Plain English with Derek Thompson - Sperm Counts Are Falling All Over the World—Why?

Episode Date: December 9, 2022

In the last 50 years, average sperm counts have fallen by 50 percent. This isn’t just happening in the U.S. or Europe or Asia. It seems to be happening everywhere. If the current rate of decline continues, researchers concluded, the average male sperm count will fall so low that the typical guy in every advanced economy will be infertile by 2050. Harvard's Jorge Chavarro, a professor of nutrition and epidemiology, breaks down the data on declining sperm counts and tells us what it means, what might be causing it, what men can reasonably do to avoid it, and how bad it could get. If you have questions, observations, or ideas for future episodes, email us at [e-mail]. You can find us on TikTok at www.tiktok.com/@plainenglish_ Host: Derek Thompson Guest: Jorge Chavarro Producer: Devon Manze Learn more about your ad choices. Visit podcastchoices.com/adchoices

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You can listen to This Blue Up on Spotify or wherever you get your podcasts. So I was on Twitter the other day, where it seems like the only thing people talk about anymore are Elon Musk and Twitter. And I came across a study that really blew me away. It was an analysis, a meta-analysis, actually, of male sperm counts. In the last 50 years, it said, average sperm counts have fallen by 50%. And this isn't something that's just happening in the U.S. or Europe or Asia. it seems to be happening all over the world. In fact, if the current rate of decline continued,

these researchers concluded, the average male sperm count would fall so low that the typical guy in every advanced economy would be considered infertile by the year 2050. That is, a child born this year, would, by the age of 28, live in a world where the average man is technically infertile.

That seemed, at the very least, pretty damn interesting, pretty damn concerning. And frankly, kind of under-discussed in certain circles. Maybe declining sperm cancer talked about a lot in your other favorite podcast, favorite YouTube person. But I think about it at the policy level. This country had a national discussion about lead poisoning. We had a national discussion about clean air, clean water. But if environmental factors are slowly decimating the human population's ability to procreate, that does seem like something policymakers should talk about more.

So I started reading about this subject. And as I learned very quickly, this is a place with unbelievably high amounts of controversy. There are some researchers who have major issues with this analysis and the scientist behind it. There's some people who think this issue is vastly underrated, as in people aren't talking about this enough, and there's other people who think it's vastly overrated. People are talking about it, and it's fundamentally much ado about nothing. But in all these articles that I read, a researcher whose perspective I really enjoyed throughout all of it was a Harvard professor Jorge Chavarro. He is a professor of nutrition and epidemiology. He knows the sperm count

meta-analysis team, but he also researches diet and male and female fertility. And Professor Chavaro is today's guest. In this episode, we break down the data on declining sperm counts. We tell you what it means, what might be causing it, what men can reasonably do to avoid it, how bad it could get. And finally, we fold this story of declining sperm counts into the broader story of declining fertility in the Western world, which is fundamentally a story of not just biology, but also of society and the choices that we make as people. I'm Derek Thompson.

This is plain English. Professor Chavaro, welcome to the podcast. Thank you for having me. I want to start with this new meta-analysis that I described in the open. Do you know these researchers, and can we trust these numbers? Yes and yes. So I know the authors very well.

I've collaborated with them. We've written papers together. We've written research proposals together. And looking at the author list, I've written at least one paper with about half of them on different aspects of male fertility. So the research community on male factor in fertility is relatively small. So there's a really good chance that we all know each other.

And these are very senior people, very serious people, and that can be trusted in giving unbiased professional opinion on the work they do. So I trust them in general. And I think as it relates to this paper in particular, it really is the fourth iteration of trying to get at this same. question, right? Especially from the senior author, Dr. Shana Swan. She's been involved in four papers starting the early 1990s to try to address this question. Is there a downward trend in human quality? And each progressive iteration, including this one, addresses some of the

limitations of the previous iteration and gets closer to a good answer to that question. Let's talk about the most important finding of this meta-analysis. And you can draw back from that and tell me what the most important fact in this field is. What should we know? Is it the decline in sperm quality, a decline in sperm reduction? What is your headline here? Right. So I think the headline is that over the course of the time period covered vitamin analysis, so roughly from the 1980s to 2020, so over a 40-year period or so, the sperm count, so sperm concentration, total sperm count, have come have reduced by about half worldwide. And this is after taking into account some issues

that weren't addressing previous versions of these studies, including is this only happening in rich countries, right? This is only happening in North America and Australia, New Zealand, and Western Europe. So these numerous analysis includes data from South America and Central America, from Africa, and from Asia, and it appears that the same thing is happening worldwide. Does it have to do with the type of people who end up giving a cement sample for research?

