Are humans running out of sperm?
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It's Unexplainable. I'm Noam Hassenfeld. And this week on the show, we're talking about sperm. Because according to a whole lot of media outlets, we're in the middle of a sperm crisis. On tonight's show, the world is running out of sperm. What's going on? Well, here's part of the answer. Over the past four decades, sperm counts in men in the West have declined by more than 50%, and they're still dropping. In 2017, some researchers claimed that sperm counts might be falling.

and falling pretty quickly. - And what are the other reasons for this steep decline in sperm concentration and sperm count? - Chemicals in plastic that interfere with our body's natural hormones. - Smoking nicotine, tobacco, other recreational drugs are bad. - Do you keep a cell phone in your pocket? Use a laptop computer on your lap?

Both can possibly raise testicular temperatures enough to hurt sperm production. It's feared that the planet could be facing what scientists are calling a "spermageddon" by the year 2045. If it carries on at its current rate, humans could end up extinct. The whole thing sounds pretty scary. So our reporter Bird Pinkerton dug into "spermageddon" to find out just how worried we should be.

But the more she looked into this crisis, the more questions she had about the story of disappearing sperm and about the science of sperm itself. Spermageddon did not appear out of nowhere. It has a long, wiggly historical tale. So in the 1970s, there were

a bunch of studies that were very concerned about men's sperm dropping in numbers and quality. This is Rachel Gross. She's a friend of mine, but she's also a health and science reporter who has spent lots of time digging into questions about reproductive health. I love all things eggs and sperm. Rachel says that back in the 60s, 1970s, sperm banks were pretty new. Cultural ideas about sex were changing.

And so scientists were asking questions about how healthy sperm was and how they could actually measure that.

And they came up with methods for measuring sperm health that are still used today. We really look at three things, shape, movement, and number. So scientists would get a rough count of sperm in a sample, and then for shape and movement, they would look for stuff like... Are they normally shaped? Do they have a head and a tail? Are they swimming along fine and not just kind of wriggling in circles? And they thought that these traits could teach us about how good or bad sperm are at fertilizing an egg.

So for decades, researchers were taking these kinds of measurements, they were publishing them. But the studies they were doing had some pretty significant flaws.

They were very inconsistent in who they were looking at. You might be looking at men presenting at fertility clinics, so those would be men that already have infertility issues. A lot of studies didn't control for men's ages. And there are other factors that impact sperm number and quality, like has a guy masturbated in the past week? That actually has a big effect, and if you're not controlling for it, then you're going to get numbers that are all over the place.

And so in the late 2010s, a bunch of researchers decided to kind of zoom out a bit and do what is called a meta-analysis.

which is basically a reanalysis of many different studies that tries to control for factors that might be different and get rid of any not-so-great data. The authors were not collecting new data, but instead they took data from 40 years' worth of studies, studies from all around the world, and looked at sperm counts from over 40,000 people.

And they did their best to kind of correct for anything that would skew the data one way or another.

So the 2017 study just cut out any men that were presenting for fertility problems or that had known testicular issues. And they tried to control for age, which is very important. And finally, in 2017, when they were done with all their analysis, they published their big finding, which was over time, on average, in some Western countries, sperm count was going down. Sperm is on the decline.

It doesn't look like it's stopping anytime soon, and we might run out of sperm. This 2017 meta-analysis is what kicked off the most recent round of Spermageddon. And after it was published, one of the co-authors, she's an epidemiologist named Shauna Swan, she wrote a book called Countdown, where she said, quote,

quote, the current state of reproductive affairs can't continue much longer without threatening human survival. Here she is on the TV outlet Deutsche Welle. If you were to project the decline that we're seeing, which is the rate of more than 1% a year, you can extrapolate from 2011 and see that by 2045, you would get very close to no sperm at all. She clarified that the decline in sperm would probably level off at some point, but she

She claimed that the sperm levels would eventually be very low. And this argument got picked up by everyone from The Guardian to the Joe Rogan podcast to commenters in the Reddit red pill community. Okay, so my favorite comment was, TLDR, feminism is creating low sperm counts. Rachel spent a whole bunch of time reading through these Reddit threads. So the general idea is that

feminism, second wave feminism, started taking hold around the 1970s in America and some other Western countries. And it's kind of, according to these men, done away with like rigid gender roles with traditional masculinity. And

That impacts testosterone and sperm levels, according to them. So they actually think that the advent of feminism has created, quote unquote, betas and, quote, betas don't produce sperm. So I actually asked Dr. Swan to talk to me about the paper and the response to it, but she declined. She said that she didn't want to discuss what she called a manufactured controversy, which

But as this paper was kind of making the rounds, getting a lot of attention, causing some panic, other sperm researchers started to look into it. And one of those researchers is Alan Pacey. He's a scientist at the University of Sheffield. He's been studying sperm for 30 years. And just to be clear from the start, like, Alan thinks it's totally possible that sperm counts are on the decline. I'm not a denier.

