No Decline in Sperm Counts After All, Danish Data Show

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In the dystopian book and film Children of Men, human beings suddenly stop being able to have children. There’s no real explanation given for the change — though there’s talk of a precipitous decline in sperm count quality — but the loss of the ability to reproduce essentially robs humanity of its future, and leads to the utter collapse of society.

As the protagonist Theo Fanon puts it with characteristic despair:

I can’t really remember when I last had any hope, and I certainly can’t remember when anyone else did either. Because really, since women stopped being able to have babies, what’s left to hope for?

Our world isn’t quite there yet, but some scientists and environmentalists have worried for a while about a possible decline in global sperm quality, and a resulting reproductive crisis. Much of that fear was fueled by a 1992 Danish study that seemed to show that sperm counts had declined by 50% globally from 1938 to 1991, and that the trend would continue into the future.

(More on TIME.com: Could Lab-Grown Sperm Help Infertile Men?)

The study was criticized by many researchers for its flaws — including the length of time men abstained from ejaculation before their semen was collected, which can affect sperm quality — but it was also extremely influential, cited by more than 1,000 subsequent studies. Though other attempts to gauge sperm quality returned conflicting results, the original Danish study was scary enough to support theories that endocrine disrupters and other chemicals in the environment could threaten the future of the human race.

But new and better data has just been released on sperm quality — and suddenly the future isn’t looking quite so apocalyptic. (Er, at least in this area.) The same Danish group that published the original study has been collecting and analyzing annual semen samples from 18-year-old Danish men who were being examined for their military fitness. Over the past 15 years, 5,000 men have provided semen for analysis — a much better data pool than was involved in previous research, as Gina Kolata describes in the New York Times:

That design was an improvement over older studies, Dr. [Dolores] Lamb said. The data are from men of the same age and from one geographic area (sperm numbers and quality can vary from one region to another). Analysis of sperm is better now than it was in years past. And with 15 years of data, she said, any decline in sperm numbers or quality should have been evident.

The data, which were published in the American journal Epidemiology, show virtually no evidence of declining sperm quality, with levels barely changing from 1996 to 2010. As Jens Peter Bonde, Ceciia Host Ramlau-Hansen and Jorn Olsen wrote in a commentary in Epidemiology, these results create problems for researchers who’ve been insisting that we face a sperm crisis:

First, the Danish data provide a new perspective on the claim that human sperm counts are declining. Alleged links between declining sperm counts and exposure to environmental toxicants have generated immense media and public interest while fueling research programs to address the “estrogen hypothesis” and more recently the broader “endocrine disruption hypothesis.” The Danish data obviously do not reflect semen quality in earlier years, but it is a puzzling coincidence that a decline in sperm counts would end exactly at the time when a proper monitoring program is initiated.

Now the story behind this data is frankly weird. The data wasn’t released by Dr. Niels Erik Skakkebaek of the University of Copenhagen, who initiated the study; he hasn’t commented yet on the results, pending publication in a scientific journal. Instead, the research group’s current leader, Niels Jorgensen, sent the data to the Danish Ministry of Health, which helped fund the study. The journal Epidemiology then got the data off the health ministry’s website and published it in a commentary.

(More on TIME.com: Environmental Toxins Cost Billions in Childhood Disease)

Obviously this isn’t the ideal way for potentially explosive data about a very hot topic to make its way into the public realm. The Danish study, while reassuring, is far from conclusive. Again from Epidemiology:

Recent developments in the sperm-count story emphasize the continuing need for good prospective data — not only of semen quality and reproductive hormones, but also of indicators of female fecundity and couple fecundity, such as time to pregnancy. In searching for possible etiologies of reproductive disorders, we need to pay attention not just to environmental toxicants, but to the wide range of behavioral, medical and other factors that have potential to damage human reproduction.

The dystopia hasn’t quite been averted yet.

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