And therefore, there's data that eventually gets published on cement quality. And that is addressed. That was a bigger problem, the first version of the paper. And it is absolutely a non-issue in this version of the paper. And then what was probably the biggest criticism, in the 1990s version, the 1992 version of this paper and then the 2000 version of your paper is, is what you're seeing differences in what's happening at the lab, right? So there really wasn't a consensus on how is it that you should analyze a sample

until the late 90s and more recently there's, the WHO has established guidelines, lines that have become pretty much universal on how you process and how you evaluate as even sample. But in the other papers, people were using all sorts of different methods for counting sperm. So that was a big issue in some of the earlier versions of the paper. The newest one uses only includes papers in the mental analysis that use the standard WHO protocol. So again, it is less of an issue. That's really interesting because I did a lot of reading about this subject in the last few days, and it seems like there's been a controversy in the last few years. Some people think that this issue is being catastrophized, that it's not nearly as bad as the meta-analysis suggests. And you're saying that in previous years or even previous decades, people said, this is self-selection bias. People are only turning in sperm samples if they have reason to believe that they might have less sperm concentration or if they're having some difficulty with fertility. Maybe it's self-selection bias. Maybe it's mostly a phenomenon of lab analysis. But you're saying this meta-analysis, we should trust because it resolves or directly answers

a lot of these criticisms that had been levied against this kind of research. Is that basically right before we move forward? That's basically right. So I think with each different version of the paper over the last 30 years or so, this group has gotten closer to answering the question, is there a downward trend in sperm counts? And I think this most recent version of the paper gives us the best possible answer to date, which is probably yes. Probably sperm counts are going down. And tell me about when this declined happened, because you're looking at a difference between the 1970s, early 1980s.

You said there was a meta-analysis published in 1990. Now we've got an update published in 2020. When did this decline happen? And is it a decline, is it a decline that's over or are sperm counts continuing to fall in these populations? So when is it happening? I guess it depends on how much do you want to believe previous versions of the meta-analysis, right? So if you think that if you're willing to say, okay, maybe the methodological problems that were present in the 1992 version of the paper are not that big of a deal, and we can use that. as an initial guide of how long has this been going on,

that paper included papers from the late 1930s. So you could say, like in the worst-case scenario, the answer is it may have been happening since the late 1930s all the way through today to the 2010s. The last paper published that's including the meta-analysis in 2019, so it's probably 2010s. If you want to be more stringent as to what is the type of data that we should be allowing to be included in this type of analysis, then you would say, well, it's probably something

that's been happening from the 1970s forward. So definitely at some point in the second half of the 20th century, maybe a little later, I'm sorry, maybe a little earlier. definitely still happening today. Now, the question is, like you said, is it going to continue to happen and we don't know? And it really,

in my mind, it really depends on why. So yes, it's going down, but why is it going down? And is it going down because of whatever is explaining this downward trend has

saturated the possible effect, that it can have on a population level on civil quality, or is it something that can continue to impact sperm concentration and sperm count at a population level? And that question, in my opinion, cannot be answered today

because we do not have a very good understanding of what is causing this downward trend. We're going to get to your dealing with, grappling with the why question, which, as you said, is not something we have a firm answer on. But I think a question that might help to focus us on the why is the where, right? If every population around the world was having the exact same rate of decline, well, then we would have one set of answers.

But if different countries are experiencing different rates of decline, that presents a different set of answers. So tell me, is every region that's being studied seeing the same rate of decline when it comes to sperm counts? Or are some countries in some regions declining fast? than others. For that particular question, the current meta-analysis, the one that was just published, is the one that provides the best possible answer. So obviously, there's not, there are not studies from every single country in the world, included in these meta-analysis. It includes data from about 50-something countries. So it's a pretty reasonable number of countries, including the study in the meta-analysis, 53 countries.

