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Sperm counts worldwide are plummeting faster than we thought
“We may cross a tipping point when most men will be sub-fertile,” says one expert. Here are environmental and lifestyle factors that could increase a man’s risk.
Five years ago, a study describing a precipitous decline in sperm counts sparked extreme concerns that humanity was on the path to extinction. Now a new study shows that sperm counts have fallen further and the rate of decline is speeding up, raising fears of a looming global fertility crisis.
The initial study, published in July 2017, revealed that sperm counts—the number of sperm in a single ejaculate—plummeted by more than 50 percent among men in North America, Europe, Australia, and New Zealand between 1973 and 2011. Since then, a team led by the same researchers has explored what has happened in the last 10 years.In a new meta-analysis, which appears today in the journal Human Reproduction Update, researchers analyzed studies of semen samples published between 2014 and 2019 and added this to their previous data. The newer studies have a more global perspective and involved semen samples from 14,233 men, including some from South and Central America, Africa, and Asia. The upshot: Not only has the decline in total sperm counts continued—reaching a drop of 62 percent—but the decline per year has doubled since 2000.
The 2017 report also revealed that sperm concentration (the number of sperm per milliliter of semen) dropped by an average of 1.6 percent per year, totaling more than a 52 percent among men in these regions over the previous four decades.
“The decline is not tapering off—it’s steep and significant,” says study co-author Shanna Swan, a reproductive and environmental epidemiologist at the Icahn School of Medicine at Mount Sinai in New York City. “Overall the drop is similar in magnitude but when we look at recent years, we see that it’s speeding up.”
Study lead author Hagai Levine, a medical epidemiologist at Hebrew University of Jerusalem’s Hadassah Braun School of Public Health, calls the results “worrisome as we were hoping that at some point the decline would be leveling off. The opposite may be true, and we may cross a tipping point when most men will be sub-fertile or when the causes of this decline will also manifest by other adverse health trends.”
Contrary to common perception, infertility impacts men and women equally, says Amy E.T. Sparks, a reproductive physiologist and director of the IVF and Andrology Laboratories at the University of Iowa Center for Advanced Reproductive Health. “I think the perception that infertility is primarily a woman’s problem may be due to the tendency for women to initially seek medical care for infertility rather than men.” In the scientific community, the prevailing view is that male and female fertility challenges are each responsible for about one-third of infertility cases; the remaining cases are due to a combination of male and female factors.
But the new data suggests a “substantial increase in the proportion of men with low sperm counts which leads to a reduced capability for fertilizing their partners,” says David M. Kristensen, a molecular toxicologist at Roskilde University and Copenhagen University Hospital in Denmark who was not involved in the study. “This is of concern for not only the families that are affected but also for societies in general, as many countries such as Italy and Japan are already suffering from shrinking populations.”
Beyond reproductive matters, there’s also a concern that reduced sperm counts are associated with a variety of health problems in men. “There is an association between semen quality and overall health—studies suggest that impaired semen quality is associated with a higher risk of testicular cancer, cardiovascular disease, and [premature] mortality,” notes Michael Eisenberg, director of male reproductive medicine and surgery and a professor of urology at the Stanford University School of Medicine, who was not involved in either meta-analysis.
“One can view the decline in sperm counts as a biomarker for male health in general,” says Kristensen.
In fact, a study in a 2018 issue of the journal Andrologyfound a higher risk of hospitalization among men who had lower sperm concentrations. Those with sperm concentrations below 15 million/mL—considered low—had a 53 percent greater risk of being hospitalized for any reason over the course of 36 years than those with more robust sperm concentrations between 51 and 100 million/mL. This effect persisted even after the researchers controlled for body weight, smoking, and other factors.
It’s important to note that the decline in sperm counts isn’t happening in a vacuum. Low sperm count often goes hand in hand with low testosterone levels and changes in male genital development while in the womb, says Swan, author of the book Count Down: How Our Modern World Is Threatening Sperm Counts, Altering Male and Female Reproductive Development, and Imperiling the Future of the Human Race.
In a man, the production of sperm requires a certain level of testosterone as well as the testes’ ability to regulate the temperature of the tissue in which sperm are made, Sparks explains. “Levels of testosterone have been reported to be declining during the same period of time that the sperm production rates were measured in this meta-analysis.”
It’s also important to recognize that it’s not just a matter of what a man is exposed during his lifetime that can affect his sperm quality. What an expectant mother is exposed to while she’s pregnant can affect the sperm concentrations of her male offspring: During early pregnancy—what’s called the “reproductive programming window”—certain environmental chemicals can affect women in ways that could permanently alter the reproductive development of their male babies, Swan explains. “Whatever disruption to reproductive development that occurs in utero is permanent,” she says.
By contrast, the damage done to a man’s sperm during his life—such as by smoking or being exposed to pesticides—can be reversed if the exposure to the harmful chemical stops. It takes about 75 days for sperm to mature, says Swan, which means that men essentially have regular opportunities for a do-over for their sperm quality every two and a half months.
What’s driving the decline?
Neither the 2017 nor the 2022 meta-analyses examined what is causing the drop in sperm count, but other research suggests environmental and lifestyle factors may be to blame. These include exposure to endocrine-disrupting chemicals (which mimic or interfere with the body’s hormones), smoking, and obesity. For example, a study in a 2022 issue of the journal Toxicologyfound that occupational exposure to pesticides was associated with sperm found in lower concentrations, sperm that were poor swimmers, and sperm with more DNA damage. And a study in a 2019 issue of the journal Human Reproductionfound that men who are overweight tend to have reduced sperm concentration, lower total sperm count, and fewer motile sperm.
