SHANKAR VEDANTAM, HOST: From NPR, this is HIDDEN BRAIN. I'm Shankar Vedantam. In the summer of 1968, a scientist created a small metal room at the National Institute of Mental Health in Maryland. It was known as Universe 25. And if you got close enough, you could hear its eight tiny inhabitants...(SOUNDBITE OF MOUSE SQUEAKING)VEDANTAM: ...Mice.(SOUNDBITE OF MUSIC)VEDANTAM: Universe 25 was a mouse utopia. There were tunnels to play in and strips of paper to nest in. Food and water were plentiful. There were no diseases to get sick from, no cats to fear, no rain, no snow, no wind. All there was to do was eat, play and reproduce. And reproducing was key because the scientist, John Calhoun, had a question. What would happen as the population of mice grew larger and the dimensions of the room felt smaller? Would a change in numbers set off a change in behavior? Soon, the mice began to have babies.(SOUNDBITE OF MOUSE SQUEAKING)VEDANTAM: Then those babies grew up and had their own babies. And those babies grew up and had their own babies. And on, and on, and on - ballooning from dozens of mice to hundreds of mice to thousands of mice, all crowded together in a 9-foot-by-9-foot metal room, twitching, scampering, screeching.(SOUNDBITE OF MICE SQUEAKING)VEDANTAM: Soon, it didn't feel like a utopia anymore. The details of what happened are disturbing. There were too many young mice and too little space. There were few places to nest and not enough room at the top of the mouse social hierarchy. Eventually, a horde of lower status male mice turned into outcasts, spending all day huddled in the center of the room vacant and listless. With so many males withdrawn and distracted, nursing females had no one to defend their nests, and they became aggressive. Social order broke down.(SOUNDBITE OF MOUSE SQUEAKING)VEDANTAM: At one point, nearly every mouse in the enclosure had a raw, injured tail from being bitten by other mice. After a while, the mice stopped reproducing. Eventually, the entire population was dead.(SOUNDBITE OF MUSIC)VEDANTAM: John Calhoun watched closely as his rodent heaven became a rodent hell. And then he reached a controversial conclusion. Universe 25 was a metaphor for us. It was every city, suburb and village projected into the future all around the world. With too many people competing for partners, shelter and social standing, we would eventually destroy ourselves just like those mice.(SOUNDBITE OF MUSIC)VEDANTAM: This week on HIDDEN BRAIN, we revisit that dire prediction made half a century ago. And we find a twist in the doomsday story far stranger than anything John Calhoun imagined.(SOUNDBITE OF MUSIC)VEDANTAM: In the decades since John Calhoun created his mouse utopia, the world has changed dramatically. Many more people live in cities. They work at jobs that didn't exist in the 1960s. And their choices about whether to get married and have kids are often very different from those of earlier generations. Gerontologist Sarah Harper looks at the connections between these changes and what's happening on a large scale with the planet's population.In her book, "How Population Change Will Transform Our World," she shows how large-scale demographic shifts shape intimate aspects of our lives from the number of children we choose to have to how we care for aging parents. They also have enormous implications for our economic lives and the well-being of the planet.Sarah Harper, welcome to HIDDEN BRAIN.SARAH HARPER: Thank you.VEDANTAM: In 1968, Sarah, a Stanford University biologist named Paul Ehrlich published a book. And it was titled "The Population Bomb." What was its central message?HARPER: The central warning of that book was that we were going to have massive, out-of-control population growth - so much so that we were going to increase to about 24 billion people on the planet and that that was just going to be totally unsustainable.VEDANTAM: So I want to play you a little tape about the kinds of fears that books like "The Population Bomb" spurred. This piece of tape I'm going to play you is from a recent documentary. Take a listen.(SOUNDBITE OF TV SHOW, "RETRO REPORT")HOWARD K SMITH: Overpopulation, so long predicted, has stolen upon us. It's getting worse week by week.MASUD OLUFANI: In the 1960s, a new kind of fear began to spread across America.SMITH: The U.S. could be busting out at the seams by the end of this century. If we do not, by humane means, limit our numbers, then numbers are going to be limited by more famines and shortages and consequent social conflicts.VEDANTAM: I want you to talk about this idea for a moment, Sarah, which is that - the fear was that overpopulation could actually destroy us. And this fear influenced not just, you know, people reading books, but it influenced policymakers and planners all over the world, right?HARPER: That was the time when we actually thought that maximum world population was going to hit about 24 billion by the end of the century. If you remember, there was the old Malthusian idea that had occurred in Europe and that had occurred a couple of hundred years earlier that where you have sex, you have babies, and because people wanted sex, they were always going to have babies. And as a consequence, the population was going to outgrow the capacity of the world known at that time to feed itself. And that was a slightly more sophisticated argument that was appearing in the 1960s. But basically, the idea was the same, that