What's the Deal with Declining Sperm Counts?

Emily: [00:00:00] Hey, Perry.

Perry: [00:00:01] Hey, Emily.

Emily: [00:00:03] So, Perry, when you were in medical school studying the kidney and other body parts, did you ever imagine that people would get so much of their medical information from TikTok?

Perry: [00:00:14] No, I did not. Um, but of course, full disclosure TikTok did not exist when I was in medical school. Um, but broadly, social media was out there, but it was like people sharing recipes and stuff. It was not how people got medical information. And it's a completely different world now. It is an influencer's world, and we're just living in it.

Emily: [00:00:36] It's a little bleak.

Perry: [00:00:38] I think. It is bleak. And you're more social media savvy than I am, Emily. Like, I'm just sort of getting into this space, and, um, it's very eye opening.

Emily: [00:00:50] I think the high for me recently was a whole social media era in which people were going on to talk about how if you feed your baby butter. It makes them sleep through the night, which was such a great example. You probably haven't gotten this.

Perry: [00:01:04] I have not gotten that one yet. My my algorithm is turning pretty weird, but not that weird.

Emily: [00:01:10] And what was interesting about that, what's interesting about this whole space is I think it's so hard for people watching that to know, is this real? People will say, oh, evidence says studies show, but they don't have any citations because it's TikTok or Instagram and it's not the New England Journal of Medicine.

Perry: [00:01:27] That was.

Perry: [00:01:28] 100%. As I start, you know, I was on Instagram and stuff and my feed before we started this podcast was like, you know, like improve your golf swing and, you know, here's a great bread recipe and then started doing research. And all of a sudden, now my feed is like this wellness influencer style feed, which I'm sure you're familiar with. And the thing that stuck with me as I was watching this, as someone who comes from academics is like, oh, you can just say like, there's no there, there's no peer review. You can just say whatever you want. And what I kept coming back to, especially with things like that that sort of seem ridiculous, you know, was like, wait, does this person really believe what they're saying? And they're a true believer in this and they are like a butter advocate, or do they not believe what they're saying and they're just saying it for the clicks, or they're grifting and they want to sell you their butter supplement. And like, it's really hard to tell the difference. And I don't even know which one is worse.

Emily: [00:02:26] I agree. I mean, I think often, you know, it's, you know, comment supplement for my 12 step routine for clearer skin where, you know, well, probably they're they're selling you something. But it's really challenging when people watch this kind of content to figure out what's real, what's not real. And, you know, for me, I always think even if they're referencing studies, a lot of times those studies confuse correlation and causation. You will hear me say that a million times on this podcast, one of my life's passions.

Perry: [00:02:58] It's going to be on your tombstone.

Emily: [00:03:00] One of my life's passions is the confusion of correlation and causation, by which I mean that a lot of times the people who are doing one health behavior are really different in many other ways. So sure, they're taking a vitamin D supplement, but they're also exercising and not smoking and doing all kinds of other stuff. And you don't really know that it's the supplement as opposed to all the other things. And we'll talk about that a lot.

Perry: [00:03:25] The type of person who takes the supplement. Yeah, I get it.

Emily: [00:03:28] But for me, that's one of the core confusions. Even if we are taking people at face value and assuming they're not trying to sell me supplements, comment in the comments.

Perry: [00:03:37] That is. That is why I was so excited when we decided to do this podcast that we could look at some of these claims. And actually, you know, you and I are both people who can read a medical study, like who know how to go into the literature and.

Emily: [00:03:51] Our job.

Perry: [00:03:52] Figure that's what we do. That's our day jobs. Right. And like figure out what's true. Figure out what's not true. And tell people because some of the stuff out. It's not all fake, right? Some of the stuff out there really can improve your health and some of it can't. But we're all super busy. And if you don't know which is which, you're going to, like, spend 29 hours a day, you know, juggling eight different supplements and shining red light therapy on your face.

Emily: [00:04:16] Absolutely. I think people need to understand both. Will this make a difference and how big is that difference? So I can decide what to spend my very limited amount of time on.

Perry: [00:04:26] And selfishly, I will say that between the two of us, even though I'm the medical doctor and you're just whatever other kind of doctor, uh, you are the healthier one of the two. So, selfishly, um, I am excited to learn how to take care of myself even a little bit better.

Emily: [00:04:41] I'm going to teach you so much. Perry. Okay, so, uh, this is our first episode. Uh, what do we have on the menu for today?

Perry: [00:04:49] I wish you hadn't phrased it like that because it's sperm.

Emily: [00:04:53] Amazing. I'm Emily Oster, I'm an economist and a data expert.

Perry: [00:05:00] And I'm Perry Wilson. I'm a medical doctor.

Emily: [00:05:02] It's Thursday, February 12th, 2026. And this is Wellness, Actually.

Perry: [00:05:08] Because you're getting a staggering amount of health and wellness information nowadays from every source imaginable. And some of it is awesome.

Emily: [00:05:16] And some of it is, well, actually, bulls--t. Fortunately, we're both people who know how to read studies, how to parse the data, and can tell you what's worth thinking about and what you can safely ignore.

Perry: [00:05:29] But before we dig in a note that this podcast is for educational purposes and should not be construed as medical advice. We don't know your unique situation, so talk to your doctor for personal health decisions.

Emily: [00:05:40] This week we're asking what's the deal with declining sperm counts? And then we'll get to your question of the week. But first let's do the health news roundup after the break. All right. Perry. So last week, RFK Jr, the secretary of Health and Human Services, claimed that a ketogenic diet could cure schizophrenia. He said the quote is a doctor at Harvard had cured schizophrenia using the keto diet, which is a high fat, low carbohydrate diet, and also claimed that there were studies showing that people lose their bipolar diagnosis by changing their diet. So how much of this is rooted in fact, as opposed to just in nothing?