And the way they try to deal with these regional variation, or is the same thing happening across the world, is they divide their samples, their analytical samples between high-income countries, essentially. So North America, Western Europe, Australia, and Asia, and then everywhere else in the world, right? There's fewer studies in everywhere else in the world, but there's enough to make estimates of what's happening outside of Western Europe, North America, Australia, New Zealand. And even though there's a decline in sperm concentration and total sperm count all over the world, the decline appears to be less steep. outside of North America, Western Europe, and Australasia. I'm sorry, Australia and New Zealand.

So, yeah, so that part appears to be slightly different. It's still a downward trend. It's just not as severe. And just a really quick question. Is there a particular country where we see sperm counts are declining the fastest? Like, is it faster in the U.S. than in Europe? You said that it's, it tends to be richer, higher income. seems to me, countries that are seeing the steepest decline.

Is there some place that is just seeing the steepest decline? So that is not specifically addressed in these meta-analysis of country-specific estimates, but a lot of the initial interests had to do with trends that were identified in Northern Europe. Mostly in Scandinavia, which coincidentally is where some of these investigators are based. But, and then the question was evolved to, is this only a Scandinavian thing, or is this something happening more generally? And that's how most of these research evolved and has continued to evolve. And it does appear that it's a more generalized phenomenon. And then the issue is why and is there a bottom, right?

So are we racing to zero or is it going to stabilize somewhere above zero? And that's exactly how I want to structure the rest of the conversation. I want us to talk about why, and then I want us to talk about how bad is it and how bad could it get. So you've already said that the why here is complicated, and we don't have a crystal clear answer as to what is causing sperm counts to decline, especially in some of these Western countries. But give me some of the nominees. What are some of the more popular explanations for why this is happening? So when you, I think there's always a little bit of discipline bias when you try to look for answers as to why something's happening. And I think what has happened with the same in quality decline is a really good example of that.

So there's a lot of the investigators involved in this particular study, and more generally, a lot of the people who do research on human fertility come from, an environmental health background for historical reasons. And so if you look through the literature as like, why is this happening? What people have always tried to do is look for answers in some not entirely specified environmental factor.

So maybe it's chemicals entering into the world's Maybe it's air pollution, maybe it's this, right? So what has been clear from that research is that there is not any one environmental factor that is explaining these overall downward trend. So it is possibly true that there are a variety of adverse environmental exposures from environmental chemicals that get that enter broadly into exposure in human populations through to air pollution or other factors that are associated. that contributed a little bit to this decline, but I don't think that's the entire story. Most of my research has to do with nutrition and lifestyle factors as they relate to fertility. So I tend to approach this problem from a different perspective. So from when I see these data, the one thing that catches my eye is that the downward trend

has coincided more or less with rates of obesity worldwide. And so, and we know from individual, from other studies, not necessarily trying to look at the downward trend, but trying to look at what might be the impact of nutritional factors and male factor of fertility, that BMI is a really, really strong predictor of cement quality of sperm concentration, especially at really high levels of obesity. It is known to decrease sperm counts quite substantially. So it is quite positive. that worldwide changes in the frequency of obesity are also contributing to these. And other things that have happened at the same time that obesity rates have gone up is that how people eat across the world has changed quite dramatically. So the amount of food that we eat that is prepared outside of home has changed quite dramatically. The amount of sugary beverages that we drink has changed quite dramatically and all over the world.

And each one of these factors has been associated in a few studies with lower sperm concentration. So I think that we will never find the one answer of why is it that sperm counts are going down? Because there is not a one answer. It will be one of these things where there is, it's many little, a wide variety of small, problems, which when you put them all together, weigh down testicular function and is manifested as lower sperm counts. I want to offer what I take as a kind of synthesis of that answer and then throw it back to you to let you tell me if I'm understanding this right. It seems like you're saying there's sort of two groups here. Group number one are the ecologists, the environmental researchers.