What I am concerned about is whether the evidence presented to date actually supports that hypothesis. And in my head, it doesn't.

For Allen, the problem with making big claims about running out of sperm or running out of sperm by 2045 is just how little scientists currently know about sperm. He explained this to me by making kind of a comparison. He said Western scientists discovered sperm around the same time as they discovered Saturn in the 1600s. They've had the same amount of time to research both.

But even though Saturn is many millions of miles away... I would argue that we probably know more about Saturn than we do about sperm.

That lack of knowledge creates sort of two broad categories of problems with the 2017 meta-analysis. There's the data in the study itself, and then how that data is being interpreted. But let's start with just the data. The 2017 paper looked at data from just one method of counting sperm. It's sort of the method that's considered the sperm counting gold standard, so that's good.

But it still has some problems. The gold standard method for counting sperm is to use a device called a hemocytometer. Researchers will take a sample with sperm in it, and they'll put it between two pieces of glass, one of which has sort of a grid etched into it. And then you wait a while, 15 minutes or so, and then the sperm will settle to the bottom of the grid.

and you can then look down a microscope and count the sperm. All this might sound super straightforward, but in Alan's experience, the reality is much more complex. He actually taught his master's students how hard it really is by giving them some fluid and asking them to measure the sperm in it.

The results were that you could get a number from 10 million sperm to 100 million sperm from a different person in the same class. Allen's students were looking at fluid from the same sample, but some of them were getting results that were 10 times bigger than others. These were untrained individuals who hadn't yet appreciated the subtlety of what they needed to do.

So with training, we can get people to count sperm within a 10% margin of error. And that's about as good as we can get, a 10% margin of error. Alan has been introducing this kind of training and quality control over the last three decades.

But that's why he's suspicious of data that was collected 30 years ago or even earlier in the 1970s when the earliest studies that were included in this meta-analysis were being done. And again, for that reason, I don't find the study convincing because it has no way of accounting for that type of variation.

Shauna Swan told me in an email that she had not seen any evidence suggesting that changes in quality control methods or training would create a systematic bias in the data that she and her colleagues had used. And other researchers that I reach out to, they said that no data set is completely perfect, which is fair. But still, like, before we get too worried that humanity's sperm is sort of running low, it might be worth collecting a better data set.

And the way to do that would be to do a prospective study where you set out to look at sperm quality in men year after year after year after year. And it wouldn't take longer than 10 or 15 years for us to know whether or not this was a real change or not. But let's say we do accept that sperm count has declined. Even then, we might not actually have a reason to panic because we don't fully know what declining sperm counts mean for reproductive health.

The Mayo Clinic says that a normal sperm count is between 15 and 300 million sperm for every fluid milliliter. And above that line, as far as scientists currently understand, it doesn't really matter if you're at like 40 or 200, your fertility is fine. What I see from the

sperm count decline data is a change from normal to normal. And there isn't any consensus out there that having significantly more sperm is better or that having less sperm is worse. It's really difficult, I think, to extrapolate

along a line to say that an X percent reduction in sperm quality will have a Y percent reduction in fertility. I did talk to one researcher who pointed to different animal models that do indicate that lower sperm could mean less fertility. But then talking to Alan again... We know from other studies that animal data isn't always representative of the human condition. So I do worry when I hear people say, but the animal data shows this.

It's possible that if sperm counts are declining, they could keep declining, right? They could dip below the normal range. But that's based on the assumption that trend lines are just going to continue in the exact same direction for a long time. And in a journal that Allen edits, a group of researchers proposed kind of an alternative idea that sperm counts simply vary a lot over time and region. So they could go up, they could

up, they could go down, but there's no reason to assume that they're going to go outside the range of normal. And Alan isn't necessarily saying that that's the right interpretation, but both he and Rachel, the reproductive health reporter that we heard from earlier, they say that we just don't know enough yet about sperm to be sure. We don't know if more is better. We don't really know the threshold beneath which less is worse.