Perry: [00:06:32] So this is really typical of Rfk's style, which is like there is a kernel of fact in here that then gets just like expounded to 100 fold higher, which is fine if you're maybe an influencer on TikTok, but it's not great if you're the secretary of Health and Human Services. So Ketogenic diets. And what they're talking about here is not like I'm keto. Like, I, you know, I try to avoid carbs. A ketogenic diet is really hard to do. Like it's it's you have special foods and stuff. You are in the state of ketosis, and we do use it to prevent seizures in people who have refractory epilepsy that no drugs work, so it does affect the brain. There was a small study which showed a ketogenic diet had modest effects on schizophrenia symptoms in a subset of people with schizophrenia. The author of that study was asked about RFK Jr's quote and said themselves, no, this is not this is not a cure. I mean, it's clearly not. It's interesting. Um, but this is just, you know, it speaks to this thing that keeps happening where it's like your medical problems are kind of your fault. There's always this subtext that's like, oh, well, if you only ate a better diet, maybe you wouldn't have schizophrenia. And it's just it's really like disabling to people with this kind of condition.

Emily: [00:07:54] I think the other piece for me here was this, this sort of taking of what should be the first step in a kind of research program and deciding that we're done. Right. The process of science. I mean, you know, this as a researcher, like the process of science is like, okay, we do an experiment with like two people and we see, huh. Like, maybe there's something interesting there and maybe there is something interesting here, but then you sort of expand it out and think about what would be feasible. Could it work on a larger scale? How big are the effects? And this is kind of taking that first step and being like, oh, we're done. We fixed it. Like just eat a lot of bacon or.

Perry: [00:08:32] Yeah, absolutely.

Emily: [00:08:33] And that's not a that's not the solution.

Perry: [00:08:35] Absolutely. Uh, and the history of these interventions across all of medicine is typically that something that works in a small group or in lab animals or something is kind of interesting. Doesn't scale up. Like it doesn't work. The literature is littered with failed interventions like this. So irresponsible to say. But let's move on. Uh, Emily, did you see this article in the New York Times reporting that there has been an increase in refusals of the vitamin K shot after birth. So what is going on with vitamin K now?

Emily: [00:09:10] So it's worth saying why we might give kids vitamin K shot right after birth, which is that it prevents bleeding. So infants who do not have vitamin K shot are at a much higher risk for later bleeding, which can be fatal. And so there are really good reasons to give infants a vitamin K shot immediately after birth. It has been done for many decades. It's extremely safe. But in our current vaccine hesitancy environment, this shot, like many others, has become sort of part of the kind of we're afraid of shots Thing and more parents are refusing it, which of course is very dangerous for their infants.

Perry: [00:09:52] But not a vaccine, right? I mean, just let's be clear.

Emily: [00:09:55] It's not a vaccine. It's a vitamin shot. Exactly. Yeah. And that's what's I mean, that's sort of amazing because people are thinking, okay, this is like a vaccine. No, it's a it's a vitamin shot. It's a vitamin.

Perry: [00:10:05] And yet we like vitamins.

Emily: [00:10:08] We like vitamins. I mean, the same. What's so interesting is, of course, the same space. They love vitamins that are useless. This is a vitamin that's very helpful. And yet people are refusing it. And I think it's just an example of where some of this vaccine hesitancy has spilled over, even into things that are not vaccines and in this case have no side effects and have very clear, obvious benefits.

Perry: [00:10:32] Right. There's obviously an element of mistrust of medicine here that's taking place. There's also this movement towards kind of, you know, a more natural experience, right. Like we didn't you know, our ancestors didn't give their kids vitamin K shots. But I will point out, as I often do, that things were really way worse in the good old days. So prior to 1900, infant mortality was 25% in the United States. So that's death before age one. One out of every four kids. Death before age five was about 50%. So that is not something we want to go back to. And the thing that has changed infant mortality rates so dramatically are interventions like this kind of, you know, straightforward dumb interventions like, oh, babies are deficient in vitamin K and they have, you know, hemorrhage in their brain. And let's give them vitamin K.

Emily: [00:11:20] Yeah. So please, if you're having an infant get the vitamin K shot. Okay. So let's turn to a I would say arguably more important topic than schizophrenia and infants which is ski jumping. Yeah. So it's the Olympics. I love the Olympics. I like the endurance sports, but I also like ski jumping. And there's a report of some mechanical doping in ski jumping, which is working like the following.

Perry: [00:11:49] Oh, I know where you're going with this.

Emily: [00:11:50] Apparently, in ski jumping, you want your suit to be very big because if your suit was, like, really loose, it would be like a sail. So when before you ski jump, they take a scan of you in your suit to make sure that the suit is not too far from your body. So there's a report. It is, admittedly from a German tabloid that some male ski jumpers are injecting their penis with acid so their penis will grow, and then the 3D scanner will make it seem like they're filling up the suit more. And so they'll be able to have a bigger suit. But then the penis goes back down before the ski jumping so they get the sail like effect. I just want to know, will this work increase your penis size? And would it have other uses beyond ski jumping that we should be considering?

Perry: [00:12:33] Um, I saw this article too, and, um. Yeah, real hard for me to read. I have to say, um, I think they, they, they really dropped the.

Emily: [00:12:43] Because you feel like you missed out on some opportunities.

Perry: [00:12:45] Yeah. You know, I could have been a I could have been a champion ski jumper. I'll let you draw your own conclusions, but, um, but no, I mean, they missed the opportunity to call this inflate gate, first of all, which is just journalistic malpractice. Um, I think if these ski jumpers have the balls to inject their penis, uh, you know, more power to them. Just to answer the medical side of this, uh, hyaluronic acid, which is what they're talking about injecting, like. Yes, it does swell tissues. I mean, so yes, it will make something bigger, transiently. It then gets absorbed by the body. There's some bodybuilders who have been accused of doing this and their pecs and stuff to make them more swole. This is a bad idea. Um, you do not want an infection in your penis or around your penis. It doesn't turn out well. If you want to never sleep again, Google Fournier's gangrene. Uh, and Don't. You will see what can happen if you do something like this. So just, uh, you know, fly through the air with your regular penis, please.

Emily: [00:13:50] I think that's a good place to stop with our health news. Uh, but after the break, we'll come back and ask, what's the deal with declining sperm counts?

