in youth. For example, longitudinal studies indicate a population-level drop in testosterone levels over the adult lifespan. One famous study from Massachusetts showed that a man’s total testosterone at age 70 is, on average, about 30% lower than it was when he was 30, even after accounting for health and lifestyle factors. In fact, researchers observed an additional secular trend: men born more recently tend to have lower T than those born decades earlier at the same age. But in any case, a typical healthy teenager should have testosterone near the top of the normal adult range, whereas a typical 65-year-old is often near the lower end of the normal range. To put it simply: a 19-year-old male usually has much more testosterone circulating than a 65-year-old male. The Mayo Clinic emphasizes that testosterone peaks in adolescence/early-20s and then declines steadily with age. Therefore, RFK Jr.’s statement completely inverts the actual relationship – teenagers do not have half the testosterone of seniors; if anything, an older man might have roughly half the testosterone of a teen. (It’s possible the Secretary was trying to refer to generational drops in testosterone, a separate issue we’ll touch on later, but as stated, his comparison is biologically implausible.)
• Girls and Puberty Timing: The claim that girls are hitting puberty 6 years earlier than before is a gross exaggeration. True, the average age of puberty onset in girls has trended slightly earlier over the past century, but nowhere near by six years. Historically, menarche (a girl’s first menstrual period) occurred around age 13–14 in the mid-20th century in developed countries. Today the average age at menarche is about 12½ years – only modestly younger than a few decades ago. For breast development (thelarche), a comprehensive 2020 meta-analysis found about a 0.24-year (3 month) decrease per decade in the age of onset from 1977 to 2013. This adds up to perhaps ~1 year earlier onset over 40 years – meaning, for example, that what was age 11 before might be age 10 now on average. No credible data show an average 6-year shift in normal puberty timing. An average American girl does not begin true puberty in first grade; the normal window for female puberty remains about 8–13 years of age. (Precocious puberty – before age 8 in girls – can occur in rare cases due to pathological factors or severe environmental exposures, but it is not the population norm.) Thus, the statement that “our girls are hitting puberty 6 years early” is unsupported. At most, one might say puberty is happening a bit earlier than in past generations, likely due to factors like improved nutrition and higher childhood obesity rates, but on the order of 1 year earlier, not six.

Bottom Line: Secretary Kennedy’s dramatic soundbite does not hold up to scientific scrutiny. Teen boys today do nothave half the sperm count or testosterone of older men. In fact, teens and young adults generally have higher fertility potential and hormone levels than their grandfathers. And while there are slight downward trends in the age of puberty in girls, it’s measured in months or a couple of years, not an alarming six-year difference. Misrepresenting these facts can cause unnecessary panic. However, there are real concerns about reproductive health trends that merit discussion – just not in the way the Secretary phrased them. The remainder of this article will focus on one major aspect of his remarks: the idea of declining sperm counts over time, which is a topic of active scientific research and debate.

Are Sperm Counts Declining? A Look at the Evidence

The notion that men’s sperm counts are much lower today than in decades past has been circulating for years, gaining traction with a series of high-profile studies. Secretary Kennedy cited two specific studies (from 2017 and 2023) as the basis for his sperm count claim. These studies, by Dr. Hagai Levine and colleagues, indeed reported substantial declines in sperm counts since the 1970s. But newer research, including a 2025 analysis by Dr. Kieran Lewis’s team, paints a more nuanced picture – even suggesting stable or increasing sperm counts in some populations. Let’s unpack the findings:

1. The 2017 Meta-Analysis (“Western World” Decline):
   In 2017, a landmark systematic review and meta-regression by Levine et al.compiled data from 185 studies(42,935 men) between 1973 and 2011. The men were grouped by region and whether they were from fertile or unselected (general) populations. The key finding was striking: sperm counts (both concentration and total count) had plummeted in North America, Europe, Australia, and New Zealand(collectively termed “Western” countries) over that period. Among men unselected for fertility in these Western regions, average sperm concentration declined by about 1.4% per year, culminating in an overall drop of 52.4% between 1973 and 2011. The average sperm concentration in Western men went from roughly 99 million per mL in 1973 down to about 47 million/mL by 2011. Similarly, total sperm count (which accounts for semen volume) declined by 59.3% in Western men, a loss of over half the total sperm per ejaculate on average. These results were statistically significant and held up across multiple sensitivity analyses. Men from other regions (Asia, Africa, South America) did not show a significant decline in that 2017 analysis, but importantly there were far fewer data from those regions at the time. The 2017 study grabbed headlines worldwide, with headlines like “Sperm Counts Halved in 40 Years.” It raised concern that environmental factors – dubbed in popular media as everything from chemicals to lifestyle changes – might be causing a reproductive health crisis in the developed world. The authors themselves called the decline a potential “canary in the coal mine” for male health, urging research into causes. This study is what initially fueled much public discussion (and likely informed RFK Jr.’s thinking about generational decline).
2. The 2023 Meta-Analysis (Global Decline and Acceleration):
   Levine’s group followed up with a new meta-analysis, published in early 2023, that incorporated more recent studies (up to 2018) and data from previously underrepresented regions. This updated review included 223 studies from 53 countries, providing a truly global picture of sperm counts from 1973–2018. The headline results reinforced and extended the 2017 findings:

• Global decline: Overall, across all continents, sperm concentrations dropped significantly from 1973 to 2018. The meta-regression estimated about –0.87 million sperm per mL per year decline on average. That corresponds to a decline from ~101 million/mL in the early 1970s to ~49 million/mL by 2018 – roughly a 51.6% overall decline in average sperm concentration. In terms of total sperm count (TSC), the global decline was about 62% over that period. These numbers mirror the earlier “Western” decline, but now it appears the drop is global, not just in Europe/North America. In fact, this 2023 analysis was the first to report declines in regions like Asia, Africa, and Latin America. For example, men from South/Central America, Africa, and Asia (SAA countries) showed a significant downward trend in sperm concentration (about –0.65 million/mL per year among unselected men) whereas no decline was evident in those regions in the 2017 analysis due to limited data.
• Post-2000 acceleration: Worryingly, the decline in sperm counts appears to be accelerating in the 21st century. When Levine et al. looked only at data from 2000 onward, the slope of decline became steeper – about –1.73 million/mL per year, equivalent to a 2.64% per year decline since 2000, versus ~1.16% per year in the late 20th century. In other words, not only did sperm counts drop markedly in the last 50 years, but the rate of decline doubled after the turn of the millennium. This finding grabbed headlines with terms like “the decline is accelerating”. If real, such an accelerated trend could mean average sperm counts are falling to levels that may impact fertility on a population level in the coming decades.
• Robustness: The 2023 study took pains to account for factors like men’s ages, abstinence period before sample, and lab methods, and it excluded men known to have infertility. The decline persisted despite these controls. Dr. Levine and colleagues concluded that the decline in sperm count is “a worldwide phenomenon” and urged action to investigate causes. They described the situation as a potential “crisis” for global reproductive health.

These two meta-analyses (2017 and 2023) are what RFK Jr. referenced as evidence that young men today have about half the sperm count that their fathers or grandfathers did – and indeed they do show roughly a halving of average sperm counts over ~40-50 years. It’s important to note, however, that these studies report population-level averages and trends. Even after a 50% drop, the average sperm concentration in 2018 was ~49 million/mL, which is still above the WHO’s threshold for “normal” (15 million/mL) – but it means more men fall into lower ranges than before. The implications could include more men experiencing sub-fertility or longer times to conceive than in the past.

3. The 2025 U.S. Focused Meta-Analysis (Stable Counts in America):
   Contrasting with the above global studies, a new 2025 systematic review and meta-analysisby Lewis et al.(Cleveland Clinic and colleagues) examined sperm count trends specifically in the United States. This review, published in Fertility and Sterility, narrowed in on U.S. men without known infertility from 1970 to 2023. Importantly, it included both men from the general population and men proven fertile (e.g. recent fathers or sperm donors), but excluded men who were known to have fertility problems. The motivation was to address heterogeneity and concern that global data might not apply uniformly to every region. Here are the key findings:

• No significant decline in sperm concentration in the U.S.: Across 58 studies representing 11,787 American men, the authors found no overall change in average sperm concentration from 1970 to 2018. In an unadjusted model, sperm concentration actually showed a tiny positive slope (+0.14 million/mL per year, which was not statistically significant). After adjusting for factors like region of the country, there was still no significant decline. Only after adjusting for both region and whether the men were confirmed fertile or not did a slight downward trend (–0.35 million/mL per year) reach statistical significance. This adjusted trend corresponds to about a 17.5 million/mL decrease over 50 years, which the authors noted is a much smaller magnitude than the declines reported globally. Furthermore, when breaking it down by subgroups, no U.S. region (Northeast, South, Midwest, West) showed a significant decline on its own, and neither did the subset of fertile men nor the subset of men with unknown fertility status. In plain language: U.S. sperm concentrations appear to be stable overall in the last 50+ years, with at most a very modest decrease.
• Increase in total sperm count: Intriguingly, among the studies that reported total sperm count (which factors in semen volume), the U.S. analysis found a significant increase in total sperm count over time. Mean total sperm count rose by about +2.9 million sperm per year between 1970 and 2018. Over decades, that adds up. This suggests that if anything, ejaculate sizes (volume × concentration) might be getting larger on average for American men. (One could speculate reasons – perhaps better overall health or nutrition improving seminal volume, etc., but the study did not pin down causes.) Regardless, this finding starkly contrasts the ~–2 million/year decline globally found by Levine’s group. It suggests no fertility apocalypse is evident in the U.S. data through 2018 – in fact, the average American man’s total sperm output may be slightly higher now than in the 1970s.
• Reassurance and caveats: Dr. Scott Lundy, the study’s senior author, said these results are “somewhat reassuring” – indicating “no cause for panic” that sperm counts are universally plummeting. However, he also cautioned that this doesn’t mean we should dismiss concerns; rather, it shows the importance of examining data carefully. The authors pointed out that their analysis doesn’t address male infertility rates (which could be influenced by many factors besides count) and that it’s possible specific subpopulations or regions might have trends that get obscured in the aggregate. They encourage continued monitoring but suggest the “sperm Armageddon” narrative may not apply uniformly everywhere.

4. Other Studies and Context:
   Apart from these high-profile analyses, the literature contains mixed findings on sperm trends, often depending on location and methodology:

• A number of European studies have echoed declines. For instance, a French study reported a ~32% decline in sperm concentration from 1989 to 2005 among men in fertility evaluation. And a recent Chinese systematic review (including >300,000 men) noted a decline of about –2.0 million/year in total count from 1981 to 2019, quite similar to Levine’s global figure. These reinforce that many countries show downward trends.
• On the other hand, several Northern European studies found little change or slight increases in recent decades. We have a great example from Denmark: Beginning in the 1990s, Danish researchers started systematically collecting semen samples from young men entering military service. This provided a relatively homogeneous, population-based sample of young adults over time. In 2012, Jørgensen et al. published results on 4,867 Danish men (median age 19) from 1996–2010. The results were somewhat surprising: sperm quality did not deteriorate over those 15 years – in fact, it improved slightly. Median sperm concentration increased from 43 million/mL in 1996 to 48 million/mL in 2010 (a statistically significant rise) and median total count rose from 132 million to 151 million. There was no change in motility or morphology, but already-high levels of abnormalities remained high (more on that later). The key point is that in this well-controlled Danish cohort, sperm counts were stable or trending upward in the 2000s, not falling. The authors did note that the levels, while stable, were not great – only 23% of these young men had optimal sperm counts and morphology by strict criteria, and a substantial fraction could face fertility delays. But the feared year-over-year decline wasn’t observed in Denmark.
• Sperm donors vs general population: Some studies of sperm donor applicants have also not shown declines, and if anything, some programs report stable or better semen quality in recent cohorts. However, donors are a selected group (they must meet minimum count criteria), so they aren’t perfect mirrors of the general population. Still, their data can be useful. One U.S. sperm bank reported no significant change in donor sperm counts over 2008–2018, though a slight dip in motility in the late 2010s was noted (possibly transient or method-related).
• Sperm quality factors: It’s worth noting that “sperm count” is not the only aspect of male fertility. Sperm motility (swimming ability) and morphology (shape) are crucial too. Some research suggests motility and morphology have also changed over time. For example, a Spanish study (infertile patients, 1997–2017) found no change in sperm count but a decline in motility and normal morphology percentage. In contrast, the Danish conscript study found motility and morphology were unchanged (motility remained high ~68% throughout; morphology abysmally low ~7% normal forms throughout – reflecting very strict criteria). The Levine meta-analyses did not examine motility/morphology, so it’s possible that even if counts are holding in some places, other quality factors might be shifting. However, evaluating those is even trickier due to differences in lab assessment techniques over time (especially morphology criteria changes). The consensus is that age affects motility and DNA integrity (older men have lower motile sperm and more DNA fragmentation), but secular trends in these parameters are not well documented.