we simply would not be able to feed, water, house the amount of people that the planet was going to expect.VEDANTAM: So demographers have a term called the replacement level. It's a somewhat clinical way to describe an intuitive concept. When women of childbearing age in a country have around two kids on average, the population stays flat. If it's more than two, the population rises. If it's less than two, the population falls. You have an amazing statistic about the replacement level in two-thirds of the countries on the planet. What is it?HARPER: Without us actually realizing this was happening, I would say about 10 to 15 years ago, the statistics began to come through that we were actually - in two-thirds of the world's countries, women were at, near or below replacement. And that was really surprising because it had taken European and the North American region around about nearly 200 years to go through the demographic transition, to go from high childbearing to low childbearing. And here, suddenly in about 25 years, we realized that particularly throughout many parts of Asia, many parts of Latin America, southern and eastern Europe, we had women who were coming down from quite large childbearing to, in many cases, under two.VEDANTAM: The change that Sarah is describing is dramatic enough that it's worth reiterating. In many parts of Europe and North America, women gradually began having fewer babies. This shift took place over a span of two centuries. But the same shift took only 25 years in parts of the world that went through the transition more recently. Sarah says that demographers anticipated the population changes we see, just not how fast they would unfold.HARPER: Because I think demographers actually get the trend right, but they get the pace wrong. And we didn't realize how quickly it was going to happen. And that was really, really important because when I say we were going to hit 24 billion, part of the reason was the pace at which we were having children and the fact that that pace would slow but it was going to take so long to slow that the numbers would be huge before we really started to reduce our childbearing. And every baby girl that's born in any part of the world, of course, is a potential mother, and so it's very, very easy for populations to just grow dramatically if you have large numbers, particularly of girls, being born.VEDANTAM: So these demographic changes have enormous implications for our economies, for social safety nets, for education, health, for immigration. In fact, it's hard to think of a domain that won't be touched by these changes. And we'll talk about the implications in a moment, but I want to start first by understanding the nature of these changes. You say there are typically four broad stages as countries make the transition from high population growth to flattened population growth to sometimes a declining population. What are these stages, Sarah?HARPER: So we start off with very high levels of childbearing and very high levels of death. And that's definitely what Europe was like 200 years ago. And then what typically happens is infant mortality starts to fall. And because infant mortality falls, then women begin to realize that if they have large numbers of babies, they're going to have large numbers of children, and so we move into the third stage and childbearing falls. And then eventually, we have low childbearing, low death rates. So people are living longer. And we reach that final stage, which is what's happening in the majority now of the high-income countries, where most babies that are born have the opportunity to live long, healthy lives.So in Africa, where we have very high fertility, we have large numbers of children. In our middle-income countries, where they've reduced their children - therefore they have a low number of child dependents - they haven't yet started aging, so their big boom is in large numbers of workers. And then we come to the high-income countries, where we have low childbearing and low death rates, and on the whole, we have an aging population.VEDANTAM: To help us visualize how a demographic curve in a country can shift over time, let's travel back to England in the 1800s. In the early 19th century, life there wasn't so good.HARPER: So definitely in England, we had a world where there was very poor sanitation. Most people ate pretty rubbishy food, to be perfectly honest. Food distribution wasn't particularly good. And as a consequence, disease was high. People got lots of infections. A lot of the infections they got was just from the environment that they lived in. Animals and people in rural areas lived very close together. We still didn't have basic sanitation in many European towns. And so death was present everywhere.But what we did across that century was quite remarkable. We started to really improve sanitation. We made sure that clean water was available. Food transport systems increased tremendously. So suddenly, people who lived inland could be eating fish, the availability of meat. We had - even the concept of being able to really properly preserve food from the summer months. And as a consequence, people just became healthier. They could ward off disease, and they began to live longer.