Emily: [00:14:03] Okay, Perry, so if your social media feed is like mine, you are probably hearing a lot about declining sperm counts in a very panicked way. So one of the things I keep hearing is like, if we are going in this direction by 2035, no one will have any sperm. And I keep thinking about this graphic novel that my husband liked called Why The Last Man, which is about a world in which there's just one guy who has sperm and all the ladies are, like, chasing him. Did you?

Perry: [00:14:33] It's classic wish fulfillment. Graphic novel.

Emily: [00:14:37] Exactly. So uh, I want to talk about whether this concern about our declining sperm is a real concern, or just a kind of imaginary panic sperm problem, like many other things. So we're going to talk about that today. But before we get into it, I think we should make sure everybody understands what sperm are, where they come from and how we would know how many we have and how many should you have? Give me a doctor. One on, one on sperm.

Perry: [00:15:07] Okay. All right. Medical school, 101 on sperm. Here we go. So sperm are the male unit of fertilization. To carry sperm carries half of your genetic information. The egg has the other half you combine, and you make a baby. Most people know this. Sperm are produced in the testes, and it's not a simple process. There are stem cells in the testes that go through multiple cycles of generation. There's multiple sort of stages of evolution of sperm in the testes, like some kind of sexual Pokemon. Um, it takes about 90 days, between 80 and 100, to go from that stem cell to a fully functional sperm that is ready to go, so to speak. Uh, testes are busy little guys. They are producing 200 to 300 million sperm cells a day. That's a lot. Um, that being said, only half are viable. Uh, in your in your typical man. So, you know, you produce anything at that rate, you're going to get some flaws. Um, so only half are going to be good enough to sort of make it all the way out. And that's where we get into sperm counts.

Emily: [00:16:14] Can I ask, uh, when you make sperm that are not used, obviously, if you're making 300 million sperm a day, is that right?

Perry: [00:16:22] A day? Yeah. They got to go somewhere. Right.

Emily: [00:16:24] Okay. They got to go somewhere. Like what? What happens? Are you peeing them out? Do they get eaten? What happens?

Perry: [00:16:30] They kind of get eaten. They just get reabsorbed. So? So they they don't have an unlimited lifespan. They'll just kind of die. And like any dead cell in your body, once a cell dies and breaks down these big, uh, white blood cells called macrophages, which are just like big cells that are specialized in eating dead stuff, come in. They live in the testes, and they eat them up and they dissolve them and all the nutrients and stuff go back into your body. So they just get recycled, basically. Yeah, no big deal. All right.

Emily: [00:16:57] No big deal. So you have these sperm. Some of them are good. Some of them are not good. When we say that we have good sperm. And I think this is relevant for talking about declining sperm counts, because really what we mean is a decline in viable sperm. But what are the things you need for a sperm to be capable of fertilization, which is its primary measure of quality?

Perry: [00:17:17] Yeah, I mean, it's a lot more than just sheer numbers, I think, as you're alluding to. Right. So there's all this social media talk about sperm counts and declining sperm counts. That's only one parameter in about ten that urologists, for example, will look at to tell you what your sperm quality is. So yes, you need a certain number. We'll get into what kind of normal numbers are in just a second, but they have to be able to move sperm swim. So they have to they have to be motile. They have to move around. You can see that under a microscope. They have to have a sort of standard morphology, that tadpole ish thing that you look at. Some sperm have two heads, some sperm have 2 or 3 tails. And those are not going to be efficient at getting into an egg. They have to contain the nuclear payload of half of your genetic material. If they don't, you're not going to get a viable embryo. So they have a lot to do. And it's you know, it's a numbers game. Basically. In the end, it's a pretty difficult trek for a sperm to make it all the way to an egg. Um, it's a hostile environment. Uh, for very good reason. The the weak are weeded out on this trek to the egg. Um, we were all traumatized as young children by watching Look Who's Talking, where we saw graphic depictions of sperm competing with each other to get to the egg. Um, and, uh, and but that's, you know, that's how biology is supposed to work. Uh, but sperm count is certainly one part of all of this.

Emily: [00:18:43] And I also think when we talk, when people talk about declining sperm counts, I think they they mean both counts. But they also often mean some of these other things. So a decline in sperm that are in decline in the number of sperm, a decline in the number of sperm with the right morphology, with the right motility. So sperm they can that have the right shape, they can swim, they can get to the egg. That's what we're that's what we're focused on. That's what we're what we're worried about.

Perry: [00:19:07] Right. Um, now, when it comes to count, though, because it's always nice to put like a simple number on things, um, there's essentially two counts that get talked about in different studies. One is actually the sperm concentration, which is the count per milliliter of fluid basically. So how much per milliliter. And then one is the total count. So if you take all the milliliters in a particular sample how many sperm are in there. Most researchers are focusing more on that concentration, the count per milliliter as as a better proxy of overall sperm health. And just to give you some numbers, most men will have, you know, who are healthy and fertile, have numbers in the sort of 60 to 120 million range per milliliter. The W.H.O. defines any sperm count above 16 million per milliliter as normal. They used to say 20. That's been reduced. And Emily, you're going to tell me in a second why or if that's for real. But, you know, most most people are actually quite a bit above that minimum threshold. Nevertheless, lots of panic about declines in the count over time. So, Emily, you tell me, what's the deal with this? Like what does the data show, and is it for real?

Emily: [00:20:23] I think what's most interesting to me here is why this is so hard to answer. So if you sort of think about what you would like to be able to see, to have a measure of sperm counts over time. Really, we would like to sort of take every, you know, person with testicles when they are 18, bring them in, you know, have them jack off into a cup, measure their sperm. And then we could have if we had that starting in like 1920, you know, we could really get a very good sense of how sperm has changed over time because you'd have a random sample or you'd have everybody, even just a random sample. I would take a random sample.

Perry: [00:20:59] Yes, yes. As any good economist would, I will point out that, uh, the time of abstinence is an important factor in sperm count. So you would need these 18 year olds to hold off on any sort of ejaculation for five days, which I think is really the hurdle that people aren't discussing. But go ahead. That's the ideal.