In summary, when asking “are sperm counts lower among men today than decades ago?”, the weight of the evidence suggests yes, on average, there has been a decline, at least in many regions. The headline-grabbing meta-analyses estimate roughly a 50–60% global drop in the last 50 years. That is substantial and has rightly raised concerns about environmental and lifestyle factors impacting male reproductive health. However, the evidence is not monolithic or uniformly agreed upon. There are credible data – especially from focused, single-country studies like the U.S. and Denmark – that show little to no decline in sperm counts in certain populations. The picture is nuanced: it may be that some areas or groups have experienced dramatic declines, while others have remained stable. It’s also possible that methodological differences (how semen samples are collected, counted, and reported) could create artificial trends. This brings us to an important question: How do we reconcile these apparently conflicting results?

Reconciling Conflicting Findings: Why Do Studies Disagree on Sperm Trends?

It’s perplexing at first – one set of analyses shows a sharp worldwide decline in sperm counts, while another analysis of U.S. data finds stability or even an increase. Is one set of studies wrong, or is there some way to make sense of this discrepancy? Scientists have been actively debating this issue. Several factors likely explain the divergent findings:

1. Population Differences (Geography and Selection):The simplest explanation is that trends genuinely differ by region or population.The global meta-analyses lump data from many countries together to find an overall average trend. It could be that in some countries (or time periods) sperm counts dropped a lot, whereas in others they stayed level or even rose – and the global average still works out to a decline. For instance, Western Europe might have seen strong declines due to certain environmental exposures or lifestyle changes, while perhaps North America’s levels held steadier (hypothetically). The 2017 Levine study indeed found that the decline up to 2011 was driven mainly by Western countries, whereas “Other” countries (Asia, Africa, S/C America) did not show a significant drop. By the 2023 update, some of those other regions did show declines as more data accumulated, but the magnitude might still vary. The 2025 U.S. analysis specifically looked at American men and found no big change, suggesting the U.S. might be an outlier compared to the global trend. It’s worth noting that even within the “West”, countries differ: Denmark’s young men didn’t see a drop in 1996–2010, but studies in France did find a drop in a similar timeframe. Differences in environmental regulations, obesity rates, maternal exposures, healthcare, etc., could contribute to why one country’s trend differs from another’s.

Another population factor is who was included in the studies. The Levine meta-analyses tried to separate “unselected” men (general population or men unaware of fertility issues) versus “fertile” men (known to have fathered a child or sperm donors). They found declines in both groups in Western countries, though slightly larger in unselected men. The U.S. study included both but excluded any known infertile men (whereas some older studies in the meta might have included men from infertility clinics). If earlier decades’ data had proportionally more subfertile men in them than recent data (or vice versa), that could bias trends. The 2025 study authors tried to minimize such bias by only using studies of men without diagnosed infertility. Still, the inclusion criteria and baseline characteristics of study populations differed study to study. For example, some older datasets might be from military recruits, others from husbands of pregnant women, others from convenience samples of college students or even prisoners. If the mix of these changed over time, it could create spurious trends. The ideal way to assess trend is to sample the same population in the same way repeatedly over time – which very few studies have done (the Danish conscripts study is one of the rare ones). Lacking that, meta-analysts have to piece together disparate studies and adjust for differences as much as possible.

1. Methodological and Measurement Variability: One major point of contention is whether the observed declines are partly an artifact of how semen analysis is performed. Sperm counting is notoriously variable. Techniques and lab standards have improved over time, especially with the advent of improved training and quality control in labs. Ironically, this could mean that older studies (say in the 1970s) might have overestimated counts compared to modern rigorous labs, or possibly the opposite. Allan Pacey, a leading andrologist, has argued that a lot of the decline seen could be due to us “getting better” at counting sperm – implying earlier labs may have been overcounting or that modern labs are more strict. Levine’s team, however, believes they accounted for this by excluding studies that used outdated methods and by focusing on studies using the gold-standard hemocytometer count throughout. They also note that even if early studies were less standardized, the fact that recent data (post-2000) show a continued decline suggests it’s not just a methodological artifact. Nonetheless, lab-to-lab variability is a concern. For example, the World Health Organization’s criteria for what’s “normal” have changed over editions of its manual, and how technicians count and classify cells can differ.

