Is RFK right about US sperm counts?

Summary

Podcast Summary: More or Less

Episode: Is RFK right about US sperm counts?
Date: December 6, 2025
Host: Lizzie McNeil (BBC Radio 4)
Guests: Professor Alan Pacey (University of Manchester) & Edif Aaron (Yale University)

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Overview

This episode investigates and fact-checks claims made by U.S. Health Secretary Robert F. Kennedy Jr. ("RFK") regarding the alleged dramatic decline in sperm counts and testosterone levels in younger American men. The show explores whether teenagers today really have "50% of the sperm count and 50% the testosterone of a 65-year-old man," dissecting the data, studies, and methodologies underpinning such assertions.

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Key Discussion Points & Insights

1. The RFK Statement: Is It Plausible?

• Claim examined: "The average teenager in this country has 50% of their sperm count, 50% of the testosterone as a 65-year-old man." (01:34)
• Host Lizzie McNeil immediately questions the likelihood:

  “Can this statement possibly be right?” (01:55)

2. Sperm Count Over Time: What Does the Science Show?

The Aging Male and Sperm Quality

• Professor Alan Pacey explains:
  • Sperm quantity in men remains relatively constant with age, but quality deteriorates.

  “Generally speaking, the number of sperm that are produced per unit time remains the same... So it's more of a quality deterioration with age, not a quantity deterioration.” — Professor Alan Pacey (01:59)

Historical Studies and Methodological Issues

• Initial suspicion around a 1992 study that suggested major declines in sperm count, but the analysis was flawed:

  • Older studies sampled fertile men; newer studies sampled men from fertility clinics.
  • Changes in what was defined as “normal” sperm count over time.
  • Counting techniques have changed, which can skew results.

  “Generally, poor counting technique overestimates the sperm concentration, which means poor counting technique in the 1940s would have automatically elevated the situation in comparison to a modern counting.” — Professor Alan Pacey (03:38)

• The supposed 50% global drop in sperm counts cited in a 2023 meta-analysis also suffers from these same methodological weaknesses (04:08-04:26)

  “That data is controversial. All of these populations are subtly different in different ways, and so comparing like with like around the world and over time is actually quite a difficult thing to do.” — Professor Alan Pacey (04:26)

Variation in Results and U.S.-Specific Data

• Recent meta-analysis (2025):
  • No discernible decrease in sperm counts for US males across generations;
  • Out of 35 studies, 21 showed no decline, 8 showed a decline, and 6 were ambiguous. (05:07)

  “If there has been a change in sperm concentration, it's been a change from normal to normal. And therefore, for the vast majority of men, this probably shouldn't have an impact if it is a real effect in the first place.” — Professor Alan Pacey (05:34)

3. Testosterone: A Crisis or a Case of Changing Measurement?

Natural Testosterone Decline with Age

• Testosterone typically drops 1% a year after age 30-40 (06:05)
• Testosterone replacement therapy (TRT) is rising in younger men, though often unjustifiably.

Influx of Testosterone Supplementation

• TRT prescriptions grew from 7.3 million to 11 million in the US between 2019–2024; fastest growth: men in their 20s and 30s (06:05-06:49)

Are Testosterone Levels Actually Plummeting?

• Edif Aaron investigates rumors of dramatic decreases.

  “I'd seen stuff on X and a lot of like influencer type people mentioning that total testosterone had... dropped over time.” — Edif Aaron (06:49)

• Closer inspection of the CDC data:

  • Apparent “spike” in low testosterone rates is largely due to changes in measurement methods.
    • Early 2000s data via immunoassays, more recent via mass spectrometry (far more accurate).
    • The historic ‘300 ng/dl’ clinical threshold for “low” testosterone became outdated with new methods; the Endocrine Society now suggests 264 ng/dl as a better cut-off. (08:39)
  • Once data are adjusted for the method change, the supposed decline “vanished”:

    “The rates are about constant. And so the overall level of people who are sort of self reported healthy who have this low value don't really change much.” — Edif Aaron (09:13)

Dangers of Overprescription

• If the higher, outdated threshold is used with modern testing, many healthy men could be misdiagnosed and wrongly prescribed TRT (09:21-09:36).
• Irony:

  "Do you know what is proven to lower sperm count to the point of infertility? Taking testosterone supplements when you don't need them." — Lizzie McNeil (09:36)

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Notable Quotes & Memorable Moments

• On the flaws of comparing sperm counts historically:

  “Generally, poor counting technique overestimates the sperm concentration, which means poor counting technique in the 1940s would have automatically elevated the situation...” — Professor Alan Pacey (03:38)