Emily: [00:21:21] My study is going to totally take care of that. I have different ideas, but at any rate, whether I would be able to do that or not, we have not. And so most of our numbers about sperm counts come from selected populations without very good controls. So often we'll see data from infertile men. Well, that's not very it's not very helpful as a metric over time for a bunch of different reasons. Sure. So we're kind of working with a lot of pretty incomplete data, which is why I think the the results are quite varied. So I would say if you look at meta analyses where they bring together a lot of different studies of sperm counts over time in a global context, mostly those suggest some moderate decline in sperm counts over time. I think the the sort of most recent data maybe is a little more reassuring and suggests maybe it hasn't declined very much. But I guess my read of the evidence I'm curious if you read it differently, is it does seem like there's some decline, but it's probably not as extreme as you would take away from some of the, like, hyperactive claims on the internet.

Perry: [00:22:35] Yeah, the sort of catastrophizing of the sperm count. Yeah. I mean, uh, the one nice meta analysis actually broke down this issue you're referring to, which is like selection bias for men who are having fertility problems. Right? Or couples that are having fertility problems. So overall, this study found that the average sperm count in men, sperm concentration in men from uh, 1973 to 2018 went from 100 million to about 50 million. And that's like okay. All right. So that's, you know, almost half that's pretty solid.

Emily: [00:23:09] That's a solid decline. It's a that's a decline. That's a very good big decline.

Perry: [00:23:13] But when they restricted to the studies that only included fertile men. So you know sperm counts for other reasons. The decline was from 77 million to 72 million, which is like really not that much. Now. Maybe there's more infertile men, right? Like, it doesn't necessarily mean that this is just total hokum, but you got to think it through it a little bit more in a little bit more detail. Right?

Emily: [00:23:38] Yeah. And I think that that, you know, you're citing sort of one particular study and I think one that got a lot of attention. But there are other meta analyses with sort of similar approaches that do more adjustment for, say, what area you're in or, or some of these fertility metrics which don't actually see sort of significant or see much, much smaller declines. So, you know, I love data so much. I love numbers and data. And in a case like this, I think what's so hard is because the data is imperfect, it matters very much what choices you are making about exactly who you're looking at, exactly how you're looking at it, exactly what you're adjusting for. And so it's not that surprising that the literature can kind of find Everything from there's no change at all to like, you know, the sperm counts down by half. And if we keep going on this trend, you know, we're we're all doomed forever.

Perry: [00:24:36] But I'm going to force you to put your nickel down here. You've read a lot about this. I've read a lot about this. Yes. Data is a little bit, you know, wishy washy here and there, but like, heart to hearts. Do you think there's a decline in sperm counts overall over time?

Emily: [00:24:50] I do, I mean I my, my read is that there's been a modest decline in sperm counts over the past, you know, 50 years. What about you?

Perry: [00:24:57] I think that's probably right. That was the vibe I'm getting to.

Emily: [00:25:00] Yeah, I think that there's enough data that points in that direction. That's an interesting thing about how we sort of look at data. Like you kind of read enough of this and you read it and you read it and you sort of get a picture. It's a little hard to put your finger on. It's not that I'm saying, like I looked at this one study, it's that when you look at all of it together and, and kind of evaluate basically with the background of being a person who thinks a lot about evidence, you kind of come out with something. So yeah, for me, modest decline. All right. So a very reasonable follow up question if we both think there's been some modest decline, is is it relevant? Right. So we talked about like the W.H.O. has got a threshold of 16 or 20. We talk about numbers like 75 million per, you know, semen unit. You really only only need one. You only need one.

Perry: [00:25:51] Yeah yeah.

Emily: [00:25:52] Yeah. So I guess the question is, is this clinically relevant? Is this the reason we're talking about a fertility decline in, you know, the entire world? Like, okay.

Perry: [00:26:02] Yeah. A lot to unpack on this, but I'll start off by saying absolutely not. This is not the reason for a fertility decline across the world. There are many more important factors in that, including deliberate choices by people who are deliberate choices, real family planning, uh, and of course, economic conditions that might influence those choices and many other things. So let's just put that aside. Let's talk physiology. We actually have pretty good data from in vitro fertilization studies to know how many sperm it typically takes to fertilize an egg. And yes, it only takes one. But as I said, they're running a gauntlet, uh, to get there. It's very it's very hard. And what these studies show is sort of a, a diminishing returns as sperm count increases. So at the very low end, yes, there's, there's less fertility, right. If we're talking less than 20 million per, um, per milliliter, you know, there's less fertility there. But once you start getting over that threshold, it really flattens out. There's like not much of a change in pregnancy rates, fertility rates, once you get kind of into the normal range. Which of course leads me to the question like, why are people so obsessed about this? Like, why is it that your social media feed like God, like I. God forbid. Searched sperm counts once, and now it's just. It was a huge mistake. It's. I need like, a I need a burner account just for this podcast. You know what? Why the obsession. And I don't want to like armchair psychologists this, but, like, is this just a manly thing? Is this totally, like, just like a, like a tech bro? Like, yeah.

Emily: [00:27:43] Yeah, I for for me, I mean, this is like a very interesting question I think really a question about psychology, but I think that it is a signal of your manhood that you have a lot of sperm. Like, sperm is like a, like it's a thing you could measure. And I think particularly for people who like to measure stuff, you know, like, I like to feel like you like to measure HRV, you like to measure your, you know, this hormone and that hormone. And like, you want to measure your sperm count because you want to have the best, um, you want to have the best sperm, but but just like to have it, you know, not not to do anything with it, just to have it around for for having and for telling people what it is.

Perry: [00:28:21] Yeah. It's like, um, it's like an online IQ test or like joining Mensa.

Emily: [00:28:28] So what about testosterone? Is testosterone or testosterone and sperm count the same? Are they correlated? What do we see in that?