(SOUNDBITE OF MUSIC)VEDANTAM: By the time Asian and African countries began the transition, the technology behind vaccines and many other public health interventions were readily available. Systems to dispose of waste and to pipe and clean water had been invented. This meant that countries in Asia and Africa could do in decades what had taken Europe a century or more. Because the drivers of demographic change unfolded differently in different parts of the world, Sarah says this has produced bulges in different populations in various countries. It turns out that your country can have very different challenges and opportunities depending on whether there are a lot of young people, a lot of working-age people or a lot of old people.HARPER: As these countries go through what we call a demographic transition, we have these bulges of different age groups, and they really have an impact on a society. So for example, if you look at many of those societies that are still in sub-Saharan Africa, they have huge numbers - up to 40%, 45% of their population will be children.And if you have a society which has large numbers of children, then a lot of those societies resources, both the national resources and the resources of communities and households and families, is focused on keeping that great big bulge of the population which are children - developing them, educating them, stopping them from dying, keeping them safe. And that means that it's actually quite difficult for those societies then to make that leap into what we call the demographic dividend, when we move from young children into being adults and able to be productive. And we can be productive in different ways. We can work in the family or household economy. We can work in the formal or the urban or economic economy. Or we can be carers. But it means that we are contributing more to our society and our community than we're taking from it.And then if you move into those societies which are ageing, once we have not so much older adults but older, frailer adults - and that's the sort of very old and, typically, in many societies now, it's people probably in their 80s and 90s - they, again, stand to draw down on society's resources rather than necessarily contributing to them. And that's why economies and communities and societies get affected by the bulge in the age group of their population.(SOUNDBITE OF MUSIC)VEDANTAM: Demographers once used the image of a pyramid to describe what populations look like in most countries of the world. There were lots of babies and kids, a sizable number of working-age people and a very small number of older people at the top of the pyramid. As people started to live longer lives, the pyramid turned into a skyscraper - more and more people living into their 70s, 80s, 90s. Increasingly, today the skyscraper is turning into an upside-down pyramid - a small number of kids, a shrinking working-age population and lots of older people.When we come back, we consider the challenges and opportunities of a world where many people will live past the age of 100.(SOUNDBITE OF MUSIC)VEDANTAM: Sarah Harper makes the case that concerns over a population explosion have increasingly been replaced in many countries by concerns over a population implosion. While many poor countries are still growing rapidly, many others are flat, and lots of rich countries are shrinking. Sarah says this story is rooted in the different choices women are able to make in various parts of the world. She looks at these choices through case studies of women, including a woman she calls Varya. She met Varya, a hotel worker, on a trip to Singapore.HARPER: Varya is lovely. I'm still in contact with Varya. So Varya was a very bright, bouncy, intelligent woman. And when I met her, she was in her early 30s, and she told me her life story. And she said that when she was in her teens, she had travelled from Malaysia to Singapore, and she had got a job in one of these very big American hotels. And first of all, she was the basic cleaner. And slowly, she'd worked her way up. And now she bought the room service.And she said to me, I've got a problem, and the problem is that for many years, one of the boys that I went to school with who lives in my hometown back in Malaysia, he wants to marry me. I didn't want to get married because I love my life in Singapore. The hotel is really good to me, and I'm earning money, and I have a great life. And I knew that if I got married, I would have to go back and live with my family and with his family, and I didn't want to do that. But eventually, a couple of years previously, she decided to get married. And she said another problem is he wants to have a baby, and this time I will not be able to stand out against the family. At the moment, I'm still working in the hotel, but if I have a child, I will have to be back in that community.But the really interesting thing is the second part of the story, which is that having had this conversation, a couple of years later I went back to that hotel and I tracked her down and I said, you know, how are you? And did you ever have your child? And she said, yes, I did. I had a little boy. And in fact, the hotel were really, really good to me. They have changed my hours, but my son is being looked after by my mother-in-law, and it's fantastic. You know, in fact, I can combine being a mother and working in Singapore. And I said, what about another child? And she said, absolutely not. I will only have one child.And that, I think, is a really, really dramatic story which shows about how quickly this transition can occur. So she came from a very, very large, rural Malaysian family, and yet she had been able to make that decision to have just one child. So in one generation, you go from six to eight children down to one. But you can see the dynamics. You can see why that might happen.