To illustrate, if older studies more often counted semen that had been stored or not properly mixed (leading to undercounting), and newer studies handled samples better, that could artificially create a rising trend – but more likely it’s the opposite scenario: in the past, some counts might have been inflated due to less stringent criteria for what counted as sperm or less rigorous counting (e.g., not accounting for duplicate counting of the same cells in clumped semen, etc.). A 2022 review by Auger et al. concluded that because of the high heterogeneity and many “hidden” confounders in retrospective sperm data, “available data do not enable us to conclude that human semen quality is deteriorating worldwide” in a definitive way – essentially cautioning that we must interpret these trends carefully. That review suggests that only in specific locales where consistent methods were used can we be sure of a decline. Thus, part of the conflict could simply be: the Levine studies combined many reports (maximizing power but also heterogeneity), whereas the Lewis U.S. study narrowed inclusion and perhaps got a cleaner, more internally consistent dataset for one country. Different methodological choices can lead to somewhat different outcomes.

1. Changes in Lifestyle and Health Factors: Why would sperm counts decline in the first place? Researchers hypothesize many contributing factors: environmental chemicals(endocrine disruptors like phthalates, BPA, pesticides), testicular heat (from sedentary lifestyle or perhaps even global warming), diet and obesity, stress, smoking, alcohol, drug use, etc. These factors have not been definitively proven to cause the global trend, but many are plausible. If the prevalence of these risk factors differs by country or time, that could explain divergent trends. For example, obesity rates have skyrocketed in the U.S. over the last 40 years. Obesity is associated with lower testosterone and often lower sperm counts (due to both hormonal and thermal effects). Yet the U.S. didn’t show a decline in counts – which might be due to other improvements counteracting it (e.g., reductions in smoking rates, better prenatal care, or perhaps the U.S. data included more fertile men who maintain decent counts). Europe and other regions have also seen lifestyle shifts. It’s possible that earlier decades had more heavy smokers (smoking can impair spermatogenesis), whereas recent decades have more obese individuals – two different negative factors. The net effect on sperm count might vary.

Furthermore, changes in selection pressure in the data might be at play: In the 1970s, many men getting tested might have been those already suspected of fertility issues (because routine testing wasn’t common). In the 2000s, more studies include volunteers and general population. So a selection bias could make earlier averages look higher if only very healthy men (e.g., sperm donors or men with many children) were typically studied, or conversely, it could make earlier averages look lower if only subfertile men were in clinics. Levine’s meta tried to mitigate that by separating out fertile vs unselected groups. They indeed showed the biggest decline in unselected Western men, but fertile Western men also had a decline (~0.8%/year).

1. Statistical Considerations and Publication Bias: Meta-analyses are only as good as the data available. It’s worth asking: are studies with certain results more likely to be published? If, say, researchers were more interested in reporting declines (because it’s an interesting positive finding) than reporting no change, there could be publication bias. Levine et al.did test for that and claimed it was unlikely to explain the result. Another angle: small studies with high variability might muddy the waters. The 2025 U.S. review had far fewer total men (~12k) than the global (~60k+), yet it found no decline. It could be a matter of statistical power or lack thereof – but actually the U.S. study would easily detect a large drop if it were there. Instead, it confidently refutes anything more than a mild decline. So likely not a power issue, but it’s possible that by focusing on one country, they removed some sources of variation that global studies couldn’t avoid.

Finally, chance and time windows: If most of the global decline happened from the 1970s to 1990s and then plateaued, a study focusing on 1996–2010 (like the Danish one) might show flat trend – because the big drop had already occurred before 1996. There is some hint of this: Levine’s data suggested a steady decline even post-1995, but others speculate maybe we hit a floor or slower phase around the 1990s. The U.S. data from 1970s to 2010s showed flat overall; maybe the U.S. dropped earlier and then stabilized? We don’t have great data from pre-1970s in the U.S. to be sure. Some historical reports going back to the 1930s (e.g., a famous 1992 analysis by Carlsen et al.) implied a decline from 1938 to 1990 as well, which if real, could mean a long multi-phase decline. Alternatively, perhaps the U.S. truly maintained steady counts thanks to unknown factors.