Perry: [00:28:35] Yeah, I've seen a lot of people say like, oh, sperm count is this proxy for like, everything about male health, including testosterone. Um, not true. Actually true with a tiny caveat. So testosterone levels are correlated with sperm counts in the first three months of life. So apparently, um, for for babies age 0 to 3 months, the testosterone level there is important for proper testicular development. But that's it. Now you will see studies that show that men who have very low sperm counts, like less than 20 million, are more likely to have low testosterone. So when you have sperm counts that are low enough to cause a fertility issue, you are also more likely to have low testosterone. Those things can be related, but in the normal range of sperm count, there is no correlation between testosterone and sperm count. So a sperm count of 40 million versus 80 million, those testosterone levels have no effect on that. And so if you're using sperm count as a readout of your testosterone, it's a bad readout. Or maybe it's only a readout of when you were a tiny baby. Um, if you want to know what your testosterone level is, you can measure your testosterone.

Emily: [00:29:43] Test for that. One question that I want to come back to about what you said. So we talked about the the fertility environment being hostile to sperm. Yeah. Uh, what is what do you mean? Like the the vagina eats the sperm in a negative way?

Perry: [00:30:00] I mean, kind of the big issue is acidity. So, um, the the vagina, the fallopian tubes, these whole areas are quite acid. Um, I think the pH. I'd have to check, but it's like 4.5 or something like that. Um, which is certainly toxic to sperm. The semen that they come with is basic. So there's a little, uh, acid base reaction there. Like when you do the baking soda and vinegar. It's less dramatic than that, fortunately. But that's. But that's the primary reason. And and as I said, it's like it's for a good reason because you want to kind of select for the sperm that is most motile, most robust, because that's probably the one that has the best genetic material inside.

Emily: [00:30:48] Yeah. So you're really using you're basically using the hostility as an evolutionary mechanism to kind of find, find the good sperm that are just producing just a bill. You know, so many of these and we're kind of have a million different ways that we're trying to get sort them out, as opposed to taking more time to produce good ones in the first place, which doesn't seem to be the approach that we've taken.

Perry: [00:31:06] You got it. So let's go through now. Okay. So we're going to take it on on a little bit of faith and, but some data that that sperm counts really are declining. We both think that this is happening. And of course, then the next question is why? And even before we get to some of the like really rigorous studies on this, I think we need to address just a handful of the things that I have heard on social media about this. Um, let me start with a clip from the Huberman podcast about this issue.

Huberman: [00:31:39] You might have heard that carrying your phone in your pocket can reduce your testosterone levels and sperm count. And guess what? That is true. The data contained within this meta analysis and other meta analyses clearly point out that it can reduce sperm count and maybe testosterone levels significantly, but certainly sperm count and motility significantly. It reduces sperm quality. So should you avoid putting your phone in your pocket? Certainly your front pocket I would suggest. Yes. Right. If you are somebody who is seeking to conceive.

Perry: [00:32:09] Emily, before I get your response to that, I just want to say that that was one of the most epic eye rolls I have ever seen. Your opinion?

Emily: [00:32:19] I do not think we have good evidence that cell phones reduce sperm counts. We have a little bit of, like, confounded evidence that people who talk on the phone more maybe have lower sperm counts. But of course, that's not a piece of causal evidence. There's this idea you often hear about, uh, like when you have your cell phone in your in your pocket, it reduces your sperm count. But again, we sort of look at the better evidence on that, where they know where people are carrying their phones and so on. You just don't see any impact on sperm count. There's also no reason to think that your cell phone waves would would impact your spread. There's no reason for that.

Perry: [00:32:55] Yeah, yeah. There's no biologic plausibility, something we know.

Emily: [00:32:58] Biological plausibility.

Perry: [00:32:59] Always, always talk about. Right. You want, at the very least the sort of floor the table stakes are. There has to be like a reason to believe this would be true. And then you need to look for evidence that it actually is true. And I agree, I don't know why, but this is all over the place. I actually saw a clip of a woman responding, I like to this Huberman clip, and it was like. It was like reacting to the Huberman clip. And I was like, oh, this is going to be one of my people who's going to be like, what the hell is he talking about? And it was a woman being like, good job, Andrew Huberman. And also, don't put your cell phone in a fanny pack if you're a woman, because that can affect your eggs. And I'm like, oh, man. Like, now we can't even use fanny packs.

Emily: [00:33:40] And they were coming back. I mean, I think that this is an example where people are pulling causality of time trends, where it's sort of easy to say, oh, look, you know, we think sperm counts have gone down over time. Cell phones have gone up over time. And so, you know, those two things move together. Of course, many things have gone up over time, not just cell phones. And so the idea that two trends have moved in the same direction is not a way that we generate causality ever. Yeah. Terrible, terrible way to do causality.

Perry: [00:34:07] My nickel down here that the actual reason for declining sperm counts is the rise in political violence. I. What about okay, what about laptops on laps? Because okay, here we got some biologic plausibility. Yeah. Laptops are warm. You know, the reason that the testes are outside of the body is because sperm development is inhibited at 98.6 degrees, um, at body temperature, they have to be slightly cooler than that. Um, I saw a nice study looking at like the that measured scrotal temperatures. This is in a lab. So measuring scrotal temperatures of people with a laptop on their lap. And it found about two and a half to three degrees Celsius higher with the laptop on their lap. And like, okay, so more biologically plausible, what do you think about laptops?

Emily: [00:34:59] Yeah. So I think it actually is pretty I want to put a pin on the heat thing because actually one of the things that does reliably impede sperm production is having your testicles be very hot. And there are other things more important than laptops for that, like very tight underwear. Um, so like, heat is an important consideration here. The laptop explanation, my guess is it's like by it is biologically plausible. My guess is it's not very important just in terms of total magnitude, partly because I don't actually think people have their laptops on their laps like that much. And those effect sizes are not that big. And I would be surprised if, you know, this explained much of of a time trend. But I would say like from an individual standpoint, if you are worried about your sperm count, if you're trying to get pregnant, if you're worried about your fertility, you're thinking about your sperm count and you're carrying your laptop on your lap all the time, maybe don't do that. Put it on a table. Yeah. All right, so we got we're not we're going to carry our cell phone, but we're not going to put our laptop. Uh, what about pesticides? Toxins.

