Our latest Freakonomics Radio episode is a rebroadcast called “Why Do People Keep Having Children?” (You can subscribe to the podcast at iTunes or elsewhere, get the RSS feed, or listen via the media player above.)

At issue: Even a brutal natural disaster doesn’t diminish our appetite for procreating. This surely means we’re heading toward massive overpopulation, right? Probably not.

Below is a transcript of the episode, modified for your reading pleasure. For more information on the people and ideas in the episode, see the links at the bottom of this post. You’ll also find credits for the music in the episode noted within the transcript.

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Hey podcast listeners, question for you today: why do people keep having children? Is it because we like the idea of building a family? Is it essentially a vote in favor of the future of civilization? Is it an altruistic impulse — to give someone else a shot a living in this wonderful (or, perhaps, not-so-wonderful) world? Is it strictly a biological imperative — or, even further down the ladder, is a baby nothing more than a very substantial byproduct of the sexual urge? It’s a hard question to answer, and the answer surely varies a lot from parent to parent. As for me, personally, the answer’s pretty easy. I had kids just so I’d have material for this podcast.

MONTAGE of the voices of Anya and Solomon Dubner

Today we’re bringing you an episode from our archives called “Why Do People Keep Having Children?” Thanks for listening.

[MUSIC: Madrona Music, “Stomp It”]

EMILY OSTER: My name is Emily Oster. I am an economist. I work at Brown University. STEPHEN DUBNER: Cool. Let me ask you this easy question, we’ll start. Why do people have kids, Emily? I mean biological imperative apart, unless it’s all the biological imperative. OSTER: I think this is probably an open question for debate. I think many people would tell you that it’s the biological imperative. I think that some people would tell you, kids are enjoyable. I think some people would tell you, particularly in developing countries, people have kids as an investment in their old age or even to work on their farms when the kids are young. So, I think those are probably the leading candidate explanations. DUBNER: OK, and those are all good and believable in theory. Do we have any data that suggests that we actually know anything about this question, or not really? OSTER: We have some data and I think that probably you’d see all of those things show up as explanations. I mean, we certainly see people having remittances from their kids and telling us that remittances from their kids are an important reason to have children. And certainly, many people will tell you, “I enjoy having my kids — maybe not at every single moment but that broadly I like them.” And that that was a reason to do it. But I think it’s hard to separate the biological imperative because, of course, your biology is telling you that that’s something you want to do and once you’ve done it you hardly want to say, “actually, that was all a mistake.” DUBNER: That was all biology. OSTER: That was all biology. Exactly.

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[MUSIC: Malibu Manouche, “The Blue Boat” (from As The Crow Flies)]

We’re talking today about fertility, and then mortality, and then how fertility is affected by mortality. We might even get to overpopulation. The economist Emily Oster has done a lot of research on fertility; she’s also the author of a book called Expecting Better: Why Conventional Pregnancy Wisdom Is Wrong — And What You Really Need to Know .