They're looking at what's happening in the outside world that might be affecting our hormones, our endocrine system. And they're saying it might be all. of these man-made products, the plastics and the creams and the cosmetics and the cleaning products and the drugs that we're putting into our system or coming into contact with, and they're messing with our hormones, they're messing with our bodies in such a way that it's having some kind of hard-to-discangle process that is cashing out as a steadily declining sperm count. As if, and it's a dosage effect. The more we interact with these substances, the more our sperm counts continue to decline over time.

You're saying there's another explanation, which is that it's not so much about the chemicals. It's about obesity, that obesity itself is having this direct relationship to sperm count. Is it possible that both groups are right? That what's happening is they're actually telling the same story, that there are chemicals in our foods that are making us obese and those same chemicals are having a secondary effect on our hormonal system that's having this other outcome of declining sperm count. So yes, obesity might be directly causing the declining sperm count,

but also there's the chemicals in our foods that are making us obese or having this secondary effect of reducing our sperm count. Is something like that possible? Yeah, I guess I wasn't entirely clear on my previous time on the microphone, but I think that that's exactly what I meant, is that environmental contamination is not the only thing that that's the answer. It's not the only answer as to why this is happening. But I would frame it slightly differently than you did, right, which is, you're saying environmental chemicals, some of those are abysogenic, and obesity ends up directly affecting spermatogenesis. And these environmental chemicals also may have a direct effect on spermatogenesis, independently

of obesity. I think that does happen. But at the same time, there are many cause of obesity that are independent of obesity of obesity chemicals and many lifestyle factors that have an impact on spermatogenesis, independent of obesity, right? So I think that there's, what I wanted to say, I guess, is that there is no one cause, right? There's not going to be one lever that you're going to pull and you're going to say, we're done, right?

There's probably hundreds of them. Oh, you made that point very clear. I did not want to suggest that you didn't make that point clear, that there's definitely multiple causes here. And you did a great job of summarizing my hypothesis. Is there a possibility that the chemicals are having this forking effect? on one fork they're causing obesity, and another fork they're causing a decline in sperm count.

And you're saying maybe that's one of many things that's happening, but the most important thing is that it's just a wide portfolio of effects that we think are causing this. As someone who's concerned about this, if I'm listening to you and saying, all right, there's a scientist that's telling me that there are many different features of the modern world that might be causing declining sperm count.

It's the products that we buy and we come into contact to, and it's also the food and drink that we're putting into our bodies. You know, I can't realistically, look, I like face creams. I like putting on face creams. I like buying soaps at CVS and shampoo at CVS that might have some of these substances. I use plastics like every other normal person. I'm not going to stop doing these things. If you had to sort of rank order for me, the causes that you have the most confidence in that are causing this decline in sperm count, you know, with the understanding that this is

unsettled science, what do you think we have the most confidence is a driver of this phenomenon? So I think, and again, my ranking will be different from, maybe different from the ranking of some of my colleagues that I, that I work with closely. But I think if I were to weigh it in terms of quantity of evidence, I would say that probably excess body weight is the biggest one. And in terms of having an impact on sperm counts. And that was a little tricky, though, because one of the things that we've seen is that, yes, definitely. men with obesity, especially men with severe obesity, have significantly lower sperm counts.

Then the question is, does losing weight improve sperm counts? And the literature on that is actually quite confusing. So probably the best thing that you can do is stop, is modify your life in a way that prevents you from gaining weight in the first place. Right. And fortunately, in a way, Most of the people who are interested in these are relatively young, right? And therefore, still have a lot of potential weight to gain in their future. So just preventing that additional weight from being gained is, it may be the most important thing they can do to protect their testicular function. I want to talk about how bad this news is and what it means. So I read in the course of my research for this that average sperm count has dropped more than 50% in the last few decades, but it seems to still be on average in the normal range, which means that it could still fall further and it could get into abnormal ranges, but overall it seems to be in the normal range.

So if sperm counts are still within the normal range, what is the most significant implication, would you say, of declining sperm count? Okay, so I think there's two important implications, one. One is, what do declining sperm counts tell us about men's fertility potential? What does it tell us about their future potential to father a pregnancy when they want to father a pregnancy? That's number one. And one that's probably the one that gets most attention. The second one that's probably often overlooked is what does this tell us about men's overall health?