Perry: [00:36:06] Okay. Toxins. So, you know, this is interesting because toxins is such a huge term. It's such a huge umbrella and it can range from everything from like, yeah, the stuff we spray on crops to like automobile exhaust and particulate matter. And depending on who you ask, like, uh, corn syrup, like. Yeah, right. Like all this stuff. And so it's so difficult to pin down exactly what's happening. Um, I will say that there is some interesting data in pesticides. I'm not quite ready to, like, poo poo this entirely. Um, so, uh, the one that people will have probably heard of most is called atrazine. And the reason you see a lot about this is because this was the pesticide that in like a lab animal or like in a frog embryo study, like, was able to change the gender of the frogs, right? This was like the pesticides are turning frogs gay. Type of thing. So like a lot of hype, I think a lot of misunderstanding of what these studies actually looked at. But some of these pesticides do have hormonal activity like can be hormonally disruptive, changing levels of estrogen and testosterone in animals at least, and potentially humans. Um, there was a small ish case control study looking at infertility. So I'm moving away from sperm counts here. I'm just talking about infertility of 50 men who were infertile, 50 men who weren't infertile, where the levels of pesticides were measured in their blood, and there was a signal that the men who were infertile were more likely to have higher levels of pesticides in their blood. Now, like, does that mean it's the pesticides? Does it mean they tend to eat foods that contain more pesticides? Or like they don't wash their foods off with pesticides? We have no idea. But in contrast to cell phones, which always feels like a little like wishy washy woo science to me. You know, pesticides are chemicals. They kill bugs. They may do something to us, right?

Emily: [00:38:18] So let me just say, I hear you on the biological plausibility. I find this study you just described to be ridiculous. It is a kind of study I hate called a case control study. And in a study like this, I'm sorry, I'm going to nerd out for people. But what we're doing is we're sort of taking people who have some condition and then in this case, infertility, and then finding some other people who don't have the condition and then asking them or looking at different exposures, look different at different features of these people. And the problem with the study like that is twofold. One, there are many other potential differences across people. So it's definitely not randomized. And if you look at those two groups, they're going to be different on all kinds of other things, not just pesticides. And so we're potentially confusing correlation between pesticide exposure and infertility with causality. No reason to think it's causal. The second thing is, in that particular kind of study, you also have a problem because you're selecting the people totally differently. And so that makes your kind of correlation problem worse. So I hear you on the biological plausibility I think case control studies are so deeply stupid. It's like one of my just total pet peeves that anyone ever uses them. Sorry.

Perry: [00:39:36] I'm quickly going to search my Google Scholar record for all case control studies that I may or may not have engaged in. Um, I run randomized trials. Emily. Totally.

Emily: [00:39:48] That's the gold standard. And it probably doesn't show this. Okay. So, I mean, look, it's very difficult with something like toxins. I think in the end, it's going to be very difficult to say. Well, it's not like because that means kind of anything. Um, but I would be surprised.

Perry: [00:40:02] It also perpetuates the sort of like decline of civilization narrative that I think goes along with the sperm count decline. Right. It's like there's this weird overlap almost from, you know, like everything is getting worse, right? The sperm counts are going down and the pesticides are going up. And, you know, of course, the data frogs are gay. So I think we are we are definitely in that sort of time period. And it's going to be hard to disabuse people of, of any notions like that.

Emily: [00:40:32] All right. So what about last thing I think we hear about is, is soy. So soy bad for your sperm.

Perry: [00:40:39] So if your sperm count is like the true measure of your masculinity than the like, opposite of that is how much soy you drink.

Emily: [00:40:48] Soy. Totally.

Perry: [00:40:49] Okay.

Emily: [00:40:50] Um, 100%.

Perry: [00:40:51] So the reason people even talk about this, this soy boy phenomenon. Right? The reason people even talk about this is because soy, like many plants, Contains chemicals which are called phytoestrogens, which just means plant estrogens, something like that. Um, these are chemical compounds that have a similar chemical structure to human estrogen, and in some extraordinary cases, in very high doses, can mimic some of the effects of human estrogen. You can see I'm qualifying this quite a lot. That is that's it. Right. Like that's all that's based on is like there's a little molecule in soy and lots of other plants, by the way, that like looks, you know, if you squint kind of like the same chemical structure as estrogen, um, there is no good data to suggest that soy intake has any effect on sperm count whatsoever. Really? None. In fact, there's there's quite good data that shows that it has no effect whatsoever. There's even, Emily, randomized trial data where they took men and randomized them to have, like a high soy diet versus not a high soy diet and measured their sperm counts, and there was still no difference. So it's not soy.

Emily: [00:42:07] It's not soy.

Perry: [00:42:08] It's not.

Emily: [00:42:08] Soy. Okay, so it's not any of these things. I think we should talk about what it is. Yeah. Potentially. Like what might be responsible for this change. And to do that we basically like for something to explain this trend, it needs to both impact sperm count and be trending over time. And I actually think those are like a sort of both pretty important. So there are some things that are that do affect your sperm count but are not trending over time. And then things that are trending over time, which we've talked about, many that don't affect sperm size, you kind of need something to intersect to, to matter there.

Perry: [00:42:48] Yeah.

Emily: [00:42:49] So what is it.

Perry: [00:42:50] Well all right let me let me go for low hanging fruit. You mentioned earlier one when we were talking about laptops, that he is a major consideration here. So talk to me about heat. What's going on there?

Emily: [00:43:02] Yes. I think heat is very important because it is the one of the pieces here where when I talk to people about sort of infertility and about male factor infertility, it's actually something not everybody thinks about and is potentially quite important. So as we mentioned, your testicles need to be slightly cooler than body temperature in order to efficiently produce sperm. Which means if they're very hot, either because you're making them hot with some external source like a sauna or a hot tub frequently, or if they're too close to your body, that can reduce sperm count. And actually, one of my favorite studies of this is one in which researchers took men and they gave them these, like, super tight underwear. So they had them wear super tight underwear all the time for several months. And you can actually see this for science, and you can actually see the sperm count go to zero. So their sperm count, they're sort of like Effective sperm count basically goes to zero, and then it comes back up after they take off the underwear. So you know, this is a great system for birth control that I think we're underutilizing. Um, but also it really illustrates like this actually potentially quite, uh, quite important. And so this can apply to again, tight underwear, not clinically study tight, but just in general, uh, tighter underwear may lower your sperm count. Uh, people who are doing, like, a lot of, like, cycling, you know, not like I ride my bike to work, you know, every day kind of cycling, but, like, you're in the cycling peloton wearing the tight bike shorts on your bike for nine hours a day, that's not going to be good for your. It's not gonna be good for your sperm. So heat is a I think actually heat is a big factor. I don't think it is likely that it is a factor that explains much of the trend over time, because I'm not sure why people's testicles would be hotter now than they were.

