Are sperm counts and testosterone really falling? Experts weigh the evidence

The Short Version

Probably, yes, on both counts. The two experts here agree on more than the headlines suggest; they differ mainly on how certain the sperm-count story is and on what's causing it. Reproductive epidemiologist Shanna Swan tested the famous 50% drop in 50 years claim expecting to debunk it, and found the decline survived every confounder she could model [1]. She blames hormone-disrupting chemicals such as phthalates, bisphenols, pesticides and PFAS, with the womb as the most vulnerable window [1]. Stanford urologist Michael Eisenberg calls the sperm decline "very controversial" because of how older studies were measured and weighted [3]. Yet he finds the evidence for falling testosterone "pretty convincing" and says rising obesity explains only about a 10% drop out of a reported 50% [3]. Andrew Huberman adds that microplastics now turn up in human testes and semen, where they correlate with lower sperm counts and motility. He stresses these are correlations, not proof of cause [2]. The practical upshot is the same from every side: get tested rather than guess, cut the exposures you can control, and treat semen quality and testosterone as markers of overall health [1][2][3][4].

Table of contents

What They Said

Shanna Swan: a skeptic who became convinced

Swan is a reproductive epidemiologist and professor of environmental medicine at Mount Sinai, with a background in math and statistics [1]. Her focus is endocrine-disrupting chemicals, a framing she likes because it tells researchers where to look: which hormone, when, and how [1].

How she tested the sperm-count claim. In the late 1990s she was the only statistician on a National Academy of Sciences committee asked whether hormone-active chemicals threatened human health [1]. The committee wanted her view on a Danish study by Elisabeth Carlsen, "Which claimed that sperm count had dropped 50% in 50 years" [1]. Her first reaction: "As I mentioned I'm skeptical and I looked at it and I thought yeah, I don't think so" [1]. She knew there were ordinary explanations, such as changes in counting methods, obesity, smoking and who volunteered for studies [1]. So she gathered the 61 underlying studies, extracted every factor that might explain the decline, and built a multivariable model [1]. The result: "to my astonishment When I was done The slope of the decline was exactly the same to the first decimal place It had not explained anything" [1]. Her conclusion: "This looks like it might be real" [1]. She repeated the analysis with her own selection of studies, going further back and further forward, and got the same answer [1]. Because the decline happened over about two generations, too fast for genetics, she concluded the cause must be environmental [1]. Today, she says, the picture is worse: "That's what we're seeing worse than that by the way now" [1].

Why sperm count matters. Swan cites a Danish study of couples trying to conceive. Below about 40 to 45 million sperm per milliliter, the chance of conception drops steeply. Above about 75 to 100 million, extra sperm makes no difference [1]. Counts range from zero to around 400 million, shaped by age, genetics and exposures. So sperm count matters a lot when it's low and not at all when it's high [1].

The chemical case. Swan's path to phthalates began on a flight to Japan, when CDC chemist John Brock told her to look at them [1]:

• The CDC could now measure them, and they were found in everyone, including women of reproductive age.
• Scientists had described a "phthalate syndrome" in rats. Male pups exposed during a narrow window of pregnancy were born incompletely masculinized: smaller penises, undescended testicles and a shorter distance from the anus to the genitals, called anogenital distance (AGD) [1].

It took her 10 years to show the same pattern in humans. Mothers with higher levels of the three most anti-androgenic phthalates (DEHP, DBP and BBzP) had sons with significantly shorter AGD, smaller penises, less testicular descent and smaller scrotums [1]. A second, better-designed study called TIDES, in four US cities, replicated the result [1]. The effect runs in both directions: daughters of women with PCOS, who have higher androgens, had longer, more male-typical AGD [1]. In her words, the measure "is A look inside the womb At the androgen level that the fetus is exposed to at that time" [1].