The fact that the sperm count decline is also occurring in countries in South and Central America, Africa, and Asia, according to the new meta-analysis, suggests that the lifestyle factors and environmental exposures that are likely to blame are present globally, Swan says.
As for what’s accelerating the sperm count decline found in the new meta-analysis, no one knows for sure. Levine suggests it may be due to “mixture effects” with chemicals—meaning that when various individual chemicals are added together in the environment, they can have a bigger, more detrimental impact by magnifying each other’s negative effects. Or, he says, the decline may result from “cumulative exposure over time.”
Given that the latest meta-analysis included data from 50 years, Swan suspects the acceleration stems from the cumulative impact of environmental chemicals over generations. Remember: While in the womb, a male fetus is exposed to the same chemicals and lifestyle factors—such as poor diet, smoking, and obesity—that his mother is exposed to while she’s pregnant. But the transmission of these exposures doesn’t stop there—the epigenetic effects of these exposures may be transmitted from one generation to the next, not just from the mother but possibly from the father too. It may be due to factors in the father’s sperm that disrupt the reproductive development of male fetuses in the womb, Levine notes.
As more generations are exposed to these environmental chemicals and harmful lifestyle factors over time, the effects may become additive.
A wake-up call
More research needs to be done to determine what is tanking sperm counts. In the meantime, men and women can try to protect their reproductive health—by consuming a healthy diet, exercising regularly, maintaining a healthy body weight, and avoiding smoking—all behaviors Eisenberg advises his male patients to adopt.
Swan also recommends reducing exposure to endocrine-disrupting chemicals by being a savvy consumer. These chemicals include: phthalates (in plastics and personal-care products such as nail polishes, shampoos, and hair sprays), bisphenol A (in hard plastics, adhesives, and the lining of some food cans), flame retardants (in furniture and carpets), perfluoroalkyl substances (in nonstick cookware and stain-resistant carpets), and pesticides (in plant-based foods and lawn-care products).
Ultimately, Levine and Swan say that local and global actions are needed to reduce or get rid of these chemicals in our environments. “We should find ways to prevent further decline and even reverse the trends,” Levine says. “We must avoid being complacent about it and fool ourselves that assisted reproduction is the solution.”
Over the past 50 years, human sperm counts appear to have fallen by more than 50% around the globe, according to an updated review of medical literature.
If the findings are confirmed and the decline continues, it could have important implications for human reproduction. Researchers say it would also be a harbinger of declining health in men in general, since semen quality can be an important marker of overall health.
The review, and its conclusions, have sparked a debate among experts in male fertility. Some say the findings are real and urgent, but others say they are not convinced by the data because the methods of counting sperm have changed so much over time that it’s not possible to compare historical and modern numbers.
Nearly all experts agree that the issue needs more study.
“I think one of the fundamental functions of any species is reproduction. So I think if there is a signal that reproduction is in decline, I think that’s a very important finding,” said Dr. Michael Eisenberg, a urologist with Stanford Medicine who was not involved in the review.
“There is a strong link between a man’s reproductive health and his overall health. So it could also speak to that too, that maybe we’re not as healthy as we once were,” he said.
Others say that while the review was well-done, they are skeptical about its conclusions.
“The way that semen analysis is done has changed over the decades. It has improved. It has become more standardized, but not perfectly,” said Dr. Alexander Pastuczak, a surgeon and assistant professor the University of Utah School of Medicine in Salt Lake City. He was not involved in the review.
“Even if you were to take the same semen sample and run it and do a semen analysis on it in the 1960s and ’70s versus today, you’d get two different answers,” he said.
Pastuczak says that in more contemporary studies of semen analysis, ones that rely on samples analyzed by a different method, “you don’t see these trends.” In fact, some studies in Northern European regions show sperm counts going up over time, not down, he said.
Updated review adds data from more countries
The new analysis updates a review published in 2017 and for the first time includes new data from Central and South America, Asia and Africa. It was published in the journal Human Reproduction Update.
An international team of researchers combed through nearly 3,000 studies that recorded men’s sperm counts and were published between 2014 and 2020, years that had not been included in their previous analysis.
The researchers excluded studies that featured only men who were being evaluated for infertility, those that selected only men who had normal sperm counts and those whose study participants were selected based on genital abnormalities or diseases. They included only studies published in English, those with 10 or more men and those with participants whose sperm was collected in the typical way and counted using a device called a hemocytometer.
In the end, just 38 studies met their criteria. They added these to studies included in their previous review and extracted their data, which was fed into models.
Overall, the researchers determined that sperm counts fell by sightly more than 1% per year between 1973 and 2018. The study concluded that globally, the average sperm count had fallen 52% by 2018.
When the study researchers restricted their analysis to certain years, they found that the decline in sperm counts seemed to be accelerating, from an average of 1.16% per year after 1973 to 2.64% per year after 2020.
“It’s really remarkable that actually the decline is increasing,” said study author Dr. Hagai Levine, an epidemiologist and public health researcher at the Braun School of Public Health and Community Medicine.
On a population level, the average sperm count dropped from 104 million to 49 million per milliliter from 1973 through 2019. Normal sperm counts are considered to be over 40 million per milliliter.