Perry: [00:44:52] This isn't a global warming argument that you're making.

Emily: [00:44:54] Not a global warming argument. This is just like this is an individual, actionable thing. And I will say there's two other kind of individual, actionable things, which I also don't think are anything about the trends, which are heavy alcohol consumption and smoking. So both consumption of of alcohol at a very high rate. And cigarette smoking definitely affect sperm counts and sperm quality. But those two things have both declined over over time globally, so unlikely to explain trends in sperm counts, even if they are potentially important changes for some individual looking to improve their sperm. So we got to look for something else.

Perry: [00:45:31] So we need to look for something that affects sperm count and has been rising since the 70s.

Emily: [00:45:37] And what do you think it is?

Perry: [00:45:40] I will tell you, as I was digging into the medical literature, scientific literature on this, thinking that there would be more controversy just based on what I saw on social media, I was like, oh, there are 8 million studies that show that higher BMI is associated with lower sperm count. Obesity is clearly a major factor in sperm counts. It was like not subtle. It wasn't one of those situations where it's like, oh, well, this study says this, and this study says that it's like a consistent finding. Did you find the same thing? I'm like, why? Why isn't this just like the obvious answer?

Emily: [00:46:15] I found exactly the same thing. I actually would take it one step further, which is you can see both the impacts of obesity on sperm counts. And you can see on the flip side, with weight loss, particularly with GLP one, you can actually see men who go on a GLP one. Sperm parameters improve sperm counts, improves sperm motility, improves as a result, presumably of weight loss. And so this to me this jumped out very obviously as a clear explanation, something we know has trended up over time, something that's very clearly associated with sperm counts. And it kind of left me as I'm often in these situations being like, okay, given that the decline in sperm count is not very big, this probably explains almost all of what we have seen. So when we go into these like cell phones and this and that and soy and all this other stuff, there's like an obvious explanation staring you in the face that almost certainly explains like 95% of anything that we're seeing. Yeah, it ended up being kind of it was like a little bit.

Perry: [00:47:14] Right, right. It's anticlimactic. And maybe that's why you don't see it on social media as much as you should because, uh, you know, it's more it's it's more interesting to say, oh, it's this thing you haven't thought of. Right. Like, you didn't realize that fluorescent lights are, you know, talking to your spermatocytes in some weird way?

Emily: [00:47:32] Yeah. I also think people are looking for solutions. If you sort of if this is something you're worried about, you're looking for a solution, potentially. It's sort of interesting or different or like, you know, and things get clicks. The solution that's like people, you know, obesity is a metabolic issue that we should be addressing in, in our health choices. That is something people have been hearing for all kinds of reasons, not just for this reason. And so just this is like yet another reason. It's just like, that's boring. Like just another. Another like shamy. Well, you lose weight. It's like, yeah, hear that? We hear that all the time.

Perry: [00:48:05] We all know that already.

Emily: [00:48:06] Yeah. So, uh, we talked before about biological plausibility in this explanation about obesity. Is this a biologically plausible explanation and why.

Perry: [00:48:16] It is definitely biologically plausible, although, like all things, it's complicated. Um, so fat is a hormonally active organ. So. So the more body fat there is, the more levels of certain hormones, including estrogen like hormones, even in men get, um, get generated by fat tissue. And so there's this sort of endocrine phenomenon that will happen with overweight and obesity that could affect sperm production, but like that has not been totally well elucidated. Um, there may well be other reasons, whether it's like glucose metabolism and things like that, which might be important for keeping sperm alive longer and stuff like that. Um, whether it's vascular, you know, there needs to be adequate blood flow and things. That all remains to be seen. Um, but we talk a lot about correlation versus causation. We kind of throw that out. Um, but I just to get metaphysical for a second, I think the way I define this for like my students, people say, oh, you know, a cause is B that's important to know the reason we care if A causes B or if A is just associated with B is because if A causes B, then changing A changes B, and that's where the rubber meets the road in terms of health. Right. And so what you pointed out was like if obesity overweight causes low sperm counts, then changing your weight will increase your sperm counts. And that is the level of evidence we have from the GLP one trials. So that's like, you know, I'm sure a philosopher would take issue with that particular definition of causality. But as a doctor, that's all I care about. Can I change this and make you better? Yes. Great.

Emily: [00:49:57] Yeah, yeah. I think when we think about causality as, as economists, also, we're actually much more focused on the idea of if I change this, will this other thing change as opposed to exactly why? Right. So I sort of asked you about like what is the mechanism for this? But in practice, one of the things we get out of that GLP one evidence is if we change this, then sperm count would change. And that's the behaviorally relevant thing. It is interesting to think about, well, why, you know what would be the mechanism there. Partly because it might suggest, you know, variations on on the behavior change or it just might help us understand people better. But causality is really about if I change x, would y change. And that is very different from when we talk about correlation. And we just see like these two things are moving together, which could be because x causes y. It could be because of y causes x. It could be because some other thing z causes x and causes y. And that's that's not super helpful. That's not super helpful for for decision.

Perry: [00:50:55] So I want to drill down. Now we've talked about the population. We've talked about millions of people and the average sperm count and stuff like that. We've said how, you know, a lowish sperm count at the very low end can impact fertility. But beyond the very low end doesn't seem to matter too much. Um, but let's take an individual like I would probably argue, if a patient came to me and said, you know, my sperm count is 60 million, I'd like to get it up to 100 million. I think my answer would be like, why? Like like if you're not having fertility issues, like sperm count doesn't correlate with, you know, how strong you are, how fast you can run a mile, like how good you are in bed, like none of these things correlate. Nevertheless, people seem to care. So what is your sort of set of recommendations for like, like, let's do that thing where you say, oh, get your sperm count up for some reason.