Linking the womb to adult fertility. In Rochester, New York, she measured AGD in college volunteers and collected semen samples. Men with longer AGD had higher sperm counts [1]. Eisenberg later found that fathers at an infertility clinic had longer AGD than men who had never fathered a child [1]. Swan also studied partners of pregnant women in four US cities [1]:

• Men in agricultural central Missouri had half as many moving sperm as men in Minneapolis.
• Within Missouri, men with poor sperm quality had significantly higher urine levels of five pesticides, including the widely used herbicide atrazine.

The womb matters most. Asked whether early exposures can be reversed, Swan says no. Men whose mothers smoked during pregnancy have about a 50% reduction in sperm count that can't be undone, whereas a man who smokes as an adult can quit and recover [1]. "Whatever happened in the womb stays in the womb" [1]. Because a fetus already carries the germ cells for its own children, effects may span at least three generations [1].

Fertility beyond sperm. Using World Bank data, Swan notes that fertility rates have fallen about 50% in 50 years, similar to sperm counts [1]. Replacement requires about 2.1 children per woman. The US is below that, Japan is around 1, and South Korea is about 0.78 [1]. She pushes back on explanations limited to choice, contraception and delayed childbearing, pointing out that fertility is also falling in other species, which can't be blamed on rent prices [1]. She also notes a substantial body of evidence that these chemicals affect the thyroid: "there is a whole field of the effect of these chemicals on the thyroid system And there's a lot of evidence that it's adverse" [1].

Where she draws the line. Swan is careful about what she doesn't know. On cell phones and electromagnetic radiation: "I honestly have to say just gonna say I don't know, but I do know that Heat is related to fertility and sperm count" [1]. On behavior, she and Huberman stress that effects like less male-typical play in boys exposed to phthalates in the womb are shifts in overlapping distributions, "more likely, not absolute," and that "dimorphic does not mean better worse" [1]. She reports a small study in which six infertile couples replaced their products with safer ones and reported feeling better and sleeping better. She stresses this isn't yet hard data and won't discuss pregnancy outcomes until they're published [1].

Michael Eisenberg: probably real, but the sperm data are messy and the causes uncertain

Eisenberg is a Stanford urologist with more than 300 published papers on male sexual function and fertility [3]. Asked whether sperm quality is actually declining, he says: "I think it's very controversial" [3].

The measurement problem. He explains that the landmark Carlsen paper pooled studies from around the world. The early studies from the mid-20th century showed higher quality, and the more recent ones lower [3]. "Now that study was very controversial," he says, citing questions about how the studies were weighted, since there weren't many early studies to draw on [3]. Critics also point to changes in measurement techniques over time [3]. On the other side, a recent meta-analysis of tens of thousands of men, including new data from Africa and Asia, supports the decline [3]. Sperm quality also varies by region. In a Danish study of military conscripts, quality was stable over about 20 years, but only about a quarter of the men had normal semen quality [3]. He'd like the CDC to add semen analysis to its national health surveys, which would settle the question with better data [3].

Testosterone: more convincing. Here Eisenberg is less hedged: "I think there is pretty convincing evidence that that is happening. And I think the reason for that again is probably not certain" [3]. Well-designed cohort studies recruited men in the 1980s, 1990s and 2000s, and "matching age for age, these testosterone levels tend to be lower" in more recent groups [3]. He also cites CDC NHANES data [3]. Testosterone typically peaks in the early 20s and declines about 1% a year, with huge individual variation; he's seen 80-year-olds with some of the highest levels in his clinic [3].

What obesity explains, and what it doesn't. Fat tissue contains aromatase, which converts testosterone to estrogen, and larger thighs can warm the testicles [3]. But Eisenberg did the arithmetic on the sperm trend: "the reported decline in semen quality is about 50%," yet given how much body fat has actually increased, obesity "actually explains about a 10% decline" [3]. That leaves most of the reported drop unexplained by weight alone. He lists chemical exposures such as phthalates and BPA, especially in the womb, among the proposed causes [3].

You can't tell by looking. Asked whether obesity predicts low testosterone, he says "I would say that you cannot predict" [3]. Some fit men who exercise five to seven days a week have very low testosterone, so "it's hard to just predict, you know, based on appearance" without objective data [3].