Causes of the decline are unknown
The study authors say they didn’t have enough data from different regions to be able to tell whether some countries had lower average sperm counts than others or whether sperm counts were declining faster in certain areas. Data from 53 countries was included in the review.
The authors also didn’t look at what might be causing the decline. “It should be studied,” Levine said.
In other research, Levine says, he and others have teased out some factors that are associated with lower sperm counts.
Damage to reproductive health may begin in the womb.
“We know that stress of the mother, maternal smoking and especially exposure to manmade chemicals that are in plastic, such as phthalates, disrupt the development of the male reproductive system,” Levine said.
Lifestyle may also play an important role. Obesity, a lack of physical activity and diets high in ultraprocessed food may all be culprits, he said.
“The same factors that harm health in general usually are also harmful to semen quality,” he said.
One expert said that, ultimately, trying to do this kind of study is fraught with problems that complicate the findings.
“The paper is very scientifically or statistically robust and does a good job of summarizing the data that is available in our field. But it’s important to recognize that that data is still very limited in how it was collected and how it was reported,” said Dr. Scott Lundy, a urologist at the Cleveland Clinic who was not involved in the research.
Standards and methods for counting sperm have changed greatly over time, Lundy says, making it difficult to compare modern counts to historical data.
Still, he said, that historical data is all that’s available to the field.
“While it’s not a cause for panic, because the counts are by and large still normal, on average, there is a risk that they could become abnormal in the future, and we have to recognize that and study that further,” Lundy said.
For decades, researchers have been asking if sperm counts are decreasing worldwide, and if so, whether this presages a global decline in male fertility. Most recently, a large systematic review and meta-regression analysis sought to identify trends in sperm counts between 1981 and 2013 and found that sperm counts appeared to be declining rather than stabilizing. One of the complicating features of relying on sperm count as a marker of fertility is that a low sperm count does not guarantee an inability to conceive. A large variety of factors, including tobacco, alcohol, and drug use; psychological stress; obesity; insufficient sleep; and environmental factors such as air pollutants and heavy metals, have been identified as potential risk factors affecting semen quality. Initial investigations recommended for a patient presenting with fertility concerns include a detailed history, physical examination, investigations based on the clinical context, and semen analysis for most patients. Although the evidence is conflicting, our review suggests that the potential decline in male sperm counts does not necessarily translate to a decline in male fertility.
A broad summary of the published evidence on sperm-count and fertility trends.
Concerns regarding declining sperm counts have existed for over 50 years, and even predate the use of a standard means to assess semen characteristics. The scientific characterization of semen samples has also changed significantly since the World Health Organization’s (WHO’s) first publication of its Laboratory Manual for the Examination and Processing of Human Semen in 1999, with the 6th edition released in July 2021.
Dozens of articles have been published about a global reduction in male spermatogenesis, with varying results reported. However, the articles that report declining sperm counts are the ones more often picked up by news outlets, which leads to a widespread belief of globally declining sperm counts as fact. For example, the BBC released a podcast on this topic in 2021, stating, “the fall in men’s sperm counts is more alarming than first thought.” This type of coverage has led to far-reaching claims of an imminent fertility disaster facing the world. However, in the field of reproduction, the contention that sperm counts are declining is not universally accepted and has been a hotly debated topic over the last several decades.[1,5-9] Although the evidence for declining sperm counts is conflicting, our review suggests that a decline in sperm counts does not necessarily translate to a decline in male fertility.
Evaluation of male fertility
Approximately 15% of male-female couples around the world will experience infertility. The male is found to be the sole cause in 20% of those cases and a partial contributor in 30% to 40%. Male sperm production (i.e., spermatogenesis) occurs in the seminiferous tubules of the testes. The 70-day process includes proliferation of spermatogonia, meiosis, and then acquisition of a head and tail structure to become mature spermatozoa. Endocrine drivers of male sexual function include follicle-stimulation hormone, which stimulates Sertoli cells in seminiferous tubules, and luteinizing hormone, which drives testosterone production in Leydig cells. Endogenous testosterone production is the single most important endocrine driver of spermatogenesis. Men can produce tens to hundreds of millions of sperm each day.
History taking for men presenting with fertility concerns should include questions about general health, testicular symptoms, duration of infertility, sexual mechanics, difficulty with erection and/or ejaculation, coital frequency and timing, prior conceptions, medications, environmental exposures, and drug and alcohol use. It is also important to ask specifically about consumption of muscle-building supplements or exogenous testosterone, as these substances can decrease or completely eliminate sperm production via negative feedback on pituitary follicle-stimulation hormone secretion. It is important to obtain a medical history of prior injury (e.g., torsion), illness (e.g., mumps), surgery affecting the scrotum or testes, and childhood and pubertal development (e.g., delayed testicular descent). A family history of infertility could suggest potential chromosomal or genetic causes of infertility such as Kallmann syndrome, myotonic dystrophy, androgen receptor defects, gonadotropin and gonadotropin receptor defects, cystic fibrosis, or chromosomal rearrangements.