DUBNER: Let me ask you this: When you bring up the different economic parts of the reason why people have kids, or maybe explain after the fact why they’ve had kids, what do we know generally about the factors that make a given person or family more or less likely to have kids? Let’s say socioeconomic status? Maybe the political environment in the place you live? Personal philosophy or religion? Access to technology? I mean those are just a few of the ones we can think of. OSTER: Yeah, I think many of those things are going to matter. I think certainly what kind of birth control you have access to is going to influence how many kids you have because your ability to control it is influenced by birth control. I think income is an important component of this. One of the most striking facts about fertility over the last several decades is that as many countries have gotten richer, fertility has gone down really dramatically, even in places that used to be very poor. And I think many people interpret that as an effect of income; that as you get richer you want fewer children for various reasons. So I think that looks like a very strong driver of this effect. DUBNER: You wrote a paper that I love about India and the introduction of TV in different areas of India which allowed you to measure well the changes that TV brought. Can you talk about that in terms of fertility? OSTER: Yeah, so in that paper we look at what happens when people get access to cable television and we find something which actually has also been echoed in this study in Brazil, which is that when people get access to cable TV, which really lets them watch soap operas, it actually decreases their fertility. And one interpretation of that is that people see the people on TV, they have fewer kids, and they have this really fancy life, presumably because they’re on television. But you know, maybe if I had fewer kids I could have that also. And I think that’s sort of one interpretation of that. DUBNER: I guess another interpretation might be that soap operas are not as sexy as previously thought? OSTER: No, true, and also people are too busy watching TV to have sex is another interpretation we considered. DUBNER: What do we know if anything about — I don’t know quite what to call it, I guess “mood” or “affect” — whether, let’s say, someone who is optimistic about their own future, maybe their country’s own future, is more or less likely to have children. OSTER: Yeah, I think the best evidence we have from that is about what happens in recessions, which is that fertility goes down. So even though broadly, as countries get richer over periods of decades — fertility goes down as money goes up — in the short-term, when we see a recession we tend to see declines in fertility. And that sort of suggests that when people are less optimistic and, you know, maybe things are more complicated economically, that that’s a time when people will choose to delay. DUBNER: But how can you tease out the economic optimism versus the economic reality. In other words, maybe I want to have kids right now, but I or a partner just lost a job or just got downgraded somehow. How do you tease out the mental effect? OSTER: That is a very hard thing to tease out. I think you can do it by looking at people who haven’t lost their job; on the other hand those people may be more at risk of losing their job, so separating those things out is hard and looking at people’s affect by asking them, “how do you feel about this?” and so on is of course confounded by a lot of other things. You want to find something that changes people’s mood about, say, their economic circumstances or something else, without actually changing those circumstances, and that thought experiment is very hard.

[MUSIC: Louis Thorne, “Waltz for Robin”]

DUBNER: So to me, one of the most interesting, if not necessarily broadly compelling arguments for the relationship between, let’s say, optimism and fertility is contained in a paper you wrote that’s so fascinating. We talked to you about it earlier, about Huntington’s Disease and whether people who are genetically predisposed towards carrying it will want to get the test, and if they do or don’t, how their lives may change, or not change in light of that. So, one surprising finding – at least surprising to me – was that people who know that they may be genetically predisposed to Huntington’s Disease aren’t less likely to have kids, yes? Why is that? Why in your view, and from the data, do you believe that’s true? OSTER: So, I think our view when we looked at the data on this, and again, this is something where we can see the fact but it’s harder to see the interpretation. Our view is that people are not interested in facing the possible negative reality and they would like to, sort of, take actions which make them feel like things are going to be OK. And if you want to have kids, you want to have kids and that is something you’re going to do anyway. And better to do it sooner so you’re healthy for longer with these kids than you would be otherwise. And so, I think that was our interpretation in that. DUBNER: So if I’m an H.D. carrier, or I may be and I don’t know yet, there are two potential consequences to consider if I’m thinking about having kids. One is I as a parent may die relatively early, relatively young, and I as a parent may bequeath to my offspring this same genetic predisposition to this disease. Do you have any idea how those two potentialities are weighed in the minds of people with H.D.? OSTER: I think we don’t, although, you know, from some anecdotal stuff I think one of the pictures you get is that many people are optimistic — and I think we can all be hopeful about this — that there may be a better treatment or a cure for Huntington’s disease in the future. And when you think about a disease like this which has onset in adulthood, like in the 40s or 50s, and you think about having a child now, it’s easy to imagine that maybe something will be different about treatment or cures later on. And so, I think that the feeling of salience about passing on the gene is perhaps less strong than it would be if it were something which you knew would manifest right away, like Tay-Sachs. DUBNER: It strikes me, as we’re talking about this, that optimism, or pessimism, are almost impossible, really, to measure, aren’t they? I mean you can ask people but that doesn’t really get you very far does it? OSTER: Yeah, I agree. I don’t think it gets you very far, and I think people will tell you things which are not right or are very person-specific. My impression from some of the psychology of this is that: people’s level of optimism is pretty similar within a person, no matter what’s going on. So, you know, if something really good happens, you’re happy for like a few days, but you know, then you go back to baseline. And sort of similarly if something really bad happens you’re unhappy for a while but you go back to baseline. And I think that’s sort of something that means it’s hard — a lot of what you’re going to pick up is sort of constant differences across people rather than changes over time that might affect their behavior. DUBNER: Gotcha. OK, so considering that, and considering everything that you, Emily, know about fertility and pregnancy and mortality, let’s say that there’s a terrible, terrible natural disaster that kills hundreds of thousands of people including many, many, many, many children who are very vulnerable to a natural disaster. What would you expect to happen in that kind of circumstance? That people would hurry to repopulate, or that they’d be so stricken that they’d not want to have kids? OSTER: It’s an interesting question. It feels like a question for the data. I think both of those things would happen. My instinct, based on what we know about fertility and high-mortality environments, is that people would try to replace lost children because one of the explanations for why there’s such high fertility in the developing world many decades ago, is that child mortality was so high, you want to have a lot of kids to replace the children that do not survive. And so, an extension of that would be if you lost a child, or God forbid children, in a disaster, that you would want to have more children to replace them. But, as you say, on the other side there’s this terrible thing has happened and there’s sort of incredible pessimism, and so that might affect fertility in the other direction. DUBNER: But it’s not like you can come up with an experiment to answer this question, right? It’s not like you’d want to randomly kill a thousand children in one state and not kill any children in the next-door state and see what fertility looks like in those two states. OSTER: I agree that does not sound like a great idea and I think this is a hard thing to imagine testing for that reason.