A mirror of overall health. Eisenberg's most distinctive point is that these numbers matter beyond fertility. "Men with lower testosterone levels of higher risk of, you know, heart disease, diabetes, mortality, the same studies exist for semen quality as well" [3]. Men with more health conditions, "obesity, hypertension, hyperlipidemia," which "sort of stack up," show lower testosterone and poorer sperm quality [3]. He thinks semen quality should be treated as something like a sixth vital sign. Men with higher semen quality tend to live longer, see doctors less and have lower cancer rates [3]. About 15% of men have low semen quality and about 1% have no sperm at all, often without knowing it, since normal ejaculation doesn't mean normal sperm [3].

Why the question gets less attention than it should. When couples seek fertility care in the US, the man is bypassed about a third of the time, even though male factors contribute to roughly half of infertility [3]. Because IVF with sperm injection can succeed with a single sperm, he thinks research on male fertility has been dampened [3]. He reminds listeners that "fertility itself is just team sport" [3].

A puzzling counter-trend. Eisenberg's team analyzed clinical measurements from about 55,000 men and found that penile length has increased over roughly 30 years, the opposite of what they expected given the sperm and testosterone trends [3]. Possible explanations include earlier puberty or exposures in the womb [3]. The trends don't all point in one simple direction.

Andrew Huberman: microplastics are everywhere, but the human evidence is correlational

In his solo episode on microplastics, Huberman tries to be informative without being alarmist [2].

Where they end up. Microplastics and nanoplastics have been found in nearly every human tissue examined, including brain, testes, ovarian follicles, placenta and a newborn's first stool [2]. They cross the blood-testicular barrier, which protects the cells that carry DNA to offspring, and a widely reported study found them in every human testicle analyzed [2]. They also turn up in semen, where they correlate with lower sperm counts and motility [2]. Better imaging keeps raising the numbers. A 2024 study found 110,000 to 400,000 particles per liter of bottled water, averaging about 240,000, far above the earlier estimate of about 30,000 [2].

Hormone disruptors riding along. Microplastics can carry hormone-disrupting chemicals [2]:

• BPA and BPS can mimic estrogen or block its receptors, and may bind androgen receptors.
• Phthalates, added to make plastics flexible, are also endocrine disruptors.

He describes a study linking urinary phthalate metabolites to lower serum testosterone in men, women and children. The strongest effect was in women aged 40 to 60, and in men the only significant associations were in that same age range. His interpretation is a cumulative effect over decades [2].

His caveats. Huberman is explicit that there are still no causal data linking microplastics to specific human diseases, only a growing body of correlational and animal evidence [2]. He points out that a widely shared claim that we each eat a credit card's worth of plastic a week was later judged an overestimate by roughly a millionfold [2]. And he stresses that lower sperm counts don't necessarily mean infertility [2].

Canned food. One finding changed his own habits. In a randomized crossover trial, one serving of canned soup a day for five days was associated with "more than a thousand percent increase in urinary bPA" [2]. He now avoids canned soup unless he has to [2].

Huberman's fertility episode: counts falling, and what to do about it

In his fertility episode, Huberman notes that population sperm counts appear to be declining and mentions Swan's work on phthalates and BPA as one possible factor [4]. Clinicians like to see at least about 15 million sperm per milliliter and more than two milliliters of ejaculate, though counts vary enormously [4]. In about one in five couples with fertility problems, the issue lies with the sperm [4]. Sperm take about 60 days to make and roughly two more weeks to travel, so any change takes a 90-day window to show [4]. He singles out heat as a major, controllable factor: "Seat heaters in cars, for instance, terrible idea" [4]. His summary of the whole topic: "the things that we can do to optimize our fertility are the exact same things that we should all be doing to optimize our vitality and our longevity" [4].

Where They Agree, and Where They Don't

Where they agree.

