A strange thing has happened to men over the past few decades: We’ve become increasingly infertile, so much so that within a generation we may lose the ability to reproduce entirely. What’s causing this mysterious drop in sperm counts—and is there any way to reverse it before it’s too late?
Men are doomed. Everybody knows this. We’re obviously all doomed, the women too, everybody in general, just a waiting game until one or another of the stupid things our stupid species is up to finally gets us. But as it turns out, no surprise: men first. Second instance of no surprise: We’re going to take the women down with us.
There has always been evidence that men, throughout life, are at higher risk of early death—from the beginning, a higher male incidence of Death by Mastodon Stomping, a higher incidence of Spiked Club to the Brainpan, a statistically significant disparity between how many men and how many women die of Accidentally Shooting Themselves in the Face or Getting Really Fat and Having a Heart Attack. The male of the species dies younger than the female—about five years on average. Divide a population into groups by birth year, and by the time each cohort reaches 85, there are two women left for every man alive. In fact, the male wins every age class: Baby boys die more often than baby girls; little boys die more often than little girls; teenage boys; young men; middle-aged men. Death champions across the board.
Now it seems that early death isn’t enough for us—we’re on track instead to void the species entirely. Last summer a group of researchers from Hebrew University and Mount Sinai medical school published a study showing that sperm counts in the U.S., Europe, Australia, and New Zealand have fallen by more than 50 percent over the past four decades. (They judged data from the rest of the world to be insufficient to draw conclusions from, but there are studies suggesting that the trend could be worldwide.) That is to say: We are producing half the sperm our grandfathers did. We are half as fertile.
The Hebrew University/Mount Sinai paper was a meta-analysis by a team of epidemiologists, clinicians, and researchers that culled data from 185 studies, which examined semen from almost 43,000 men. It showed that the human race is apparently on a trend line toward becoming unable to reproduce itself. Sperm counts went from 99 million sperm per milliliter of semen in 1973 to 47 million per milliliter in 2011, and the decline has been accelerating. Would 40 more years—or fewer—bring us all the way to zero?
I called Shanna H. Swan, a reproductive epidemiologist at Mount Sinai and one of the lead authors of the study, to ask if there was any good news hiding behind those brutal numbers. Were we really at risk of extinction? She failed to comfort me. “The What Does It Mean question means extrapolating beyond your data,” Swan said, “which is always a tricky thing. But you can ask, ‘What does it take? When is a species in danger? When is a species threatened?’ And we are definitely on that path.” That path, in its darkest reaches, leads to no more naturally conceived babies and potentially to no babies at all—and the final generation of Homo sapiens will roam the earth knowing they will be the last of their kind.
If we are half as fertile as the generation before us, why haven’t we noticed? One answer is that there is a lot of redundancy built into reproduction: You don’t need 200 million sperm to fertilize an egg, but that’s how many the average man might devote to the job. Most men can still conceive a child naturally with a depressed sperm count, and those who can’t have a booming fertility-treatment industry ready to help them. And though lower sperm counts probably have led to a small decrease in the number of children being conceived, that decline has been masked by sociological changes driving birth rates down even faster: People in the developed world are choosing to have fewer children, and they are having them later.
The problem has been debated among fertility scientists for decades now—studies suggesting that sperm counts are declining have been appearing since the ’70s—but until Swan and her colleagues’ meta-analysis, the results have always been judged incomplete or preliminary. Swan herself had conducted smaller studies on declining sperm counts, but in 2015 she decided it was time for a definitive answer. She teamed up with Hagai Levine, an Israeli epidemiologist, and Niels Jørgensen, a Danish endocrinologist, and along with five others, they set about performing a systematic review and meta-regression analysis—that is, a kind of statistical synthesis of the data. “Hagai is a very good scientist, and he also used to be the head of epidemiology for the Israeli armed forces,” Swan told me. “So he’s very good at organizing.” They spent a year working with the data.
“We should hope for the best and prepare for the worst,” said Hagai Levine, a lead author of the study. “And that is the possibility that we will become extinct.”
The results, when they came in, were clear. Not only were sperm counts per milliliter of semen down by more than 50 percent since 1973, but total sperm counts were down by almost 60 percent: We are producing less semen, and that semen has fewer sperm cells in it. This time around, even scientists who had been skeptical of past analyses had to admit that the study was all but unassailable. Jørgensen, in Copenhagen, told me that when he saw the results, he’d said aloud, “No, it cannot be true.” He had expected to see a past decline and then a leveling off. But he couldn’t argue when the team ran the numbers again and again. The downward slope was unwavering.
Almost all the scientists I talked to stressed that not only were low sperm counts alarming for what they said about the reproductive future of the species—they were also a warning of a much larger set of health problems facing men. In this view, sperm production is a canary in the coal mine of male bodies: We know, for instance, that men with poor semen quality have a higher mortality rate and are more likely to have diabetes, cancer, and cardiovascular disease than fertile men.
Testosterone levels have also dropped precipitously, with effects beginning in utero and extending into adulthood. One of the most significant markers of an organism’s sex is something called anogenital distance (AGD)—the measurement between the anus and the genitals. Male AGD is typically twice the length of female, a much more dramatic difference than height or weight or musculature. Lower testosterone leads to a shorter AGD, and a measurement lower than the median correlates to a man being seven times as likely to be subfertile and gives him a greater likelihood of having undescended testicles, testicular tumors, and a smaller penis. “What you are seeing in a number of systems, other developmental systems, is that the sex differences are shrinking,” Swan told me. Men are producing less sperm. They’re also becoming less male.
I assumed that the next thing Swan was going to tell me was that these changes were all a mystery to scientists. If only we could figure out what was causing the drop in sperm counts, I imagined, we could solve all the attendant health problems at once. But it turns out that it’s not a mystery: We know what the culprit is. And it’s hiding in plain sight.
This story originally appeared in the September 2018 issue with the title “Sperm Count Zero.”