[MUSIC: Louis Thorne, “Mon Verrerie”]

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Sometimes nature provides a tragedy that, on top of being a tragedy, is also an opportunity to answer this kind of question. In this case, it happened on Dec. 26, 2004.

MONTAGE of news reports about the Indian Ocean tsunami.

The tsunami killed more than 200,000 people, the vast majority of them in Indonesia.

ELIZABETH FRANKENBERG: One of the things that the tsunami did, in terms of its impact on mortality, was create large gaps in the population. Women were more vulnerable to being killed in the tsunami, and young children and older people as well. And so in the communities that were very badly damaged you could actually see gaps in the age structure and the sex composition of the population.

That’s Elizabeth Frankenberg.

FRANKENBERG: I’m a demographer and a sociologist at Duke University. I teach in the Sanford School of Public Policy, and for the past 20 years or so I’ve been studying various facets of demography and economics, particularly in Indonesia, but in developing countries. And most recently, my colleagues and I have been working on a project to understand how the tsunami that occurred in 2004 affected well-being, both in the short term and the longer term.

[MUSIC: Pearl Django, “The Magpie” (from System D)]

Frankenberg has been going to Indonesia for many years.

FRANKENBERG: Well, the very beginning was way back in 1985. I was an undergraduate at the University of North Carolina, and I — you know my dad was curious what would I do when I graduated. And I said, well, I really want to work on issues in developing countries. And his advice, which was good advice, was, “Well you probably ought to spend some time in one first before you commit too much to that.” And I really love Indonesia, I love traveling there. I’ve made probably one trip a year or so, in many years two or three or four trips, so I’ve gone a lot. I go regularly. I have great colleagues there — a lot of my work is with Duncan Thomas, an economist here at Duke. We worked with our — DUBNER: Now you ended up marrying him a little bit, didn’t you? FRANKENBERG: I did marry him, 100 percent married.

After the tsunami, Frankenberg and her colleagues had a specific set of issues to explore.