(SOUNDBITE OF MUSIC)VEDANTAM: This story reveals why you see vast changes in fertility rates within a short timeframe. To be sure, these changes are uneven across the planet, not just between countries but within them. But the concerns raised in books such as "The Population Bomb" or in John Calhoun's terrifying experiment with mice - that we wouldn't be able to feed ourselves, that famine and war were inevitable - that horror story has largely not come to pass. In its place, however, the outline of a new problem is taking shape - aging populations with shrinking workforces.HARPER: What we have to understand is that the 21st century is a real time of demographic transition, that across this century, we're going to see - we think - nearly all countries in the world ending up with smaller, older populations where we are living longer and having fewer children.VEDANTAM: I'd like to spend a few moments looking specifically at the United States. It seems to be something of an outlier in the demographic trends that you're describing. It's a wealthy country that does not seem to be shrinking in population.HARPER: So the United States is particularly interesting. It is rich. It is economically still vibrant. It still has a reasonably high birthrate for a developed economy. But it's not shrinking because it still has a pretty buoyant in-migration policy. If you compare that with two European countries - so when we look at the aging countries in Europe, there are two countries that are less old - the U.K. and France. Why is that? It's because we've always compensated for our aging by having immigration of both skilled and less skilled peoples into our population. That has boosted our childbearing rate, and it's kept our economies buoyant. And that is exactly what the United States has been able to do.I used to live in Chicago, and I loved living in Chicago. My children went to a school where - they used to call it, you know, the rainbow school because there were so many different peoples from all over the world. And I think the really important lesson is to say that demographically, moving around the world is actually a very natural balancing act. And, you know, one should welcome that kind of opportunity both to be able to go out into different places but also to accept peoples from across the world. It drives economies, and it changes cultures, and it is the future.(SOUNDBITE OF MUSIC)VEDANTAM: Sarah, in our research for this episode, we accidentally stumbled on another Sarah Harper, who also happens to be from Oxford - well, actually, Oxford House. It's a community in Canada. I want to play you some tape.(SOUNDBITE OF ARCHIVED RECORDING)UNIDENTIFIED GROUP: (Singing) Happy birthday, dear Sarah.UNIDENTIFIED REPORTER: A celebration as Sarah Harper turns 109 years old. She thanks the crowd in Cree.VEDANTAM: So Sarah Harper died a couple of years ago after she crossed the age of 110, which made her what's known as a supercentenarian. So this question is for you, Sarah. Are we going to see many more Sarah Harpers in the decades to come?HARPER: I think that's wonderful, and I do hope so. Yes. All the evidence at the moment is that our populations are living longer, and our oldest old are also living longer. So that was always a big discussion. Why do we live longer? We live longer because death rates start falling across the life course. And at one point, what we thought was that we would push back death, which was - you know, on average in the 40s, when - you know, half the population of Europe was dead by their mid-40s 150 years ago. We thought we would push death back, but actually, once we got to 65, most people would be dead by 70 or 80. Now we know that as we push death back further and further - so we're really beginning to increase longevity, so much so that, for example, some recent research suggested that half the babies that are currently born in Japan are likely to make it to 107. Half the babies currently born in the US will make it to about 104.VEDANTAM: Wow.HARPER: So I think this idea - I mean, you know, my grandmother died in her early 70s, and everyone went, well - had a great life. She died in her early 70s. Nowadays, if you - if people die in their early 70s, you go, gosh, they were young because we have pushed back in our high-income countries so that we expect most people to make it into their 80s. And we know that, you know, many, many people make it into their late 80s or even 90s. And the number of centenarians is increasing dramatically. I mean, we did some modelling of centenarians in the States, and by the end of the century, you will have well over 6 million centenarians living at the same time in the U.S. So some people call the 21st century the century of the centenarian.VEDANTAM: One of the major implications around the demographic changes you describe has to do with something called a support ratio. What is this number, Sarah?HARPER: So the support ratio is typically the number of people who are of working age - and, therefore, if you like, producing and being productive - to those people who are over that age. And people get very worried about that because they say, look; historically, we had five workers to one older person, and now we're only