So does this tell us anything about men's future cardiovascular risk or future risk of dying early? So let me start with the first one, right? Because that was the one that probably, not only the one that probably receives the most attention, but also probably the one that we have slightly more information to answer. So the first question is, is it going to matter in terms of affecting fertility? And then, so the answer is maybe, but we're probably not at a, we're probably not yet at the population that this would be a major emergency at a population level. So what do I mean by that?

So the two outcomes that they're looking at this meta-analysis, and in part they are because they're the ones that can be more objectively ascertained and replicated across different labs, right? Our sperm concentration and total sperm count. And we know from studies as far back, as the late 80s, that sperm concentration, especially with sperm concentration, sperm count mirrors this relationship to an extent. It is related to the probability that a couple who's trying to get pregnant will get pregnant, right? But this relationship is strongest at the lower end of sperm

count. So what do I mean by that, right? So the difference between having one million sperm per milliliter and having 10 million sperm per militer so those two counts would be considered abnormal that difference is enormous in the in the in the in the in the in the in terms of the the probability of father in a pregnancy the probability that the the difference between let's say 10 million sperm per militer and 20 million sperm per millimeter yeah so we're moving in 10 million increments that difference is also in enormous right so those 20 million sperm, they're super, they're super important. Now, how about the difference between 50 million and 60 million sperm? So it's the same 10 million sperm per mulberry difference. That

difference is a lot less important. So what we know is that within the first 50 million sperm per millimeter or so for total sperm count, and any gain makes a huge difference in terms of the probability of getting pregnant. From the range from about 50 to 100 million, it makes a difference, but it's not that important. And once you get past 100 million per sperm male, it makes essentially no difference. If you have 100 million or 150 million or 200 million or 200 million sperm per male, it doesn't change at all your probability of father in a pregnancy. And just to quickly to ground us, what is median, right? What is normal? So it used to be, and if you look at the results of the meta-analysis,

that the automatic response would be, well, somewhere around 100 million sperm per male, right? That would be kind of your automatic response. Like your average guy on the street without any problems should have around 100 million. What we're seeing is that that's actually no longer the case. It's closer now to 50 to 60 million. Like your average guy on the street probably has 50 to 60 millions per and per meal count. So it now appears that the average guy in the street is getting close to the range where it might start making a difference in the probability of fathering a pregnancy

if they are trying to get pregnant. Now, the question is. is how much lower can you get to start making a difference and how big a difference is that going to be, right? So is that going to be a difference? Like if you look at couples who are trying to get pregnant on their own and they do not have any known fertility problems, any history of infertile or anything like that, the average couples takes two to three menstrual cycles to get pregnant, right? That's your average couple of couple. And then from then on, there's couple of stakes, slightly longer, slightly longer, slightly longer.

So what is it going to be the difference? Is it going to be the difference between taking two to three menstrual cycles and taking three to four? So that's a shift of one month, but probably not a big deal for most couples. Or is it going to be a difference between taking two to three menstrual cycles and taking seven to eight menstrual cycles, right? So still technically not in fertility. But that might start getting a lot of people more concerned, especially once you layer on top of that worldwide trends in well. and how people decide to reproduce. So in general, people are deciding to have fewer children,

to have children at later ages. So once you start later and these things on top, maybe a difference of, let's say, three or four months in median time to pregnancy is a big deal. And the problem is that we really do not have a good answer to what are the fertility implications of these, right? So, yes, there's more men who are now in the range where this may be a concern,

but we don't know what is. exactly the concern and how concern should we be. I really appreciate that breakdown. And now I'm going to do something that's a little bit dangerous. I'm going to try to express a piece of statistics, even though I've never taken a class in statistics. So you tell me how catastrophically wrong I am here. Even as the average sperm count seems to be generally in a normal range,

the lower the average gets, given the fact that there is going to be across the male population a distribution of sperm counts, the lower the average gets, the more we should expect people, men, to be below average, right? So it could be the case that the average is being brought down, not because, say, half the population is getting more infertile, but rather because something like 5% of men, for whatever reason, are seeing significant, significant declines in sperm count, while the rest of men are not necessarily seeing that same significant decline. And so this makes me think, are we seeing this effect, like, across the population

relatively evenly? Or are these declines being driven by a subset of the population? Yeah, no, that's actually a great question. And that is a question that I wish they had addressed in this paper. So maybe that'll be the 2027 paper or something like that. And I think it has to do with the way in which they collected data, right? So they were collecting averages and they were collecting average measures per year from each of the studies that contribute data to the study.