FRANKENBERG: Well, we wanted to look at two questions really: what happens to women’s fertility in the aftermath of a shock of this nature, that has several different components to it. One is mortality — mortality in communities but also mortality within families. Another is the trauma that people were exposed to as a result of what had happened, because the people that survived, many of them were caught up in the water or experienced other traumatic experiences. And comparing at an aggregate level, fertility for women from two different types of communities — communities where the mortality impact had been very large relative to communities that had no tsunami mortality, and then within those communities, fertility of women who had lost a child relative to fertility of women who had not lost any children. DUBNER: Had this kind of idea linking mortality — kind of a mortality shock — and the resultant fertility been studied before? Or was this sort of the first of its kind? FRANKENBERG: This is fairly unusual. Really, quite unusual. So, people have certainly been interested in how fertility changes in the aftermath of large-scale social upheavals — wars and things like that. But there have been relatively fewer efforts to look at fertility directly in relationship to levels of mortality. DUBNER: So, what’d you find? FRANKENBERG: So, we found a couple of things. One is that if you start from the perspective of communities, we compared fertility before the tsunami in communities that would later be strongly affected by tsunami mortality, to fertility before the tsunami in communities that did not later experience these high death rates. And before the tsunami, fertility was a little bit lower in the communities that would later experience such high mortality. DUBNER: Which we shouldn’t draw any conclusion from. That’s just random, I assume. FRANKENBERG: Well, yes. I mean that’s — DUBNER: If not random, then maybe is it a socio-economic — FRANKENBERG: Yeah, it’s more of a socio-economic thing. DUBNER: OK. FRANKENBERG: So then we look at what happened after the tsunami and what we find is that fertility rose in the places that had very high levels of community mortality from the tsunami, but fell for women from communities that had not had high levels of tsunami mortality. And so, they really respond in different ways, with respect to fertility. And that introduces this gap. So we see this big fertility increase in the places where mortality from the tsunami was very high. DUBNER: And can you just specify a little bit here. We’re talking about mortality of any and everyone? Or mortality particularly of children? FRANKENBERG: We looked at it from both perspectives, but I’ve been talking about mortality of any and everyone. The results are the same if you limit it to mortality of children. DUBNER: OK, so, so far the baseline of what you’ve established and told us is that in the areas where the mortality from the tsunami was higher, fertility rates went up. FRANKENBERG: Yes, exactly. DUBNER: Alright, tell me more. FRANKENBERG: So, then we also wanted to know, for individual women, whether their characteristics were linked to their fertility behaviors after the tsunami. And so demographers have long wondered about whether when women lose a child — a child dies — whether they become pregnant again, sort of to replace that child — effectively having a birth that would not have otherwise occurred. So, we looked at whether losing a child in the tsunami predicted a birth after the tsunami. And the answer to that question was yes. Women who had lost a child in the tsunami were about ten percentage points more likely to have another birth after the tsunami than women whose children had survived. DUBNER: Ten percentage points. And how many percent would that be? I assume that’s quite a big rise. FRANKENBERG: Yeah, that was about a 37 percent increase. DUBNER: Wow. And did that surprise you, or no? FRANKENBERG: I would say that it did not. We didn’t know what we would find, but we thought that it could have perhaps gone either way. I mean, perhaps women would be stressed and sad and grieving and not ready to do this again. But it might also be natural to have another child. So, we thought it could go either way. So I would not say I was surprised by the finding, but I didn’t have a strong prior that it would turn out that way. So that was one piece of it was that women who had lost a child to death in the tsunami were considerably more likely to have a baby again after the tsunami. And then the other question was whether women who had not yet had children, after the tsunami had children. And what we find then is that as the mortality rate rises in the community, women who had not yet had children were more likely to become mothers. So there’s an interaction between the amount of mortality in the community and women having their first birth. DUBNER: That’s a timing question? Meaning that Woman A in Province A where there was high mortality is going to have a first birth sooner than a woman in Province B where there was low mortality, or would have more children over time, or do you not know that yet? FRANKENBERG: That’s a great question. We don’t know that yet. We know that they have the children earlier than would have otherwise been the case, but whether they will ultimately go on to have more is a question we can only answer when we have data from later waves. We have a ten-year survey in the field right now and that will help us answer that question. But that’s absolutely an important question to know the answer to that we don’t know yet. DUBNER: Right, so let me ask you the most obvious and maybe the hardest question, and maybe there’s no answer for this, or maybe you don’t have the answer for this, but why? So, if you observed a strong boost in fertility among communities where there was a lot of death from the tsunami, and you make a point of noting that there are two kinds — there’s women who lost children, who are quote “replacing,” and women who haven’t yet had children who are having them sooner — is there anything you can say about the “why” other than typical human speculation? FRANKENBERG: So, some of it’s speculation and some of it we know a little bit about. We did ask people after the tsunami whether they wanted to have more children and you know, the women we observe having more children indeed articulated a desire to have children after the tsunami. So. it certainly doesn’t seem to have been an accidental result or a result of poor access to contraception. DUBNER: Right, it could be that the tsunami wiped out all access to contraception. But then on the other hand, there’s available mates, too, how does that factor into it? FRANKENBERG: So, marriage and childbearing are certainly tightly linked. But because so many more women were killed in the tsunami than men, more of the men in our study lost spouses than women lost spouses. So marriage is certainly a part of it and part of that fertility increase is younger women marrying and going ahead and having children. As to the why, we do ask people whether they want more children, we don’t ask them why they want more children, and then you start to be more speculative. But I have to say as I’ve travelled in Indonesia and watched the recovery in Aceh unfold, one of the things that strikes me about Aceh is it’s a place with such a strong sense of identity, of ethnic identity as Acehnese, and so I do think that there was a tremendous will to rebuild. And part of that is rebuilding families and population, but it’s also rebuilding homes and livelihoods, and the province overall. And I think that’s a really important theme of both the fertility results but also in some of the other work we’ve done — recovery more generally.