Physical examination should include evaluation of body mass index, secondary sexual characteristics such as presence of facial and chest hair, and the presence of gynecomastia. The genitourinary exam includes a scrotal exam, assessment of testicular size (normal length is 4.5–5.1 cm, 15–19 g, or 20 mL), consistency, and presence of nodules, epididymal abnormalities such as dilatations and cysts, abnormalities of the vas deferens, and inspection/palpation for varicoceles.[12,14]
Investigations are selected based on the clinical context. A semen analysis is a reasonable initial test to assess total sperm count (≥ 39 million per ejaculate), semen volume (≥ 1.4 mL), sperm concentration (≥ 16 million/mL), total motility (> 42%), and normal morphology (≥ 4%).[3,12] For men with oligozoospermia (concentration < 16 million/mL), a hormonal workup is indicated, including assessment of serum luteinizing hormone, follicle-stimulation hormone, prolactin, estradiol, and total testosterone.[12,15] In cases of severe oligozoospermia (concentration < 6 million/mL) genetic testing in the form of karyotype and Y chromosome microdeletion testing is indicated.
As most pathology in the scrotum is palpable, scrotal ultrasound is reserved for cases where the physical exam is difficult, inadequate, or ambiguous, or when a testicular mass is suspected. Other specialized testing such as retrograde semen analysis, transrectal ultrasound, and cystic fibrosis transmembrane regulator gene testing is typically arranged by those with subspecialty training given the specific instances in which they are indicated.
Are sperm counts really declining?
The question of whether sperm counts are declining remains controversial. A prominent article from Carlsen and colleagues in 1992 brought about concern about declining sperm counts between the 1930s and 1990s. Significant criticisms and limitations in the article were subsequently identified, including variability in sperm collection and measurement protocols, lack of control for abstinence durations, inability to control for seasonal variation, inability to control for lifestyle factors, failure to include studies not showing sperm count decline, failure to account for geographic variation, and inappropriate statistical considerations. Most notably, geographic disparities have been shown pertaining to the United States. Significantly more semen samples from New York were included than from other regions. In Carlsen’s article, 80% of the included studies were derived from New York prior to 1970, but only three studies were derived from the United States after 1970. After removing the New York semen analyses, the declining trend was reversed and not statistically significant.
More recently, Levine and colleagues conducted a large systematic review and meta-regression analysis to identify more contemporary trends in sperm counts, between 1981 and 2013. They found that an overall decline in sperm counts was observed from 1973 to 2011. Sperm concentration declined 52.4% (~1.4% per year) and total sperm counts declined 59.3% (~1.6% per year) in studies completed in North America, Europe, Australia, and New Zealand. Sperm counts appeared to be continuing to decline rather than stabilizing. The systematic review did not assess for other indicators of sperm quality, such as motility or morphology. The study concluded that declining sperm counts are a reason to be concerned about worldwide subfertility or infertility in the future. Some further concluded that low sperm counts may serve as a canary in a coal mine, indicating a global decline in the general health of males because low sperm counts have been associated with increased morbidity and mortality in males.[19-22]
A critique of Levine and colleagues’ meta-analysis (conducted in 2021 by Boulicault and colleagues) highlights several limitations that may provide alternative explanations for the trends they identified. Levine offers what Boulicault and colleagues call the Sperm Count Decline Hypothesis. In the hypothesis, sperm count is a marker of male health. Causes of the decline in sperm count are attributed to endocrine disruptors, environmental pollutants, and lifestyle. Boulicault offers an alternative framework called the Sperm Count Biovariability Hypothesis, in which sperm count naturally varies widely and fluctuations are not always pathological and are in fact typical for the human species. A central tenet of this theory is that a higher sperm count (above a certain threshold) does not equate to better health or fertility. The WHO’s reference intervals for semen characteristics are based on men who conceived a child within 1 year of trying. Based on those parameters, the lower reference limit for total sperm count was 39 million/ejaculate and sperm concentration was 16 million/mL. Levine and colleagues report total sperm counts of 212 million and sperm concentration of 66.4 million/mL in 2011. Although sperm concentration and total sperm count decreased from 1973 to 2011, the values still fall above the normal lower reference limit for fertility by a significant margin.
One of the complicating features of relying on sperm count as a marker of fertility is that a low sperm count does not guarantee an inability to conceive. Patients with low sperm counts can still conceive and patients with high sperm counts can have difficulties conceiving. Additionally, there is not enough evidence to support the claims that subfertility has been increasing over recent decades. Little evidence exists to show that sperm count is independently representative of male health status in isolation of other sperm parameters.
Do environmental and lifestyle factors affect fertility?
While it is difficult to determine if sperm counts are declining, and if so, if that will translate to an impact on fertility, it is worthwhile to understand the potential impact of environmental and lifestyle factors on sperm parameters and fertility. Tobacco, alcohol, and drug use; psychological stress; obesity; insufficient sleep; and environmental factors such as air pollutants and heavy metals have all been identified as potential risk factors that affect semen quality [Table].[28-45]
Drinking five alcoholic beverages per week was enough to show effects on sperm concentration, total sperm count, and proportion of sperm with normal morphology. The effects were most pronounced in individuals who drank more than 25 drinks per week. Smoking appears to decrease sperm counts, increase DNA fragmentation, and reduce motility and normal morphology.[20,30]
A sedentary lifestyle with more than 4 hours of sitting per day was significantly associated with higher immotile sperm.[29,33] Other lifestyle factors affecting fertility include poor sleep quality and duration, possibly contributing to abnormal sperm morphology, higher rates of oligozoospermia, and low sperm concentrations.