What I think would have been very interesting to see is, well, for example, what is the proportion of men within each study at each year, right, that have a sperm concentration or a total sperm. count that is below the WHO cut of reference value for triggering an evaluation for fertility in the context of an infertility, a couple concerned about their fertility. And we do not have that data. I agree with you that you would expect that the proportion of people falling below really low value thresholds would be increasing as the mean is decreasing.

So two things. That would be definitely true if you always had a normal distribution, but we know that cement quality, especially these two parameters, sperm concentration, and total count do not follow normal distribution. So we do not know to what extent that movement of the mean is affecting the bottom tail of the distribution. And that would have been extremely useful information to have

that unfortunately do not have. Thank you for that. I'm glad that we got into a bit of the weeds there. Let me, I have, I just want to make sure that I'm respectfully your time. We have a couple minutes left. Let me, let me ask the next question this way. I'm interested to know whether you think, even if this is not a population-wide crisis now, it will be a problem in the future. So if sperm counts continue to fall about 1% a year, at what point, according to your expertise, do we get into the danger zone? Let's just assume, like, it continues to fall at the same rate every single year. Is there a year, a decade? We're looking out,

you say, all right, that is the sperm count number that would start to get me concerned. Okay. So, again, I don't have a crystal ball, so I hesitate to prognosticate. But I'm going to try to humor you on your question. So I think that if we start to get populations where the mean is closer to 20 million sperm per male, right? That two versions of the WHO manual for similar analysis, 20 million per male was considered male factor infertility. So if on average an entire population has that low, of a sperm count, that will probably be very concerning. So I would expect that you would see a lot more infertility that is not explained by delayed

childbearing, that is not explained by early things, and that is explained just by low sperm concentration in a population. But then there's several caveats to that. One is, is it the case that we're going to continue to have a decline in sperm concentration? of approximately 1% per year, right? And probably not. My guess is that at some point, we're going to hit a plateau, right?

Where, I don't know. There might be a plateau, and it may be that we'll stop. And so that's one thing. And in terms of concerns, I think we're probably going to be seeing concerns on the long-term health of these men probably before we see concerns on the fertility of populations as a whole. What I mean by that is that there's some very interesting emerging evidence suggesting

that lower sperm count, another markers of lower testicular function like lower testosterone levels are actually associated with risk of major chronic diseases, and also with also with unspecific health outcomes, like the risk of being hospitalized for whatever reason, more diabetes, more heart disease, and more premature mortality. So the literature on that is still emerging. So for example, one of the things that is, well, if you have a lower sperm count, you're less likely to have children, to what extent these health effects that you're seeing,

are because your social structure, 20 years down the line, looks very different, and looks a lot less supportive than the social structure of somebody who had children. So whether it is because of underlying common biological pathways that led you to die earlier, or where it is because you end up having downstream consequences, like you have a very different social structure

because you have fewer children and therefore a much smaller social network 20 years down the line. We don't know. Those are things that we still need to know. But I'm actually more concerned about that part of the population health impact than on the fertility health impact. So I should make sure I understand you. You're saying that declining sperm counts might be sort of like a canary in the coal mine when it comes to health. It's not that decline, that the lower sperm counts themselves are bad for someone's health, but rather that the factors that cause sperm count declines might suggest that lots of other things might be going wrong that are bad for health? Exactly, yeah. So, I mean, you can think of the, I like to think of the testase as really two organs

that happen to share a common anatomical side, right? So there's, there's an endocrine organ that's in charge of production of testosterone, estradial, and other hormones, and there's a, and there's the reproductive hormone organ, which is in charge of making sperm, right? So you can have, you can have things that affect one part, but not the other, and you can have things that affect both. And the effects on the endocrine testis, right, on the hormone-making part of the testes, we know that hormone production, testosterone production is associated with a wide variety of other health effects, right? So especially as it related, to cardiovascular health as men start to age.

