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)

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.

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)

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)

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

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.

For any questions or feedback, contact the More or Less team at moreorless@bbc.co.uk.