• Both declines are probably real. Swan found that the sperm decline survived her statistical scrutiny [1]. Eisenberg notes that a recent large meta-analysis supports it and finds the testosterone decline convincing [3]. Huberman accepts that counts appear to be falling [2][4].
• Obesity isn't the whole story. Swan's model accounted for obesity and the slope didn't change [1]. Eisenberg estimates obesity explains about 10% of a reported 50% decline [3].
• Chemicals are a leading suspect, especially in the womb. Swan's work centers on this [1]. Eisenberg lists phthalates and BPA, especially prenatal exposure, among the proposed causes [3]. Huberman cites Swan's work and the phthalate-testosterone data [2][4].
• Heat hurts sperm. Swan says heat is related to fertility and sperm count [1]. Eisenberg calls heat the big one for sperm production [3]. Huberman lists hot tubs, saunas, laptops and seat heaters [4].
• Measure, don't guess. Swan's whole career rests on measurement [1]. Eisenberg says you can't predict testosterone or sperm quality by looking at someone [3]. Huberman urges baseline sperm analysis and hormone panels [4].

Where they differ.

• How settled is the sperm decline? Swan's view is that it's real and now worse than 50% in 50 years [1]. Eisenberg calls it "very controversial," pointing to how studies were weighted, the scarcity of early data, and changes in measurement methods, while acknowledging new supporting data [3]. The disagreement is about data quality. Swan tackled the confounders head-on in a statistical model and found them explained nothing [1]. Eisenberg highlights that regional variation and some stable populations, like the Danish conscripts, complicate a single global story [3].
• How confident about the cause? Swan argues that the speed of the decline rules out genetics and points to environmental chemicals, backed by her AGD, pesticide and phthalate studies [1]. Eisenberg says the reason testosterone is falling "is probably not certain" and lists chemicals and obesity as proposed, not proven, causes [3]. Huberman is the most cautious about microplastics specifically: widespread, correlated with worse sperm, but not yet causally linked to human disease [2].
• Is falling fertility about biology or choices? Swan rejects explanations limited to choice, contraception and economics, pointing to declines in other species [1]. Huberman acknowledges both sides in the conversation, noting a sociobiological piece in which people opt out of having children, sometimes for economic reasons [1].
• Which threshold matters? The sources use different yardsticks. Swan's Danish study points to about 40 to 45 million sperm per milliliter as the level below which conception drops steeply [1]. Eisenberg says couples trying naturally probably need 20 to 40 million moving sperm [3]. Huberman cites clinicians wanting at least about 15 million per milliliter [4]. These measure different things (concentration versus total motile count, clinical cutoff versus conception odds), so they shouldn't be read as contradictions. But a lower population average matters most for men already near the low end [1].
• Phones. Swan says she simply doesn't know [1]. Eisenberg finds the evidence unconvincing, though some lab studies show slightly more sperm DNA fragmentation with close proximity [3]. Huberman cites a 2021 meta-analysis of 18 studies finding that phone use reduces sperm quality, and advises men trying to conceive not to keep phones in their pockets [4].

Why It Works

The mechanisms these sources give for how the modern environment could lower sperm and testosterone:

• Anti-androgens during the male programming window. In rats, certain phthalates given in a narrow window of pregnancy (roughly days 9 to 12 of a 21-day gestation) incompletely masculinize males. Exposure outside that window does nothing [1]. In humans, higher maternal phthalate levels predict shorter AGD, and shorter AGD tracks with lower adult sperm counts [1].
• Estrogen mimics. BPA and its replacements BPS and BPF are estrogenic. Swan describes BPA as the evil twin of phthalates, which make plastic soft and are anti-androgenic, and says "BPA free" labels often hide analogs she considers just as harmful [1]. Huberman adds that BPA and BPS may bind androgen receptors and can act on gene expression [2].
• Particles that cross protective barriers. The smallest nanoplastics are the most abundant in brain, testes and follicles, and they're the ones that cross the body's barriers. They can also carry PFAS, BPA and phthalates with them [2].
• Fat and heat. Fat tissue's aromatase converts testosterone to estrogen, and larger thighs warm the testicles [3]. Sperm develop best about two degrees below core body temperature [4].
• Overall health drives testicular function. Obesity, high blood pressure and high cholesterol stack up to lower testosterone and sperm quality [3]. About 10% of the male genome is devoted to reproduction, which may overlap with other systems, one proposed reason semen quality tracks longevity [3].
• Clearance differs by chemical. Water-soluble chemicals such as phthalates and bisphenols leave the body within hours once exposure stops: "You don't have to do anything you just have to stop taking them in" [1]. Fat-soluble ones like PFAS and many pesticides stay far longer [1].