So my concern is that what we're seeing with sperm counts is actually a global effect on sperm function in general that will end up having long-term health consequences on cardiovascular disease, premature mortality, diabetes, before we start seeing the fertility effects in younger cohorts of men. And we are, it does. We haven't had time to get into this, but we are simultaneously seeing declines in average testosterone

levels as well. And so we're seeing this decline in both of these functions. It's interesting. I wanted to end on the concept of fertility, because we've waived at this point, but I think it's really important to go at it head on. You know, there are a lot of interpretations of declining sperm counts that say, okay, this is our explanation for why fertility is declining around the world. It's because of declining sperm counts.

But you've done a really good job in your research and in other places that I've watched, explaining that there are a lot of other social factors that contribute to the decline of fertility, especially in higher income Western countries. What do you consider the most important drivers of lower children per couple in these richer societies? Right. So I think it's important to try to define what is it that we're talking about. So when you say fertility, you kind of get an idea of what you're talking about, but you're not always talking about the exact same thing. So there's like the demography definition of fertility, which has to do with how many children, on average, do women in a population have? And is that enough to sustain the population or make it grow or make it shrink?

That's one concept of fertility. And then there's a more clinical concept of fertility, which is, are you able to have children, when you want to have children, right? And those two things are related, but they're not the same. Right? So one that you have a lot of data worldwide is the first one, is the demographic one. So you can count women of reproductive age. You can count babies, and it's really easy to say, okay, the on average women in this country

or that country having 1.2 babies per woman. So that would be something like South Korea or Japan, which is very, very low, or Spain. Or then you go and say, well, this country has a little closer to two, which is a little north of three, the replacement level. So United States is around 1.7, so we're below replacement. And you go to some parts of Asia, which are growing, and you have numbers of either two or three. So that's one concept of one way of thinking about fertility. And when you think of fertility that way, as well as when you're, you have to also think about why are people getting pregnant, right? So there's in the post-1960s world, we made an enormous transition from all of our ancestors.

So up through the 1960s, when all of a sudden we introduced to our species' effective contraception with the introduction of oral contraceptives, we now can, as a species, decide, do we want to have children? Yes or no? And if we want to have children, when do we want to have children? So what you've seen is that on average, people are deciding to have fewer children, and people are deciding to have children later. life. Why? Because it makes sense. It makes sense that if you do not have to worry about finding a

life partner and then enjoying your life and going to school, getting a profession, and do all of that without having to worry about the fact that you may get pregnant in one moment, that you will not take advantage of those opportunities if you're a woman. So women do it. Unfortunately, what I think is, especially in high-income societies, is that we haven't quite realized that contraception is not a pause button on fertility. It helps you to delay conception, but your reproductive system continues to age, even while you're not able to get pregnant because of contraception. So when you have, for example, places like Massachusetts where the median age at first birth is around, is around 35, right? Then you start having entire populations that are trying to get pregnant for the first time when we know that you're having the most serious declines in fertility

as it relates to ovarian reserve. And we also know that then you're having men who are trying to get pregnant, we're trying to become fathers for the first time in the mid to late 30s. And we also know that men's age also has an impact on the chances of getting pregnant. So I think that as fantastic for economic and professional development as contraception has been, it puts us in this quandary where we have a mismatch between cultural expectations and economic and for pursuing economic and educational opportunities and reproductive biology. And we're going to have to figure it somehow. This is a complicated subject, one that I should definitely do other podcasts about,

but I think that was really exquisitely well said. And I appreciate that you put the issue of sperm counts and fertility in this wider social context because the social context is just so unbelievably important in terms of understanding the bottom line number of the direction of fertility in this country and in others. Professor, thank you so much for walking us through the statistics, the numbers, the causes, the implications. I really appreciate it. Thank you very much. Thank you so much for having you. Thank you for listening. Plain English is produced by Devin Manzi.

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