And we don't know these things partly because we don't even know what sperm is up to when it's inside the body because we study it in cups. When a sperm is doing what a sperm does, it isn't ejaculated into a cup. It's ejaculated into the vagina close to the cervix and then sperm go on their merry way through the cervix and through the woman's body. But we have no way to study that.

So, yeah, it's possible that sperm counts are declining. It's even possible that we should be concerned. But before we declare sort of an official spermageddon, we just need a much better understanding of sperm. Because right now, a lot of roads seem to lead to one conclusion. We don't know a lot about sperm. But that raises a different question, which is...

Why? Like, why do we know so little about sperm? We spend huge amounts of money on Viagra research, other research into arousal. So why is it that we know so little about the byproduct of that arousal? More on that after the break. Support for Unexplainable comes from Greenlight. People with kids tell me time moves a lot faster. Before you know it, your kid is all grown up. They've got their own credit card.

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That's right, Dad. Mothers do bring babies into the world. But sooner than you may think now, your child will be old enough for a full explanation. So let's brush up on it and together review the human reproductive system.

Unexplainable. We're back. I'm Bird Pinkerton. And we've got kind of a weird mystery on our hands here, which is maybe human sperm is on the decline. Maybe it's not. Even if it is, it's not clear whether a decrease in sperm count would mean that people are becoming less fertile. We just don't know very much about sperm.

But as Rachel Gross puts it, it's kind of ironic that scientists know so little about sperm because they have been kind of obsessed with them for centuries. So my favorite anecdote, which feel free to cut out, goes back to this Dutch scientist, Anthony Leeuwenhoek.

Looks at everything he can possibly think of under the lens. So like dirt, fingernails, bugs, pond water. And finally, he gets around to semen and he looks at his own semen and he says, listen, I looked at my semen, but don't worry, I did not obtain it by sinful, defiling semen.

and masturbation, I had normal sex with my wife, and I looked at this droplet and I saw millions of tiny animals swimming in it. So he thought that each sperm contains the whole of life just waiting to unfold in the woman. And this became a very prominent idea that you have sperm as the seed and the female, the uterus, is the soil.

So if scientists in the 1600s saw sperm as like the core element of baby making, how are there still so many mysteries about like the basics here? Like how do we not know how sperm actually act in a body? Rachel says there isn't like a single perfect answer here necessarily, but based on her research, she sort of has two main theories.

And the first theory has to do with infertility. So historically, Rachel says that, yes, sperm were given kind of the lion's share of the credit when it came to laying out the blueprints for what a child might look like or what they might act like. But the minute the questions about infertility came up, suddenly sperm were nowhere to be found. When that happened, usually it was the woman's fault.

So, infertility was a woman's problem. It was a woman's business. And even today, when we think of reproductive health, we think of women. And this bias was actually built into the academic disciplines that were established in the 1800s. You may have noticed that we have gynecology. We do not have male gynecology. We only have urology. And urology is mostly focused on the urinary tract as opposed to sort of broader men's reproductive health.

Women's bodies ended up being the ones that got medicalized. They were considered the main reproductive vessels. They have the uterus and the plumbing. And this whole specialty and infrastructure evolved around them of doctors and surgeons who took over for midwives and were intent on figuring out what was behind infertility.

And this trend of sort of doctors and researchers looking for sources of infertility in female bodies, it continued for decades. So Rachel says this happened throughout the early and the mid-20th century.

And things did begin to change a little bit in the 1970s as we started to see sort of more andrologists. These are people who study male-specific diseases. But there was still this bias towards tracking infertility in women's bodies. I talked to a reproductive epidemiologist who said 20 years ago, you couldn't get government-funded studies to include men in fertility studies. Most researchers didn't believe men would participate, which...

often happened. So when they had to track their behavior and hormone fluctuations, they assumed that men wouldn't fill out the journal and women would. So they kept looking at women. And she said that is changing. So nowadays, it's obvious that if you're looking at infertility, you have to look at both sides of the couple. But somehow this super obvious thing was only recently implemented.