• On the evidence for sperm decline:

  “If there has been a change in sperm concentration, it's been a change from normal to normal.” — Professor Alan Pacey (05:34)

• Regarding testosterone panic on social media:

  “I'd seen stuff on X and a lot of like influencer type people mentioning that total testosterone had... dropped over time.” — Edif Aaron (06:49)

• On how reevaluating the threshold erased the decline:

  “The rates are about constant. And so the overall level of people who are sort of self reported healthy who have this low value don't really change much.” — Edif Aaron (09:13)

• On the ironic effect of unnecessary TRT:

  “Do you know what is proven to lower sperm count to the point of infertility? Taking testosterone supplements when you don't need them.” — Lizzie McNeil (09:36)

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Key Timestamps for Important Segments

• Questioning RFK's claim: 01:55
• Sperm and aging: 01:59–02:31
• Historical studies and methodological challenges: 02:58–04:26
• Summary of latest US sperm count data: 05:07
• Testosterone, aging & TRT popularity: 06:05–06:49
• Social media and testosterone panic: 06:49–07:41
• Technical discussion of measurement techniques: 07:45–08:39
• Reinterpretation of testosterone data: 08:57–09:13
• Warning about unnecessary TRT: 09:21–09:36

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Conclusion

• Host’s Summary:

  • There is no credible scientific evidence that US teenagers have half the sperm count or half the testosterone of 65-year-old men.
  • Apparent declines in both sperm count and testosterone are mostly artifacts of shifting definitions, study populations, and measurement methods.
  • Overzealous use of testosterone therapy — sometimes influenced by outdated thresholds — poses real risks, ironically including the lowering of fertility.

• Listeners are reminded to be critical of viral health claims and to seek expert interpretation when confronted by alarming statistics.

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For any questions or feedback, contact the More or Less team at [e-mail].

Transcript

[Professor Alan Pacey]

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[Lizzie McNeil]

Hello and thank you for downloading the More or Less podcast. We're the program that looks at the numbers that turn up in the News, Life and the Oval Office, and I'm Lizzie McNeil. Around the world, many countries are concerned about tackling the steady decline in birth rates and and total fertility rates. The US Is no exception. To tackle this issue, the US Government announced that it would provide subsidies for Americans seeking out IVF treatment. The announcement was accompanied by one very suspect sounding stat from U.S. health Secretary Robert F. Kennedy Jr. Today the average teenager in this country has 50% of their sperm count. 50% of the testosterone is a 65 year old man.

Can this statement possibly be right?

[Professor Alan Pacey]

A man will produce about a thousand sperm with every heartbeat. My name is Professor Alan Pacey. I'm professor of Andrology at the University of Manchester. As a man gets older, we generally see that the quality of his semen deteriorates. Generally speaking, the number of sperm that are produced per unit time remains the same as it was when he was a younger man. So it's more of a quality deterioration with age, not a quantity deterioration.

[Lizzie McNeil]

So theoretically, if there were a global shift and men born in recent years started to produce fewer sperm, then RFK's assertion could be correct. However, the there have been no studies looking at the sperm count of the average US Teen. What Robert F. Kennedy could have said is that some studies have shown that average sperm counts today suggest that there has been a fall compared to sperm counts 50 years ago.

[Professor Alan Pacey]

So the landmark study which really propelled this debate into the public domain was published in 1992. What that did was look back at sperm quality from the 1940s up until the late 80s early 90s. And that study was heavily criticized. It was heavily criticized because the Populations of men that had been looked at over time were not the same.

[Lizzie McNeil]

Studies from the 1940s looked at men with proven fertility, whereas the studies from the late 80s included samples from men attending fertility clinics. What is also important is that what we considered as a normal sperm count wasn't even defined in the 1940s.

[Professor Alan Pacey]

We didn't actually define what normal was until about 1952. We changed our minds over what was normal several times since the 1950s as well. Counting techniques had changed from the 1940s to the early 1990s. Generally, poor counting technique overestimates the sperm concentration, which means poor counting technique in the 1940s would have automatically elevated the situation in comparison to a modern counting.

[Lizzie McNeil]

RFK was likely quoting a more recent meta analysis published in 2023, and they found that global fertility rates had fallen by as much as 50%. However, the meta analysis relied on data that had the same issues as the 1992 study.

[Professor Alan Pacey]

That data is controversial. All of these populations are subtly different in different ways, and so comparing like with like around the world and over time is actually quite a difficult thing to do.