[MUSIC: Aaron Saloman, “Detroit Stroll”]

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[MUSIC: Niklas Aman, “Seaview” (from Above The Clouds)]

So Elizabeth Frankenberg, a demographer and sociologist at Duke, found that women who lost children in the 2004 Indian Ocean tsunami were much more likely to get pregnant again.

DUBNER: So, I have to say, I guess one way to interpret this finding — one way that sprang to my mind at least, for whatever that’s worth — is that it’s a very, I don’t know about inspiring, but hopeful finding, in that here’s a terrible tragedy that affects a lot of people and rather than swear off community and country and family, the opposite happened. And I’m curious what you think may have driven this on a broader scale? Do you think religion plays a role? Do you think the regional or cultural attitudes play a role? In other words, if a tsunami hit, you know, the southeastern United States do you have any idea whether the findings might be remotely similar, or South America? FRANKENBERG: Well, let me come back to the southeast United States and South America. I agree with you that there’s a strong sort of signal about hope in seeing this impact, this fertility impact. And, you know, traveling to these villages and watching these new homes be built and go from empty, to inhabited, to places where families live and families with little kids live, and these kids are out there playing and helping their parents do various things it really does create this sense of renewal in a way that’s very, very fundamental. So, I think you’re right about this sense of hope coming out of these results. And I suspect that religion is involved in it. Aceh is a very, very strongly religious place. And in addition, the Acehnese people are very proud and independent people, and so I think it’s both faith and religion and also faith in identity, in some sense. And you know, I should say, in addition to the family rebuilding there was all this rebuilding of housing and infrastructure and roads and schools. And that was a function both of Acehnese perseverance, but also of an incredible outpouring of funding for a recovery effort from all around the world, and a very strong effort by the Indonesian government to channel that money into effective reconstruction policies and assistance. And so, I think an important backdrop of rebuilding families is the fact that assets were also getting replaced, people were working, those sorts of economic aspects of recovery as well. DUBNER: OK, and answer if you feel like it, if you feel you have an answer, to the second half of that very long question I asked about whether the same event — not a different event — but whether the same event had happened elsewhere in the world, if you have any knowledge at all into whether we’d see a different effect? FRANKENBERG: Well, I do think that we would see this effect of a fertility rise in places where children are killed suddenly and unexpectedly as a result of natural disasters. There has been a little work on other earthquakes that shows something similar, for example, in Iran. And then when the China earthquake hit in 2008, I guess, schools were very, very badly damaged, and many children were killed. And many of those children were only children, and the Chinese government actually put in place a policy to try to help people who wanted to have an additional child at that point get access to assisted reproductive technology if they wanted to try to have children again. So, I don’t think this fertility response is unique to Aceh, or to Indonesia.

[MUSIC: Pearl Django, “Chutes, No Ladders” (from Modern Times)]

So, we’ve learned that fertility tends to rise after a natural disaster, which might easily be interpreted as purely good news, as a triumph of the human spirit. On the other hand, what about overpopulation? For much of the past century, one of humankind’s most pressing fears is that we are putting way too many people on the planet. Now, among population scholars, there’s a famous saying: “development is the best contraceptive” – that is, as countries get richer, their population growth tends to slow. I asked the economist Emily Oster how well this saying is supported by the data.