Where It Applies, and Where It Doesn't

Where the concern is strongest:

• Pregnancy and pre-conception. Swan says prenatal damage can't be reversed and may carry across generations [1]. Huberman says pregnant people and families with young children have the most reason to limit plastic exposure [2].
• Men trying to conceive or already near the low end. Because sperm count matters a lot when low and not at all when high, reducing exposures and heat matters most here [1][4].
• Men with obesity or metabolic conditions. These lower testosterone and sperm quality in ways that respond to lifestyle change [3].
• Agricultural regions with heavy pesticide use. Here Swan found markedly lower sperm quality [1].

Where the evidence is thinner:

• Microplastics causing specific human disease. So far this is correlational and animal evidence only [2].
• Cell phones. The experts range from "don't know" to "not convincing" to a meta-analysis says yes [1][3][4].
• Individual prediction. Population trends don't tell you your own numbers. Only testing does [3].
• Lower counts versus infertility. A lower count doesn't necessarily mean infertility [2], and fertility depends on both partners [3].
• Early findings. Swan's six-couple product-swap study is not yet hard data [1].

Anyone with fertility concerns, low libido or symptoms of low testosterone should see a urologist or reproductive specialist and get tested rather than self-treat.

What To Do With It

1. Get a baseline semen analysis and hormone panel. Huberman suggests testing in your 20s or 30s after 48 to 72 hours of abstinence, repeating about every five years, and considering freezing sperm when younger [4]. Eisenberg agrees that more information is useful and notes insurance increasingly covers fertility testing [3].
2. Cut fragrance. Swan calls fragrance a major source of phthalates in lotions, soaps, deodorants, detergents and shampoos: "if you can smell it It's probably affecting your hormones" [1].
3. Never heat food in plastic, and skip cans. "Never never put plastic in a microwave," Swan says [1]. Huberman adds hot drinks in paper cups and canned soup to the list [2].
4. Change your cookware and containers. Replace non-stick pans ("No question iron," Swan says) and use glass, ceramic or stainless steel for food and water [1][2].
5. Filter your water and switch salt. Huberman recommends reverse osmosis with remineralization for tap water and non-marine salt instead of sea salt [2]. Swan distills her water and stores it in glass [1].
6. Buy organic where you can and take your shoes off. Organic food limits pesticide and phthalate exposure. Household dust carries PFAS [1].
7. Decline paper receipts. Chemicals on thermal paper are absorbed, especially through hands with lotion or sunscreen on them [1][2].
8. Keep the testicles cool. Avoid hot tubs, saunas, heated car seats and laptops on the lap when trying to conceive, and allow a 90-day window for changes to show [3][4].
9. Work on the basics that move testosterone. Keep a healthy weight, walk more, sleep seven to nine hours, don't smoke, and keep alcohol moderate [3].
10. Take extra care before and during pregnancy. Because "Whatever happened in the womb stays in the womb," both partners should reduce chemical exposure when planning a pregnancy [1].

Sources

1. Dr Shanna Swan How To Safeguard Your Hormone Health Fertility · Huberman Lab
2. The Effects of Microplastics on Your Health & How to Reduce Them · Huberman Lab
3. Dr. Michael Eisenberg: Improving Male Sexual Health, Function & Fertility · Huberman Lab
4. How to Optimize Fertility in Males & Females · Huberman Lab