[Lizzie McNeil]

It's not just differences in counting that can affect results. Sperm counts can fluctuate even within the same individual. For example, abstinence makes sperm count go up as men produce sperm constantly, and different studies require different periods of abstinence. In all, there are so many factors that make it difficult to distinguish true declines from methodological differences.

[Professor Alan Pacey]

There are other meta analyses that show that there has been no change.

[Lizzie McNeil]

The most recent meta analysis published in 2025 showed there had been no discernible decrease in sperm counts in US men across the generations. In fact, if you look at the studies that have been done, 21 show no decline, 8 show a decline, and 6 turned up ambiguous results. Allen says that even the studies which do show a decline do not tie the drop in fertility levels to falling sperm counts.

[Professor Alan Pacey]

If there has been a change in sperm concentration, it's been a change from normal to normal. And therefore, for the vast majority of men, this probably shouldn't have an impact if it is a real effect in the first place.

[Lizzie McNeil]

So no, we do not have evidence that US teenagers have 50% lower sperm counts than 65 year old men. But what about the testosterone claim? 50% of the testosterone is a 65 year old man.

Testosterone does decline with age. Some studies estimate that this drop is around 1% a year from when a man reaches his 30s and 40s. Testosterone replacement therapy, or TRT, is a bit of a craze at the Moment. Fitness influencers and podcasters are touting the benefits of hacking their biology by taking testosterone supplements. Prescriptions for TRT in the US increased from 7.3 million to 11 million between 2019 and 2024. Robert F. Kennedy Jr uses TRT and like him, the majority of users are over the age of 40. But the fastest growing demographic are men in their 20s and 30s. And there's a concern from doctors that younger men are taking TRT when they do not need to.

[Edif Aaron]

Honestly, I'd seen stuff on X and a lot of like influencer type people mentioning that total testosterone had dropp dropped over time.

[Lizzie McNeil]

That's Adif Aaron, a graduate of medicine currently doing his research year at Yale. He decided to see if the rumors were true by looking into the data behind us testosterone rates.

[Edif Aaron]

So I just started saying like, you know what percent of people have a low toll testosterone in the United States every single year and sort of projecting it out to the population.

[Lizzie McNeil]

Traditionally you were considered to have low testosterone if you were under 300 nanograms per deciliter. Edif used data that was collected by the Centers for disease control in 2002, 2004 and then again in 2010 and 2011. According to a basic read of the data, the amount of people with Testosterone levels under 300 skyrocketed.

[Edif Aaron]

Suddenly it spikes about 100% over seven years.

[Lizzie McNeil]

EDIF took a closer look at the data and realized that the methodology had changed. The earlier tests had been conducted using immunoassays.

[Edif Aaron]

The way that the immunoassay works is you stick a compound that binds total testosterone and then you measure fluorescence or radioabsorbance of that.

[Lizzie McNeil]

It's a complicated process to wrap your head around in the time we have, but what you need to know is that this is what the 300 threshold was based off.

[Edif Aaron]

Then over the next 20 years, we sort of switched our methods, you know, multiple times.

[Lizzie McNeil]

Because of the difference in methodology, it's hard to compare testosterone levels from the 70s and 80s to even the 90s and 2000s. And since then we've started using a completely different methodology, mass spectrometry. And this is considered to be much more accurate.

[Edif Aaron]

But that old 300 level really never got changed.

[Lizzie McNeil]

Recently, the endocrine Society recommended that 264 is a much more appropriate threshold for low testosterone. As they were able to measure testosterone levels more accurately, they realized that many men who had no signs or symptoms of a deficiency had levels of around 300.

[Edif Aaron]

So we sort of did a sub analysis where we said okay, we're going to use 300 on these older generation of amino acids and then once the assay gets switched to mass spec in this US study, we're going to switch it to 264.

[Lizzie McNeil]

When they did this, the decline in testosterone levels vanished and you can see.

[Edif Aaron]

That the rates are about constant. And so the overall level of people who are sort of self reported healthy who have this low value don't really change much.

[Lizzie McNeil]

It seems that any huge drop in testosterone levels are likely to do with differences in how data was collected rather than a reflection of what's actually happening. One thing that is concerning is that if people continue to use the higher threshold but with newer technology, then people who do not need to be on TRT might be prescribed it. And there is some irony in that because do you know what is proven to lower sperm count to the point of infertility? Taking testosterone supplements when you don't need them. And that is all we have time for this week. Thank you to our experts and if you have any questions or comments, do write in to more or lessbc.co.uk. until next week. Goodbye.

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[Sarah]

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