Low levels of vitamin D were thought to be associated with decreased sperm motility and number of motile spermatozoa; however, in a randomized controlled trial assessing semen quality with vitamin D supplementation, no significant effect was observed in men who had a vitamin D deficiency.
Environmental exposures affecting sperm quality include bisphenol A (BPA), phthalates, heavy metals, and heat.[38,39,42-44,46] BPA is found in polycarbonate plastics, epoxy resin liners of aluminum cans, and thermal receipts, and was found to be associated with lower sperm concentrations. Phthalates are ubiquitous and exposure occurs via ingestion, inhalation, or absorption through the skin. Phthalates are used as plasticizers to increase the elasticity of material and can be found in materials such as cosmetics, paints, and lubricants.[43,44] Chronic phthalate exposure was associated with many adverse sperm parameters including decreased sperm concentration, motility, morphology, and increased sperm DNA damage.[44,45] Higher levels of occupational heavy metal exposure and heat have also been associated with reduced sperm quality.[38,39,45]
The debate over whether sperm counts are declining will likely be a contentious topic until better study designs are implemented. Importantly, this question is separate from whether a decline in male fertility exists, because sperm counts and fertility are not synonymous. Ultimately, the evidence is conflicting, limiting our ability to draw accurate conclusions. While it is possible that sperm counts are declining, methodologic limitations, biological variability, and technical inconsistency significantly weaken this conclusion. Furthermore, even if sperm counts are declining, this likely does not have as significant an impact on fertility as is portrayed in the media.
While researchers report that lifestyle and environmental factors are negatively associated with sperm parameters, it is difficult to interpret the impact seen in today’s society. Certainly, a healthy lifestyle and environment are important not only for fertility, but also for general health in society and the natural world.
Researchers should caution against drawing the conclusion that male infertility is on the rise based on literature showing declining sperm counts because these studies have significant methodological limitations. While the most recent meta-analysis attempts to address several confounding variables, the data remain highly heterogeneous with immeasurable confounding biases that cannot be addressed with currently available retrospective data. Furthermore, sperm counts by themselves are not a precise marker of male fertility. Research that implements prospective, meticulously designed studies is likely required to further investigate trends of sperm production and fertility.
Male scientists have long waxed poetic on the contents of their testes. “Sperm is a drop of brain,” wrote the ancient Greek writer Diogenes Laërtius. Leonardo da Vinci drew the penis with a sperm duct that connected directly to the spinal cord. The 17th-century microscopist Antonie van Leeuwenhoek claimed that each sperm cell contained within it a folded-up human being waiting patiently to unfurl.
For nearly as long, scientists have fretted about sperm’s seemingly inevitable decline. Most recently, a series of alarming headlines — as well as a new book by an epidemiologist at Mount Sinai Medical Center in New York — warned that falling sperm counts might threaten the future of the human race. “It’s a global existential crisis,” said Shanna H. Swan, author of the book “Count Down.”
Most of these headlines can be traced to an influential 2017 meta-analysis by Dr. Swan and others, which found that sperm counts in Europe, North America, Australia and New Zealand had plummeted by nearly 60 percent since 1973. The authors screened 7,500 sperm-count studies from around the world, weeded out most of them and ultimately analyzed 185 studies on 43,000 men worldwide.
They called the decline a “canary in the coal mine” for waning male reproductive health worldwide. Today, the authors would amplify that statement. “There is clear and present alarm now,” said Dr. Hagai Levine, an epidemiologist at Hebrew University-Hadassah School of Public Health and a co-author on the 2017 review, in an email. “The canary is in trouble now.” Dr. Swan, in the same email, agreed.
Now a group of interdisciplinary researchers from Harvard and the Massachusetts Institute of Technology contend that fears of an impending Spermageddon have been vastly overstated. In a study published in May in the journal Human Fertility, they re-evaluated the 2017 review and found that it relied on flawed assumptions and failed to consider alternate explanations for the apparent decline of sperm.
In an interview, Sarah Richardson, a Harvard scholar on gender and science and the senior author on the new study, called the conclusion of the 2017 review “an astonishing and terrifying claim that, were it to be true, would justify the apocalyptic tenor of some of the writing.” Fortunately, she and her co-authors argue, there is little evidence that this is the case.
The 2017 authors were “methodologically rigorous” when it came to screening sperm-count studies for quality and consistency, Dr. Richardson and her colleagues write. However, even the data that passed muster was geographically sparse and uneven and often lacked basic criteria like the age of the men. Moreover, its authors took for granted that a single metric — sperm count — was an accurate predictor of male fertility and overall health.
The connection sounds logical: Without sperm, there can be no conception. That’s why sperm count is one of the first metrics that fertility specialists measure to try to determine the cause of infertility in a couple. But beyond that truism, the science of sperm count is surprisingly slippery.
For starters, no one knows what an “optimal” sperm count is. The World Health Organization sets a range of “normal” sperm count as from 15 to 250 million sperm per milliliter. (Men produce about 2 to 5 milliliters of semen per ejaculation.) But it isn’t clear that more is better. Above a certain threshold — 40 million per milliliter, according to the W.H.O. — a higher sperm count does not mean a man is more fertile.
“Doubling your sperm count from 25 to 50 million doesn’t double your chances,” said Allan Pacey, an andrologist at the University of Sheffield and the editor of Human Fertility. “Doubling it from 100 to 200 million doesn’t double your chances — in fact it flattens off, if anything. So this relationship between sperm count and fertility is weak.”