Emily: [00:51:45] Yeah. So first of all, I think there are a set of people who would care if you are trying to get pregnant, and maybe not to go from 60 to 100, but I actually think we're probably in some sets of populations under testing people's sperm. I think when couples go to try to get pregnant, they often like women will do a fertility workup. I actually think it's a lot of value in men doing some fertility workup to begin with, partly because this is movable, right? In ways I will say. So if you come and you do a sperm workup and you find your sperm count is 20 million, there may well be things you can do to to raise it, which would be, and I think this is not too complicated, you know, don't drink heavily. So if you're doing a lot of binge drinking, try to cut down. If you are smoking a lot of tobacco or cannabis, try to cut down or quit altogether. Uh, if you are doing a lot of activities that keep your testicles very hot, like regular sauna, the most common thing would be people doing like regular sauna or hot tub usage. Try to take a break from doing those. Uh, and the fourth one is, you know, if if people are struggling with obesity, I think a GLP one is worth thinking about in that situation. But again, I would recommend these things for someone who was trying to use their sperm for pregnancy. Right? If you said I'm hoping to use my sperm pretty soon to, you know, get into somebody's egg, that's kind of when you want to be thinking about this. You shouldn't. This isn't just like, you know, for fun. I mean, you could, but, like, it's not it's not important if you're not using.

Perry: [00:53:14] Them just to drive that point home. And I'm going to geek out a little bit here now and talk about this thing called the coefficient of variation. When you measure something, there's a question you have to ask, which is if I measure this again at some time in the future, will I get the same result? And how much that changes from one measurement to another can be statistically defined as this thing called the coefficient of variation. A typical lab test like you might get in the hospital, like your hemoglobin test, has a coefficient of variation of about 4%, which means that if I measure you today and I measure you tomorrow, on average there'll be about 4% different, you know, kind of bounces around just a little bit. No big deal. There are few tests with a higher coefficient of variation than sperm count. It's about 50%. So I just want people to be on the individual level. Right. So if you measure me today and even all like taking care of all the variables, right, like remaining abstinent for five days, whatever, 50% difference on average between test and retest. So everyone needs to keep in mind when they see those people on social media who are like, my sperm count was 40 million. And then I like, you know, ate quinoa for eight meals a day and then it went up to 70 million. Like that's within the range of expected variability. So the decline of sperm count is a population wide phenomenon. Like it can be real on a population for an individual. It's actually recommended in fertility Infertility testing that men get tested multiple times because of this issue, because there's so much variability. And certainly if you're just using it as like, I want to put this on my Tinder bio or whatever, well, you should test a bunch of times and take the average.

Emily: [00:54:56] It's like retaking the S.A.T. because they only count the highest one. So just keep testing and testing until you get a number that you like. Put that in your Tinder bio, and then you're done. That's it. Good luck with that. All right, Perry, what's your one thing on sperm?

Perry: [00:55:12] Uh, here's my one thing. Here's my hot take. There's no reason to care about your sperm count if you're not trying to get someone pregnant. Full stop.

Emily: [00:55:19] My one thing is keep your balls cool.

Perry: [00:55:23] Keep it cool, keep it cool. That's it for sperm counts. We'll get to your listener questions after the break.

Perry: [00:55:32] We've got a great question here from Anna in Columbus, Ohio.

Anna from Columbus: [00:55:36] Hi Emily and Perry. I'm excited for this podcast and I have a fun question for you. Is there anything health related that you do that isn't supported by data, but you just do it out of habit or for juju like the rest of us? Thank you.

Perry: [00:55:51] I do have a belief that blueberries are way healthier than people think they are, and I have never looked it up and they just taste healthy to me. But maybe I'm right. Maybe if we do a dig into that, it does have data to support it.

Emily: [00:56:07] Like healthier than other fruits?

Perry: [00:56:09] They're the healthiest tasting food.

Emily: [00:56:14] That's so weird. Can't you just say you like blueberries?

Perry: [00:56:19] I don't. I mean, I like other things more than blueberries, but I. They the taste of health to me is somewhat tart. Blueberry, I think, but I have no data to support that. They're particularly healthy.

Emily: [00:56:31] Wow. I cannot think of a single. This is so. I feel like such a jerk. I can't I, I do all kinds of weird stuff, but I feel that all of it could be supported by data.

Perry: [00:56:45] That's the problem with this question, is that you probably can find like you could find data to support anything. Like, I can find you a study that shows that blueberries reduce the risk of cancer. Right.

Emily: [00:56:53] Like, totally. Oh, I'll tell you what I do. I hold a very strong superstition about. I am extremely neurotic about which shoes I wear, at which times for when I run. And I have, like, very strong feelings, like, this is a shoe that you have to use when you're running easy. This is a shoe. When you use for this kind of run, this is a shoe. And some of them are literally the same shoes. But like one of them is an older version, like I've run on them more. So they've gotten like downgraded to like the medium long run from the. And I think that if you open the cabinets in my family room where I keep the shoes, you would be disturbed.

Perry: [00:57:30] Okay, we will be sure to post a picture of Emily's weird shoe closet. Uh, stick with us next week when we will ask, what's the deal with peptides?

Emily: [00:57:39] Wellness actually is produced in association with iHeartMedia. Our senior producer is Tamar Avishai. Our executive producer at iHeart is Jennifer Bassett. Our theme music is by Eric Deutsch, and our content is for educational purposes only.

Perry: [00:57:53] If you like the show, help other people find us. Leave a rating and review on Apple Podcasts or your podcatcher of choice, and help us spread the word about the show. Don't give the TikTokers all the power. And don't forget, we want to hear from you. Head over to wellnessactually.fm And leave us a question for our mailbag, or suggest a topic for a future show.

Emily: [00:58:13] We'll let the influencers have the last word.

Influencer: [00:58:15] Sperm counts have dropped more than 50% in the last 50 years. If this keeps up, natural conception is going to be as rare as a Cleveland Browns Super Bowl parade. Your grandkids might need a lab just to exist.