Modern WisdomWhy Men’s Testosterone Levels Are Plummeting - Dr Shanna Swan
CHAPTERS
- 0:00 – 3:42
Signals that men’s testosterone may be declining (and why the data is messy)
Chris asks what’s happening to men’s testosterone, and Dr. Swan explains that while there are multiple signals suggesting decline, the research base isn’t as established as sperm count trends. They discuss why increased testosterone use (especially among young men) is a worrying indicator but doesn’t cleanly prove a population-level drop.
- •Research hints testosterone is in trouble, but results aren’t as definitive as sperm-count data yet
- •Testosterone use among young men is rising rapidly and may reflect multiple causes
- •Hard to separate medical need (low-T symptoms) from performance/cosmetic motivations
- •Sperm count has decades of consistent study; testosterone has far less longitudinal literature
- 3:42 – 7:49
Why testosterone matters: fetal development, genital formation, and AGD
Dr. Swan describes testosterone’s critical role very early in pregnancy, when male sexual differentiation begins. She explains how insufficient testosterone during a key early window can lead to incomplete masculinization, introducing anogenital distance (AGD) as an important biomarker.
- •Testosterone is crucial in early first trimester for male differentiation
- •Insufficient timing/amount can affect testes descent, penis size, and other traits
- •AGD (anogenital distance) reflects prenatal androgen exposure and masculinization
- •AGD was long used in animal research; Swan helped establish its relevance in humans
- 7:49 – 10:17
Testosterone beyond the womb: puberty, libido, and couple-level sexual effects
The conversation shifts to testosterone’s role across life—especially puberty and libido. Dr. Swan adds that endocrine disruptors affecting testosterone can influence women’s sexual satisfaction too, creating a feedback loop that may reduce sexual activity within couples.
- •Testosterone supports puberty-related development and male sexual function
- •Phthalates lower testosterone in both men and women
- •Higher phthalate exposure is associated with lower female sexual satisfaction and less sex
- •Lower libido on both sides can reduce sexual frequency and relationship formation
- 10:17 – 17:03
From libido to birth rates: social trends, coupling declines, and fertility-rate collapse
They connect physiology to demographic outcomes, discussing falling fertility rates worldwide—especially extreme lows in East Asia. Dr. Swan notes governments’ incentives haven’t reversed the trend, and suggests endocrine disruption may be one contributor alongside social and cultural drivers.
- •Fertility rates are falling globally; East Asia is especially low
- •Reports of reduced coupling/sex/marriage may be a major proximate driver
- •Governments trying financial incentives to increase births, with limited success
- •Swan suspects endocrine disruptors likely play a role, though direct causal links are limited
- 17:03 – 19:01
Testosterone and sperm production: why TRT can reduce fertility
Chris asks about the testosterone–sperm relationship, and Dr. Swan clarifies a key point: exogenous testosterone often suppresses sperm production through feedback loops. They caution that many people mistakenly assume TRT improves fertility when it can do the opposite.
- •TRT can suppress endogenous testosterone and sperm production via feedback mechanisms
- •Adequate natural testosterone is needed for reproductive system maintenance
- •The relationship is complex and mediated by endocrine signaling loops
- •Common misconception: TRT improves fertility; often it reduces it
- 19:01 – 21:34
The sperm-count trend: meta-analyses, worldwide decline, and acceleration after 2000
Dr. Swan outlines findings from major meta-analyses showing sustained declines in sperm count. She emphasizes that the decline appears global and that the rate has accelerated in the 21st century, increasing urgency and concern.
- •Sperm counts declining ~1% per year historically, not leveling off
- •Post-2000 decline estimated closer to ~2% per year in updated analyses
- •2022 update suggests the trend is worldwide, with more non-Western data included
- •Overall pattern: decline continues and appears to be accelerating
- 21:34 – 24:00
Geographic differences and the ‘Denmark vs Finland’ puzzle
Chris asks if any regions are ‘doing better.’ Dr. Swan notes Denmark may have leveled off somewhat due to strong long-running recruitment studies, while Finland’s trends have worsened toward Denmark’s levels—despite demographic similarities—raising unanswered questions.
- •Denmark has robust longitudinal monitoring (e.g., military recruits)
- •Denmark may be leveling off, though still with many low-count cases
- •Finland previously better but now declining toward Denmark’s levels
- •Similar populations/exposures make the Denmark–Finland divergence hard to explain
- 24:00 – 34:03
Not just chemicals: lifestyle and societal drivers of fertility and sperm health
Dr. Swan broadens the lens beyond chemical exposures to modifiable lifestyle factors and macro social forces. They cover diet, smoking, obesity, exercise, stress, alcohol, and also contraception, women’s education, and workforce participation as major contributors to fertility-rate changes.
- •Mediterranean diet associated with better sperm counts than Western diet
- •Smoking, obesity, low exercise, high stress, and binge drinking reduce sperm count
- •Female fertility is also affected by the same lifestyle factors
- •Contraception access, women’s education, and workforce participation lower fertility rates (via choice and timing)
- 34:03 – 42:34
How endocrine-disrupting chemical research took off: CDC biomonitoring and ‘phthalate syndrome’
Dr. Swan tells the origin story of her phthalates research, enabled by CDC’s ability to measure chemicals in urine at scale. She explains how prenatal exposure studies in humans mirrored animal findings, culminating in a cluster of outcomes termed ‘phthalate syndrome’ and influencing policy discussions.