Germaine M. Buck Louis, a reproductive epidemiologist at George Mason University who studies environmental influences on human fertility, agreed that sperm count is a poor indicator of fertility. “We don’t see it predicting much of anything, especially in the context of a partner with a healthy female pelvis,” said Dr. Buck Louis, who was not involved in the sperm-count studies.
The authors of the 2017 study inferred that lower sperm counts equated to lower fertility — even though the sperm-count declines they documented all took place within the “normal” range, Dr. Richardson noted. “It’s similar to the whole conversation around testosterone — more is better, and more is manlier,” she said. “That’s really a point we make, that there is no known normal or baseline for average population sperm counts.”
Sperm count has other limitations as a metric. It takes around two months for stem cells in the testes to develop into new sperm, meaning that any single count is merely a snapshot of an evolving landscape.
“Something that’s going on in a man’s body one month may be totally different from what’s happening the next month, and the effects on sperm count might be changing also,” said Meredith Reiches, an author on the 2021 paper and a biological anthropologist at the University of Massachusetts, Boston.
It also overlooks a vital piece of the infertility puzzle: women. Focusing only on the male metric leaves out key interactions between sperm, the female reproductive tract and the egg. “It’s very important, actually, to look at the couple,” said Dr. Bradley D. Anawalt, a reproductive endocrinologist at the University of Washington School of Medicine.
In her book, Dr. Swan suggests that sperm counts have plummeted largely due to the rise of endocrine disruptors, a class of hormone-mimicking chemicals found in everything from shampoo to TV-dinner packaging. (She also cites lifestyle factors like obesity, alcohol, and smoking.) Dr. Swan has shown in previous studies that exposure to these chemicals in utero can alter male and female sexual development.
Dr. Richardson and her co-authors suggested an alternative explanation: Perhaps sperm levels naturally rise and fall over time and within populations. The question has not been explored by reproductive researchers and cannot be answered easily, as global sperm counts before 1970 are largely unknown.
There are other possible explanations, as well. Sperm-counting is a tricky business and notoriously prone to human error, Dr. Pacey said. (“I say it from the point of view of someone who spent 30 years counting sperm and knows how difficult it is,” he added.) In a 2013 review article, he noted that as methodologies for counting had improved and been standardized since the 1980s, sperm counts had appeared to fall. In other words, it may simply be that earlier scientists were overcounting sperm.
Dr. Swan and Dr. Levine agreed that exploring these alternative hypotheses was important, so that threats to reproductive health could be established and prevented. “We showed evidence for decline, and raised alarm,” Dr. Levine wrote in an email. “We need to study the causes, including the unlikely possibility of non-pathological decline.”
There was one point that every author agreed on: Men’s reproductive health matters. And until now, it has been surprisingly neglected.
Male infertility contributes to at least half of all cases of infertility worldwide. Yet historically, women have shouldered most of the blame for the inability to conceive. And with the rise of reproductive technologies like in vitro fertilization, women’s bodies are the ones that have been meticulously measured and tracked by reproductive medicine.
As a result, science still lacks basic knowledge when it comes to sperm, said Rene Almeling, a sociologist of medicine and author of “GUYnecology: The Missing Science of Men’s Reproductive Health.”
“We have built up such a medical infrastructure around the fertility and reproductivity of women’s bodies that we haven’t asked some of the basic questions about men’s reproductive health,” Dr. Almeling said. “There is just so, so much basic research still to be done about sperm.”
The main qualities of sperm that infertility specialists look at nowadays — how many, what shape and how they swim — have not changed in the past 40 years, said Dr. Abraham Morgentaler, a urologist and founder of Men’s Health Boston.
Dr. Morgentaler, who worked at a semen analysis lab at Beth Israel Deaconess Medical Center in the 1980s, attributes this stagnation to the rise of I.V.F. and other reproductive technologies, which have become frontline treatments for almost any male factor fertility problem. “It almost doesn’t even matter what’s wrong with the sperm,” he said.
These knowledge gaps radiate out to all bodies. In fact, Dr. Swan said part of her motivation for writing the book was that she wanted to see the public — men and women — become more proactive about their reproductive health.
“It’s invisible,” she said. “People don’t talk about it. You talk about, ‘Oh, I’ve got a high cholesterol measure,’ or ‘My blood pressure’s up.’ But you never would say, ‘My egg count is down,’ or ‘My sperm count is down.’”
Dr. Richardson agreed that the impact of reproductive toxins on fertility deserved further investigation. “To say that we think these are alarmist and apocalyptic claims, and they’re not well founded, is not to say that we think it isn’t an important research agenda,” she said. “There is a need to center on men’s reproductive health and understand their bodies as reproductive and as porous to the environment as anyone’s bodies.”
In the U.S., nearly 1 in 8 couples struggles with infertility. Unfortunately, physicians like me who specialize in reproductive medicine are unable to determine the cause of male infertility around 30% to 50% of the time. There is almost nothing more disheartening than telling a couple “I don’t know” or “There’s nothing I can do to help.”
Upon getting this news, couple after couple asks me questions that all follow a similar line of thinking. “What about his work, his cellphone, our laptops, all these plastics? Do you think they could have contributed to this?”
What my patients are really asking me is a big question in male reproductive health: Does environmental toxicity contribute to male infertility?