OSTER: I think the data suggests that’s very true. Some of the changes in fertility over time in countries that are relatively poor have been incredibly striking. So for example, in the 1960s, India’s fertility rate was at 6; it is now at 2.5. In that period Singapore’s fertility rate was 5.6, it’s now at 1.3, which is far, far below replacement. And even in Kenya, which isn’t a particularly rich country, the fertility rate now is about 4.4, down from 8 or 9 in the 1960s. And those changes have been achieved — yes, there has been some increase in birth-control access, which we don’t want to dismiss — but I think it’s pretty clear from the timing that the development effects, are just incredibly, incredibly important in driving this. DUBNER: So this leads us to a future that doesn’t resemble at all the future that was predicted by demographers not even that long ago — 20 years ago. So, let me ask you this, Emily, if you and I went out and did a survey right now on the streets of New York, on the streets of Providence, and asked a hundred people, the following question: “The world population today is X. What would you expect the world population by the turn of the next century to be in relationship to X?” What do you think would be the median, or the average response there? OSTER: I think people would think the number is 2X or something, but I think the number is not 2X. I mean there’s actually increasingly few countries whose fertility rate is above replacement, and many countries where the fertility rate is substantially below replacement. DUBNER: So the U.N. is currently projecting that the global population will peak at around 8.3 billion in 2050 and then fall to less than the current 7.2 billion population by 2100. So, keeping in mind how terrible most predictions are, let’s take that prediction on its face value. At least when factored into everything that you’ve been telling us about fertility and development and so on, what is the kind of headline for that set of statistics? How would you describe where we’re heading globally in terms of population and how we got there and what it means? OSTER: I think the two factors that this change is likely to produce is one, the population is going to be much older. So, for most of human history the population pyramid has been more people who are young, fewer people who are old. That’s still going to be true to some extent — you’re not going to have the population primarily made up of people who are 120, but the balance between say 65-year-olds and 25-year-olds is going to be very different and that’s something that’s going to drive a lot of policy and the way the demographics work. I think the other thing that’s going to happen is, for a while at least, relatively poor countries are going to continue to have fertility above replacement while relatively rich countries do not, and that’s going to change the balance of population counts across these places even more so than now. DUBNER: So is that the driving factor in lower fertility rates, is income? When you look at the countries with the lowest fertility rates around the world is that what they primarily have in common? OSTER: When you start trying to distinguish among fertility rates all of which are below replacement the links with income become more mushy. So the places with the lowest fertility rates are like, Singapore, and Hong Kong and Taiwan – I mean those are rich countries but they’re not as rich as say Finland. DUBNER: Right, but there are other parts of Asia where the fertility rate is still relatively low, way, way, way, lower than similarly poor parts of Africa, let’s say, right? OSTER: Yes, I agree. So income sort of operates in like the places the highest fertility are very poor — Niger, Mali — but the income fertility relationship does seem to break down a little bit when you get into the very low fertility rates. The U.S. has higher fertility than say, most of Europe, even though the U.S. is relatively richer. DUBNER: That said, fertility is falling pretty much everywhere around the world, correct? OSTER: Yes. DUBNER: So why have we seen such sharp declines in fertility even in so many developing countries, where, kind of the promise of the economic future isn’t there yet? OSTER: I think the promise of the economic future is there more than we think. I mean, even places which are very poor, where things do not seem to be going that great, I think if you look sort of broadly in the last 50 years actually, the economic climate has improved, the health climate has improved. One of the reasons to have fewer kids is because fewer of them die in childhood. Once we start vaccinating people, which we’re actually getting very good at, even in poor places, then you don’t need to have as many kids to achieve the number you want to end up with. That’s something called the “demographic transition” which I think has sort of started happening, basically, everywhere. So I do think actually many, many, many places in the world things are getting better, not worse.