- •CDC biomonitoring made large-scale measurement of chemicals feasible
- •Phthalates are detectable in nearly everyone and can act as antiandrogens
- •Prenatal phthalate exposure linked to incomplete masculinization markers in humans (as in rats)
- •‘Phthalate syndrome’ became a named cluster of prenatal chemical effects, rare outside fetal alcohol syndrome
- 42:34 – 49:02
Where exposures come from: food systems, processing plastics, and ‘do not microwave plastic’
They map the main exposure routes—ingestion, inhalation, and skin absorption—then focus on food as the primary driver. Dr. Swan explains how chemicals enter food during farming, shipping, processing, storage, and heating, with plastics repeatedly contacting products along the chain.
- •Main exposure routes: ingestion, respiration, dermal absorption
- •Food is the biggest exposure source due to repeated contact with plastics
- •Phthalates can be added to help pesticides penetrate plants
- •Processing and warm contact (e.g., milking machine tubing) increases chemical leaching
- •Heating/microwaving plastic is a major ‘avoid if possible’ behavior
- 49:02 – 55:21
Practical reduction strategies: shopping, storage swaps, and the ‘Whac-A-Mole’ chemical problem
Dr. Swan and Chris discuss actionable steps: choose less packaged foods, buy simpler cuts/whole produce, use glass or safer materials, and reduce plastic contact. They also address the substitution problem where ‘BPA-free’ products may replace BPA with similar bisphenols (BPF/BPS variants).
- •Prefer unpackaged produce and minimally processed foods when possible
- •Use glass containers (e.g., Pyrex), reuse glass jars, avoid plastic storage for heating
- •Silicone is generally safer than typical plastics for certain uses
- •‘BPA-free’ can be misleading due to substitute bisphenols (Whac-A-Mole effect)
- 55:21 – 1:00:12
Water concerns: micro/nanoplastics and filtration vs distillation tradeoffs
The conversation turns to drinking water: not only container plastics but contamination within the water itself. Dr. Swan introduces micro/nanoplastics as an emerging concern and compares filtration approaches, highlighting distillation as a simple alternative while noting RO’s effectiveness depends on system materials.
- •Micro/nanoplastics may pose physical (asbestos-like) and chemical risks
- •Plastics can re-enter the body through water and air even after disposal/recycling
- •Reverse osmosis can help, but device materials and internal plastics matter
- •Distillation is presented as a low-maintenance alternative producing water into glass
- 1:00:12 – 1:05:12
Chemicals and behavior: brain sexual differentiation, language, and play patterns
Dr. Swan explains that sex hormones shape not just genitals but also the developing brain, at a later prenatal window. She reviews findings linking prenatal phthalate exposure to altered language outcomes (notably in girls) and less male-typical play behavior in boys, framing this as altered brain masculinization.
- •Brain development is hormonally influenced and shows sex differences on imaging
- •Prenatal timing differs: brain differentiation occurs later than genital differentiation
- •Studies link phthalates to language differences at age four (especially in females)
- •Prenatal phthalates associated with less male-typical play behavior in boys
- 1:05:12 – 1:10:33
Gender identity questions, twin evidence, and sensitive developmental windows
Chris asks whether these effects could influence gender identity; Dr. Swan says evidence is suggestive but not conclusive. They discuss in-utero position effects (animal and human twin findings) and outline additional sensitive periods like mini-puberty and puberty when environmental exposures may matter more.
- •Potential links to gender dysphoria are discussed cautiously (not settled science)
- •In-utero position effects: proximity to male/female fetuses can shift masculinization markers
- •Some twin studies suggest opposite-sex co-twins correlate with higher gender dysphoria likelihood
- •Sensitive windows include prenatal development, mini-puberty (first 6 months), puberty, and menopause transitions
- 1:10:33 – 1:21:44
Aging, assisted reproduction, and the future: IVF, IVG, and rising reliance on tech
They close by discussing how fertility declines with age (especially for women), but disentangling biology from choice is hard. Dr. Swan highlights Israel as an outlier due to guaranteed ART access, predicts greater reliance on assisted reproduction, and engages Chris’s points on embryo selection, stem-cell-derived gametes (IVG), and political complexity around reproductive tech.
- •Age-related fertility decline intersects with delayed childbearing and social choice
- •Israel’s fertility stands out due to broad access/guarantees for ART births
- •Prediction: increasing reliance on assisted reproduction as sperm count declines
- •Emerging tech: embryo selection and IVG (stem-cell-derived eggs/sperm) raise ethical/political issues
- •Concerns about inequality: wealth/education enable better avoidance and better access to fertility solutions
- 1:21:44 – 1:23:16
Where to learn more: Countdown, website, and advocacy resources
Chris asks where listeners can find Dr. Swan’s work. She points to her website and her book Countdown, and recommends Environmental Health Sciences for efforts to address chemical exposures at the societal level.
- •Website: shannaswan.com
- •Book: Countdown (two words)
- •Advocacy and action: Environmental Health Sciences group
- •Closing thanks and wrap-up