Male fertility decline
Infertility is defined as a couple’s inability to get pregnant for one year despite regular intercourse. When this is the case, doctors evaluate both partners to determine why.
For men, the cornerstone of the fertility evaluation is semen analysis, and there are a number of ways to assess sperm. Sperm count – the total number of sperm a man produces – and sperm concentration – number of sperm per milliliter of semen – are common measures, but they aren’t the best predictors of fertility. A more accurate measure looks at the total motile sperm count, which evaluates the fraction of sperm that are able to swim and move.
A wide range of factors – from obesity to hormonal imbalances to genetic diseases – can affect fertility. For many men, there are treatments that can help. But starting in the 1990s, researchers noticed a concerning trend. Even when controlling for many of the known risk factors, male fertility appeared to have been declining for decades.
In 1992, a study found a global 50% decline in sperm counts in men over the previous 60 years. Multiple studies over subsequent years confirmed that initial finding, including a 2017 paper showing a 50% to 60% decline in sperm concentration between 1973 and 2011 in men from around the world.
These studies, though important, focused on sperm concentration or total sperm count. So in 2019, a team of researchers decided to focus on the more powerful total motile sperm count. They found that the proportion of men with a normal total motile sperm count had declined by approximately 10% over the previous 16 years.
The science is consistent: Men today produce fewer sperm than in the past, and the sperm are less healthy. The question, then, is what could be causing this decline in fertility.
Environmental toxicity and reproduction
Scientists have known for years that, at least in animal models, environmental toxic exposure can alter hormonal balance and throw off reproduction. Researchers can’t intentionally expose human patients to harmful compounds and measure outcomes, but we can try to assess associations.
As the downward trend in male fertility emerged, I and other researchers began looking more toward chemicals in the environment for answers. This approach doesn’t allow us to definitively establish which chemicals are causing the male fertility decline, but the weight of the evidence is growing.
A lot of this research focuses on endocrine disrupters, molecules that mimic the body’s hormones and throw off the fragile hormonal balance of reproduction. These include substances like phthalates – better known as plasticizers – as well as pesticides, herbicides, heavy metals, toxic gases and other synthetic materials.
Plasticizers are found in most plastics – like water bottles and food containers – and exposure is associated with negative impacts on testosterone and semen health. Herbicides and pesticides abound in the food supply and some – specifically those with synthetic organic compounds that include phosphorus – are known to negatively affect fertility.
Air pollution surrounds cities, subjecting residents to particulate matter, sulfur dioxide, nitrogen oxide and other compounds that likely contribute to abnormal sperm quality. Radiation exposure from laptops, cellphones and modems has also been associated with declining sperm counts, impaired sperm motility and abnormal sperm shape. Heavy metals such as cadmium, lead and arsenic are also present in food, water and cosmetics and are also known to harm sperm health.
The effects of unregulated chemicals
A lot of chemicals are in use today, and tracking them all is incredibly difficult. More than 80,000 chemicals are registered in the U.S. and nearly 2,000 new chemicals are introduced each year. Many scientists believe that the safety testing for health and environmental risks is not strong enough and that the rapid development and introduction of new chemicals challenges the ability of organizations to test long-term risks to human health.
Current U.S. regulations follow the principle of innocent until proved guilty and are less comprehensive and restrictive than similar regulations in Europe, for example. The World Health Organization recently identified 800 compounds capable of disrupting hormones, only a small fraction of which have been tested.
A trade group, the American Chemistry Council, says on its website that manufacturers “have the regulatory certainty they need to innovate, grow, create jobs and win in the global marketplace – at the same time that public health and the environment benefit from strong risk-based protections.”
But the reality of the current regulatory system in the U.S. is that chemicals are introduced with minimal testing and taken off the market only when harm is proved. And that can take decades.
Dr. Niels Skakkebaek, the lead researcher on one of the first manuscripts on decreasing sperm counts, called the male fertility decline a “wake-up call to all of us.” My patients have provided a wake–up call for me that increased public awareness and advocacy are important to protect global reproductive health now and in the future. I’m not a toxicologist and can’t identify the cause of the infertility trends I’m seeing, but as physician, I am concerned that too much of the burden of proof is falling on the human body and people who become my patients.
The Doomsday Sperm Theory Embraced by the Far Right
The idea that male fertility is on the decline is an old myth dressed up as science.
The human species is in grave reproductive danger, according to recent headlines. Some scientists say that sperm counts in men around the world have been plummeting, with Western men approaching total infertility by 2045. Far-right “Great Replacement” theorists, who fear that people of color are “replacing” the white population, have taken up the research with gusto.
This all stems from a report published in 2017 in the journal Human Reproduction Update that claims sperm counts among men in North America, Western Europe, Australia, and New Zealand have fallen by some 50 percent since the 1970s. In February of this year, an author on that paper, Shanna Swan, published a book titled Count Down, which elaborates on what the phenomenon means for our future. The lengthy subtitle, How Our Modern World Is Threatening Sperm Counts, Altering Male and Female Reproductive Development, and Imperiling the Future of the Human Race, suggests things could get really bad. But we at the Harvard GenderSci Lab think there simply isn’t enough evidence to warrant doomsday predictions. The reason is that the initial finding of plummeting sperm counts relies on questionable assumptions.
