Physicians Raymond Bunge and C. M. Kinloch Nelson noticed something unusual happening at their Iowa clinic in the early 1970s: a growing number of men seeking infertility treatment. To investigate whether there might be changes in sperm in the general male population, they looked at sperm samples from 386 presumably fertile men—those seeking an elective vasectomy. Bunge and Nelson were shocked by their results: compared with men in the few studies prior, these men had lower semen volume, a higher quantity of abnormal sperm and a lower concentration of sperm. The researchers speculated that some environmental factor must be impacting the population at large.
Ever since Bunge and Nelson published their findings in a 1974 paper, there has been a jumble of investigations comparing sperm quality across studies. Some of these papers have measured sperm count, the estimated total number of sperm in a semen sample, and some have measured sperm concentration, the number of sperm in a given volume of ejaculate. (For simplicity, this article will refer to both as sperm count.) Some studies show a decrease; a few show an increase; and others show no change at all. Complicating matters, research has revealed that sperm counts vary by age, ejaculation frequency, season, geographical location, marijuana or alcohol use, type of underwear and collection in a clinic versus at home. These counts can vary even within a single sample.
These factors make it difficult to compare sperm counts across different study populations. So although some large-scale analyses of past studies have suggested a significant global drop in sperm count since the 1970s, a few researchers caution that the finding is far from settled.
A paper published in August 2022 in Nature Reviews Urology states that “available data do not enable us to conclude that human semen quality is deteriorating worldwide or in the Western world, but that a trend is observed in some specific areas.” The idea of a global decrease in sperm count is “an important hypothesis, but the data is not good [enough] to be able to draw conclusions,” says Dolores Lamb, a researcher who studies andrology—male reproductive health—at Weill Cornell Medicine, who was not involved in the study.
Lamb and others explain that many factors can affect sperm counts. For one thing, sperm move, and not all studies immobilize them before counting. The numbers can also vary within a sample because semen isn’t always well mixed. “So all of these things conspire to mean that sperm counting is a really difficult thing to do and a flawed thing to do,” says Allan Pacey, an andrologist at University of Sheffield in England. “And we know that clinically, which is why it’s so difficult to predict somebody’s fertility from a single semen sample.”
One of the most accurate ways to assess sperm count now is with flow cytometry, an automated tool that can count and analyze each sperm individually. But that method wasn’t available when many older studies were conducted. So large analyses that compare old and new studies often exclude ones that use the new methods in favor of simpler techniques that require researchers to count sperm in a chamber with a grid—a more error-prone technique.
Another problem with these large analyses is that most individual studies were not designed to determine whether or not sperm counts were changing in a given population, according to Jacques Auger, a retired andrologist formerly at the Cochin Institute in France and a co-author of the paper in Nature Reviews Urology. A notable exception is a study from Denmark, which was conducted in response to an alarming analysis published in 1992 that suggested a decline in sperm count. The Danish study took semen samples from new military recruits—a relatively uniform demographic group—for 15 years and found no evidence of a drop during that time.
In contrast, other studies do point to a decrease. Hagai Levine, an epidemiologist at the Hebrew University of Jerusalem, and his colleagues published an analysis in 2017 and an update in 2022 that evaluated a combined total of more than 200 studies that used a counting chamber. These two papers found about a 50 percent decrease in sperm concentration—most notably in Western countries—since the 1970s. Levine says that he wouldn’t expect small differences in counting methods between studies to account for the significant overall decline observed on average. In other words, he believes there are enough data showing a drop to make it convincing.
“It’s very true that when you put data from a lot of study cohorts together, there is also the concern of differences across studies,” says Lidia Mínguez-Alarcón, a reproductive epidemiologist at the Harvard T. H. Chan School of Public Health. She believes that the large analyses indicating a drop have accounted for these differences, however. The opinion of many reproductive epidemiologists, she says, “is that, in Western countries, there has been a decline in sperm counts.” For non-Western countries, she claims, there just aren’t enough data to say one way or another. The 2022 updated study by Levine and his colleagues suggests a decline in sperm counts in South America, Asia and Africa, too, although Levine notes that there are fewer studies from those areas.
Other researchers acknowledge the drop in birth rate in many countries but question the reasons behind it. “Fertility is declining, but I’m not sure it’s through biology,” Pacey says. About one in six people experience infertility, and there are many possible reasons for this. Pacey suspects social and economic factors play a key role. Couples are waiting longer to have children, and people may be having less sex. Sperm count is also an imprecise measure of fertility—some data suggest higher sperm counts are associated with faster time to conception, but it’s still possible to achieve a pregnancy with lower counts.
Some studies have noted a strong link between obesity and infertility—potentially because of an impact on semen quality. And obesity rates are increasing. Endocrine-disrupting chemicals in the environment, such as phthalates or bisphenol A (BPA), have also been linked to a drop in sperm count. Ambient temperature, too, affects sperm count, and as global temperatures rise, that could hypothetically affect fertility.
“Sperm count is largely affected by the environment, and by our lifestyle, because it’s manufactured all the time,” Levine says. And even if his estimate of a significant global decrease is off and, say, “only 20 percent, 10 percent [or] 5 percent true, we still better do something about it and not wait to see because some of the causes may be irreversible.”
Most people in the field agree that the issue needs further study. Both Levine and Marion Boulicault, director of interdisciplinary research and community at Harvard University’s GenderSci Lab, point out that male fertility research has long been neglected. Levine says worldwide, systematic studies would not only inform whether there is continued or future decline but would also help scientists better understand whether the changes are caused by factors such as obesity, alcohol consumption or chemical exposure.
“It’s shameful that for such an important problem, 30 years later, we haven’t got the answer that we all agree on,” Pacey says.