So, where does the truth lie? Most likely, there has been a real decline in sperm counts in many places, but the magnitude may have been overstated in some media narratives. The global meta-analyses used rigorous methods and a huge dataset, and their findings of ~50-60% decline are statistically robust. That said, not every population follows the global average. It’s analogous to climate change data – globally the temperature is rising, but some regions may have stable or even cooler trends in certain periods; it doesn’t negate the global signal, it just adds complexity. Here, globally sperm counts are down, but U.S. men (for example) haven’t experienced as steep a drop, for reasons still unclear.

Another important perspective is given by a 2022 review in Nature Reviews Urology (Auger et al.), which argued that when focusing on well-designed studies, declines are evident only in specific regions and cohorts, and that the idea of a uniform worldwide decline might be too simplistic. They call for prospective monitoringgoing forward, which could settle these questions by using consistent protocols over time.

In the meantime, most experts concur that even if the decline is somewhat overstated, the possibility that environmental or lifestyle factors are negatively affecting male reproductive health should be taken seriously. Dr. Shanna Swan (one of the co-authors on the Levine studies) has even warned about a “fertility crisis,” suggesting that if trends continue, we could reach a point where a large proportion of men have subfertile sperm counts. Other researchers urge caution, noting that average counts are still above fertile thresholds and that male fertility involves more than just the count. For example, despite falling sperm counts, the total fertility rate (number of children per woman) has more to do with societal choices than sperm biology – though in a sub-fertile population, more couples might need IVF or take longer to conceive.

From a clinical perspective, what matters is when do low sperm counts translate into infertility? Generally, counts above ~40–50 million/mL are considered unlikely to impair fertility, while counts below ~15 million/mL are associated with reduced chance of conception. The concern raised by Levine et al. is that as the mean drops, the fraction of men below, say, 40 or 20 million/mL increases, potentially leading to more infertility cases. This is a legitimate public health question.

To sum up the reconciliation: The seemingly conflicting results can be explained by differences in population, methodology, and possibly genuine regional variations. The global decline is likely real, but not universal – some populations (like U.S. men or young Danish men) have not seen significant declines, which suggests factors such as environment or lifestyle could be protective in some contexts or harmful in others. Additionally, improved standardization in counting might have reduced some artifactual inflation of counts in the past, making recent counts relatively lower. Researchers are actively working to identify the causes – chemical exposures (like endocrine disruptors), obesity, and in utero influences (like a mother’s smoking or chemical exposure during pregnancy affecting her son’s future fertility) are prime suspects. Each of these factors varies across time and geography, potentially contributing to why the U.S. might differ from, say, Europe in sperm trends.

Conclusion and Takeaways

Bringing it back to Secretary Kennedy’s statement: No, the average American teenager does not have half the sperm count or testosterone of a 65-year-old man. That claim is an inaccurate portrayal of current science. Teen boys, as a group, still have robust testosterone levels and, assuming they’ve gone through puberty, generally healthy sperm production. Older men, by contrast, experience a natural decline in both hormones and semen quality as part of aging. So on that point, the Secretary’s comment can be firmly debunked.

However, when considering the broader issue that likely underlies his concern – that today’s young men have lower sperm counts than young men of previous generations – the answer is more complex. The current weight of scientific evidence does suggest a significant decline in sperm counts over the last 40-50 years in many parts of the world. This includes a roughly 50% global average drop in sperm concentration and total count since the 1970s, as demonstrated in large meta-analyses. If those results reflect reality, it is indeed a concerning trend that could have implications for fertility and population health. On the other hand, not all data sets agree – for example, extensive data from the U.S. and some northern European cohorts do not show a dramatic decline and are somewhat reassuring. Thus, we can say: men today on average appear to have lower sperm counts than men decades ago, but the magnitude of this change and its universality are still debated. There is no consensus that we are in a full-blown crisis, but there is enough smoke that further research is urgently needed to identify any fire.