First, let’s talk about who is actually affected by the supposed problem of declining sperm counts. For their paper, Swan and her colleagues looked at sperm counts recorded in other published studies between 1973 and 2011. They chose to group the relatively limited historical data on population sperm counts into two big categories: “Western” and “Other.” The researchers labeled the United States, Canada, Europe, Australia, and New Zealand—majority-white areas—as “Western.” They lumped all other countries, with their majority brown, Black, and Asian populations, into the category “Other.” Without using the word white, researchers signaled—as anthropologists and psychologists have done when comparing populations around the world—that the men we should be concerned about are white men. With, effectively, white male sperm counts declining, “the future of the human race” is being imperiled. Framed this way, their conclusions have leaked into the world in troubling ways. White nationalists and so-called men’s rights activists are taking up the alarm over the decline of “Western” sperm. In online forums like 4chan and Reddit, men worry that white people are not reproducing at rates that keep up with racialized others, especially immigrants and Black people. As one comment puts it, “humans have doomed themselves” by allowing the “wrong males” to reproduce.
But this is about more than optics. Separating out majority-white nations in an analysis of historical trends in sperm counts makes it harder to parse what’s actually driving the decline—and to whom that decline is happening. “Western” and “Other” are ineffective scientific categories because they fail to capture the factors most likely to influence sperm count. For example, many hypothesize that pollutants, especially chemicals in everyday plastics, are the most likely culprit behind sperm decline. But “Other” men are just as likely to be exposed to these chemicals as “Western” men: plastics, toxic pollution, and pesticides are as concentrated—sometimes more so—in low- and middle-income countries as they are in wealthy nations. We don’t know what the results would have looked like if scientists had chosen to use “high pollution” vs. “low pollution” regions as categories of analysis instead of “Western” vs. “Other” nations. Maybe, for example, the study would have found declines only among men exposed to certain kinds or levels of pollution.
And it’s possible that there just isn’t much of an issue at all—even for “Western” males. While sperm counts in “Western” countries may have indeed declined over a few decades, in 2011—the latest data included in the study—they remained well above the infertility threshold. Though it might be intuitive that it’s better for men to be overflowing with sperm, the historically “low” sperm counts likely don’t represent an issue for even the “Western” portion of the human race. In fact, those “low” sperm counts weren’t that different from the counts in “Other” countries back in the 1970s. It’s also not clear that sperm counts are being driven ever lower by some sinister factor. Even if there have been declines in average sperm counts in some men around the globe, it may be a result of normal variation in sperm counts, as we argue in a new paper out in the journal Human Fertility. The sperm decline theory posits that “Western” sperm counts from the 1970s are an optimum from which we have declined and that this decline is something that needs to be fixed.
But zero evidence supports either of these assumptions. In fact, it would be surprising if there were anything systematically lowering the fertility of men across a large, but specific, swath of the globe. It’s more likely that sperm counts, like other measures of reproductive function such as testosterone and progesterone levels, vary significantly across individuals, time periods, and geographical locations without pathology. To distinguish between cases of normal and pathological variation, and to do justice to the health and fertility of men in all parts of the world, researchers will need to identify local factors, environmental or otherwise, that could plausibly be the cause of long-term trends in human sperm counts, and track them through rigorous study designs created specifically for this purpose. Approaching the question by dividing the world’s men into a racialized “us” vs. “Other,” and then using the resulting data to make broad-brush statements about causes and consequences, is simply unsound science.
How did these unscientific categories and assumptions make it into the research in the first place? The narrative that white, Western men are in danger of emasculation and disappearance has deep roots in white nationalist discourse. It is tied to a nostalgic cultural myth of a past in which white men held unchallenged power. It is all too easy for scientific institutions, with majority-white researchers, to center white people and further these myths, which circulate often unconsciously. This is why having more diverse voices in scientific research is so important. The recent sperm count decline research demonstrates how racist, sexist, and Eurocentric ideas can get embedded in the categories that scientists use to analyze data. When research is designed, executed, and communicated by people with varied perspectives, problematic assumptions about particular groups are more likely to get caught and addressed. The institutions that fund science, the journals that publish it, and the media outlets that publicize it need to scrutinize findings that make apocalyptic, decontextualized claims about population differences in the ability to reproduce. This is especially crucial when the stories about who is in danger and why feel familiar and convenient. The narrative of Western sperm count decline is, essentially, a myth that we’re all too used to hearing.
nationalgeographic.com, “Sperm counts worldwide are plummeting faster than we thought: “We may cross a tipping point when most men will be sub-fertile,” says one expert. Here are environmental and lifestyle factors that could increase a man’s risk.” By Stacey Colino; cnn.com, “Sperm counts may be declining globally, review finds, adding to debate over male fertility.” By Brenda Goodman; bcmj.org, “Global decline of male fertility: Fact or fiction?” By Nora Tong, BDSc Luke Witherspoon, MD, MSc Caitlin Dunne, MD, FRCSC Ryan Flannigan, MD; nytimes.com, “The Sperm-Count ‘Crisis’ Doesn’t Add Up: Reports of a decline in male fertility rely on flawed assumptions, a new study contends.” By Rachel E. Gross; theconversation.com, “Male fertility is declining – studies show that environmental toxins could be a reason.” By Ryan P. Smith; slate.com, ” The Doomsday Sperm Theory Embraced by the Far Right: The idea that male fertility is on the decline is an old myth dressed up as science.”BY ALEXANDER BORSA, MARION BOULICAULT, MEREDITH REICHES, AND SARAH S. RICHARDSON;
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