[MUSIC: Tangria Jazz Group, “Ethan’s Song” (from Mebane’s Eleven: Tunes for Two)]

DUBNER: Can you talk to me for one sec about how the reason for having children has changed over the centuries? And obviously there’s still variance around the world, but you know, from sort of captive workforce and insurance policies to, in many countries, at least, consumer durables, or luxury goods. OSTER: Yeah, I mean if you think about a place like the U.S. there was certainly a time — it wasn’t that long ago, 200 years maybe — when a lot of the reason to have kids was you have a farm, there’s a lot to do, people gotta work on it, that was a major source of motivation for childbearing. That really isn’t true anymore. I think over time we’ve sort of moved to a place where children have gotten to be more of an investment and a consumption good and even if you expect to get something out of them it’s kind of very far in the future and it’s more of an enjoyment thing than actually “this person is going to pay for me to live a life of luxury.” So I think that has really changed, and it’s changed the way we approach childhood. I mean, I think there’s a lot of discussion of childhood used to be by the time you’re six you’re up at 4:30 milking the cows, and now it’s like, “I’m taking you to soccer practice at 14 different locations so you can get into the right college” or whatever it is. DUBNER: You know, when people listen to economists, they hear two things that to me at least, sound kind of contradictory. One is if you don’t have enough people being born in a given place you’re in economic trouble, because there won’t be enough young workers to shore up the economy, and the older retiring workers and so on. But you also hear, if you have too many people born in a given place, you know, there won’t be enough jobs and funds for those people and everyone in that place will suffer. So tell me, Emily, are these two arguments as contradictory as they seem to me? And if not, or maybe even if so, is there some perfect, magic fertility rate that makes an economy hum? OSTER: It’s 2.2. DUBNER: OK see you later! Thanks. OSTER: Yeah, thanks very much. No, so, I think both of these things are true. They are true for different reasons. It is true that if you have a very old population there are not as many people to support them. It’s not that you couldn’t design policies that would allow that to work fine, it’s just that the current policies most places have in place are not designed for that. So, if you think about Social Security, it relies on people paying in so it can pay out. If you don’t have enough people paying in you’re going to struggle to pay out. You could design a different system where people pay in and they get out what they put in. That would be sustainable no matter what your fertility rate is, it just isn’t the system we have set up now. On the other side, on the population growth side, I think it is true that as places grow very quickly, in terms of population, that that can be problematic for finding jobs. But again, it’s a question of how you set up the incentives and that can also be good for places because they get a lot of new workers and new people to do stuff and that is an important aspect of economic growth as well.

[MUSIC: Ruby Velle & The Soulphonics, “Heartlite” (from Heartlite)]

DUBNER: So, tell me if the following statements are true: you, Emily Oster, along with your husband Jesse Shapiro, who’s also an economist, currently have one child named Penelope who’s roughly 3 years old. OSTER: Yeah, she’s 3 and ½ DUBNER: Furthermore, you, Emily Oster, presumably along with your husband Jesse Shapiro, once more, are expecting a second child. OSTER: Yes, we are. DUBNER: OK, so, in summary, why? Why are you having a second kid? OSTER: Well the first one is so great! No, I mean I really, having a kid is like the greatest thing that ever happened and I think, you know, we would like her to have a sibling and so we’re going to see how that goes.

Next week on Freakonomics Radio, we speak with the man who was handed the keys to the global economy just as it started heading off a cliff.

BEN BERNANKE: So, look, from a policy point of view, I mean, how big a probability of a second Depression do you need in order to act? I think 25 percent would be enough, but in my view it was probably about 90 percent that, if the panic not been arrested that, the Depression we had would have been much much worse.

Former Fed chair Ben Bernanke. That’s next time, on Freakonomics Radio.

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Freakonomics Radio is produced by WNYC Studios and Dubner Productions. This episode was produced by David Herman, Gretta Cohn, Caroline English, Suzie Lechtenberg, and Chris Bannon, with help from Joel Werner. Our staff also includes Arwa Gunja, Jay Cowit, Merritt Jacob, Christopher Werth, Greg Rosalsky, Kasia Mychajlowycz and Alison Hockenberry. If you want more Freakonomics Radio, you can subscribe to our podcast on iTunes or wherever you get your podcasts, and we’ll deliver another episode every Wednesday at 11 pm Eastern Time. Thanks for listening.