So overall, Rachel says that science just hasn't spent enough time or money studying sperm and its relationship to fertility. And that is one theory about why we might not know that much about sperm. But when she started poking around, she also found a second potential culprit. Just before midnight last night in a hospital in Lancashire, England, a baby girl was born and made medical history. She was the world's first authenticated test tube baby conceived not in her mother's body, but in a laboratory.

So in the 70s and 80s, we got a very important advancement in reproductive science, which was IVF. In vitro fertilization. Being able to unite an egg and sperm outside the body. That was really a revolutionary technology. Basically, anything that was wrong with...

the fallopian tubes or with the sperm or with them getting together could just be bypassed. She's trying a different route to get her eggs and her husband's sperm together.

So all these urgent problems about how do we figure out if someone has low-quality sperm or not enough sperm and how do we fix it kind of became irrelevant. IVF is definitely not some sort of magic cure-all for fertility. Like, many couples go through emotional, expensive rounds of IVF and they don't get pregnant. But because IVF sort of bypasses the step of the sperm needing to reach the egg,

It makes it a little less urgent for researchers to understand that process or to understand exactly how much sperm is needed or how active it should be. I was told by men's health experts and urologists that IVF was a really big reason that we have not explored sperm to its full potential.

So those are sort of two possible explanations for why we don't know enough about sperm to sort of concretely say whether or not we're running out of it. One, researchers historically focused on the woman's role in infertility. And two, IVF just sort of made studying sperm's journey through the body a little less urgent or pressing.

But while these answers are helpful, they don't fully solve the question here. Yeah, some of the answers I got were really frustrating and unsatisfying. I was like, that doesn't make sense.

Because, I mean, like, clearly there's an appetite for information about sperm, right? Like, that's why we have this whole spermageddon crisis in the first place. Like, when researchers have studied sperm count or sperm quality, even if the studies were incomplete, like, they have gotten a lot of attention and a lot of headlines. So if there's a hunger for answers...

Why hasn't more been done to sort of collect really good data on sperm? I think everybody loves a good doomsday story. Alan Pacey, the sperm scientist, agrees. As humans, we get drawn to a crisis and we get drawn to a decline. And that's

kind of natural, like it would seem in our psyche, in that steady state is not as interesting to us as a concept. And I think if you look around the world right now, you see we live in a global pandemic. There's inequality everywhere. We are ruining the planet. We've caused climate change. It looks hopeless. I'm not saying people want it to be hopeless, but that is our prevailing narrative. And this study fits right in.

So maybe, and again this is just a maybe, but

Maybe we don't have the research to definitively prove that spermageddon is happening because it's convenient to just assume that it is happening. I can see how people in my profession who don't have my perspective or just people out there in the general world love a crisis and love the fact that sperm counts may be declining and then you get a kind of self-reinforcement and a perpetuation of the hypothesis

because of human nature. I like this thought because I do think there's something very human about running around yelling "the sperm count is falling! The sperm count is falling!" Maybe we just need to rephrase things here. Like maybe we should run around and yell "is the sperm count falling? And what would it mean if it were?" Or like "we don't know very basic things about sperm! What's going on with sperm?"

That's not quite as catchy as a phrase like "spermageddon." I don't know, it might lead us to discoveries about ourselves and our bodies that are ultimately even more exciting. This episode was edited by Kathryn Wells with additional editing from Brian Resnick and Noam Hasenfeld. Noam also wrote the music.

It was produced by me, Bird Pinkerton, Richard Zima, check the facts. Christian Ayala was on mixing and sound design. Meredith Hodnot is the glue that holds us all together. Manning Nguyen is a delight who has been harvesting seaweed lately. And Liz Kelly Nelson is the VP of Vox Audio.

If you want to read more about sperm, eggs, sexual reproduction, how to get a sea urchin aroused, I really recommend Rachel Gross's upcoming book, Vagina Obscura, An Anatomical Voyage. Thank you so much to her for her reporting and to Deborah O'Brien, Dolores Jean Lamb, Renee Allmaling, Sarah Richardson, and Andrea Gore for their time and their knowledge.

If you have thoughts about Spermageddon or ideas for the show, please email us. We are at unexplainable at Vox.com. And I personally love reading through our inbox because you're all just really creative and interesting people. Unexplainable is part of the Vox Media Podcast Network. And we'll be back next week.