What explains the differences between studies like Levine’s and the 2025 Cleveland Clinic analysis? Likely a combination of methodological rigor, population selection, and true regional variability. The global studies aimed big and found a big signal – but potentially swept in more noise. The U.S.-only study aimed narrower and found little change – suggesting perhaps the global trend doesn’t strongly manifest in the U.S. or that the prior methods may have overestimated declines. We should not dismiss the global decline as an artifact, but neither should we ignore data that provide a counter-narrative. The prudent approach is to continue refining our surveillance of male reproductive health. Prospective, standardized studies (for example, routinely sampling young men across multiple countries every 5-10 years) would greatly help confirm or refute these trends in the future.

From a public health standpoint, regardless of whether the drop is 50% or, say, only 10%, the factors that potentially contribute to lower sperm counts (and also to other health issues) are worth addressing. Lifestyle improvements – maintaining a healthy weight, not smoking, limiting alcohol, managing stress, and avoiding certain environmental toxins – are common-sense measures that could benefit reproductive health. There is also a need for policies reducing environmental exposure to known endocrine disruptors (like certain phthalates, heavy metals, etc.), which are suspected in affecting fertility. These measures would be beneficial even if sperm counts weren’t falling, but the possibility that they are adds urgency.

In conclusion, RFK Jr.’s eye-catching soundbite is inaccurate in its specifics and should not be taken literally. Teenagers are not biologically half as virile as seniors; girls are not entering puberty at age 6 on average. Those claims distract from the real, subtler issues. However, his underlying concern about generational changes in reproductive health is rooted in legitimate scientific inquiry. The evidence tilts toward a real decline in men’s sperm counts over the past several decades, though perhaps not uniformly everywhere. The scientific community continues to investigate this phenomenon, its causes, and its implications.

For patients and healthcare professionals, the key message is: don’t panic, but do pay attention. Men concerned about their fertility should focus on controllable factors (diet, exercise, avoiding smoking/drugs, and limiting exposure to environmental toxins when possible). Clinicians should stay informed on the latest research, as recommendations may evolve if further evidence clarifies these trends. And as a society, recognizing that men’s reproductive health can be influenced by environmental and lifestyle factors, we have an opportunity to improve conditions that support healthier sperm – which, as a proxy, often overlap with better general health, too.

In the end, the topic of sperm counts is a reminder that our environment and behaviors can impact our biology across generations. It’s a complex story that can’t be summed up in a single sensational statistic. By sticking to rigorous data and avoiding hyperbole, we can better understand and address any real problems that exist. The truth, as usual, lies in the nuanced examination of the evidence – not in one-liners.

References:

1. Levine H, Jørgensen N, Martino-Andrade A, et al. Temporal trends in sperm count: a systematic review and meta-regression analysis. Hum Reprod Update. 2017;23(6):646-659
2. Levine H, Jørgensen N, Martino-Andrade A, et al. Temporal trends in sperm count: a systematic review and meta-regression analysis of samples collected globally in the 20th and 21st centuries. Hum Reprod Update. 2023;29(2):157-176.
3. Lewis KL, Cannarella R, Liu F, et al. Sperm concentration remains stable among fertile American men: a systematic review and meta-analysis. Fertil Steril. 2025;123(1):77-87. 
4. Jørgensen N, Joensen UN, Jensen TK, et al. Human semen quality in the new millennium: a prospective cross-sectional population-based study of 4867 men. BMJ Open. 2012;2(4):e000990. 
5. Travison TG, Araujo AB, O’Donnell AB, et al. A population-level decline in serum testosterone levels in American men. J Clin Endocrinol Metab. 2007;92(1):196-202. 
6. Auger J, Eustache F, Chevrier C, Jégou B. Spatiotemporal trends in human semen quality. Nat Rev Urol. 2022;19(10):597-626. 
7. Swan SH, Elkin EP, Fenster L. The question of declining sperm density revisited: an analysis of 101 studies published 1934–1996. Environ Health Perspect. 2000;108(10):961-966. 
8. Carlsen E, Giwercman A, Keiding N, Skakkebaek NE. Evidence for decreasing quality of semen during past 50 years. BMJ. 1992;305(6854):609-613. doi:10.1136/bmj.305.6854.609. 
9. Eckert-Lind C, Busch AS, Petersen JH, et al. Worldwide secular trends in age at pubertal onset assessed by breast development among girls: a systematic review and meta-analysis. JAMA Pediatr. 2020;174(4):e195881. 
10. Pacey AA. Is sperm quality declining? Nature Reviews Urology. 2013;10(12):653-654. doi:10.1038/nrurol.2013.242.