going to have two workers to every one older person.VEDANTAM: Sarah emphasizes that the effects of a shrinking support ratio are not in the distant future.HARPER: I think one of the things we have to understand is that it isn't going to happen. It's happening now. So I would say that in most high-income countries, most men and women - not millennials, but most men and women in their 50s and even 60s are actually already feeling the impact of the aging population. So if you go back and you look 20, 30 years ago, we didn't have huge numbers of people in their 80s and 90s. Now we do, and most people in their 50s and 60s either are caring for, helping to support or who have cared for older adults. It's happening now.I mean, the strain on particularly women in their late 40s and 50s, I think, is extraordinary. So women are asked to be in the labor market. Some of them are becoming grandmothers and are being asked to look after their grandchildren so that their children can go into labor market, but they're also caring for their older parents, who are in their 70s and 80s. And in some cases, they're caring for their grandparents.VEDANTAM: I want to talk a little bit about how the changes that are underway might affect social security nets. When we first created pensions, what did we have in mind in terms of how long they would support people? And how have the demographic changes underway changed or complicated that picture?HARPER: The thing about pensions is exactly - why did we have a pension system to start with? And when pensions were introduced, they weren't to enable people to have long periods of leisure after a working career. That's not to say that isn't something that actually is quite beneficial, but that's not what it was all about. It was just to support people in the last few sometimes months or just one or two years before they died because they were no longer able to work. And to put it into perspective, when Bismarck introduced the concept of the old age pension into Europe, half of the European population was dead by 45. And yet he took 65 at the age at which the pension should start. So if you want to extrapolate that now, actually, state pension ages in high-income countries should be 103.VEDANTAM: Other reports suggest that Bismarck set the retirement age at 70. Whatever the exact number, it suggests that a comparable retirement age in modern times would be significantly higher than it is today.HARPER: Because half our population makes it now to 80. So you can see how a system that was a safety net became, in the 20th century, if you like, a concept where people felt that they had a right to this period of funded leisure at the end of their lives. And I think nobody will push back against that. It's just that instead of it being five or 10 years, it's actually, in some cases, 20 or 30 years of funded leisure at the end of our lives. And I think people are beginning to say that that is just unsustainable when we are going to have - remember - not only people living longer but, at the moment, the large baby boom cohorts all coming up into old age at the same time.VEDANTAM: So what I'm really hearing you say is that population change has really raised serious moral questions because governments need to decide who to distribute resources to. You can obviously invest in both the young and the old, but at some point, there are trade-offs, and policymakers are going to have to confront that.HARPER: I think one of the really interesting things, which I say in the book, is to grow old in a society that is itself aging is very different from growing old in a young society. And if you look at the case, for example, in Africa, where they're - in sheer numbers, there are so many older adults. But they're a very small percentage of the population because there are so many young people, and therefore, most of the resources are invested in young people. If you look at the States and, in particular, many European countries and some of our Far Eastern countries like Korea and Japan, where they have very, very few young people and a large number - and a large percentage of older adults, inevitably, resources are going to be shifted to the older adults.And I think one of the things that we now are talking about is what we call the generational contract, and the generational contract exists everywhere. And it's the idea that when you are an adult, if you have a child, you look after that child. And then that child grows up, you become elderly, and the child looks after you. And in a traditional society, that would be done within the household of the family. You actually physically care for your older parents and grandparents. And in a modern society, we do it through health care, long-term care, pensions. And young people pay taxes, and that funds a whole system of support for older people.And I think there's real pushback at the moment, again, because of this very, very large generation of baby boomers who are really distorting the population that some young people are saying, actually, should we rethink this? And some older people are saying, maybe we need to work longer. We need to be able to save more. We need to really look at the whole way that pensions work so that we can look after ourselves in our old age much more as we go through this sort of bulge of older adults which is occurring across this century.VEDANTAM: So some rich countries are trying to reverse declines in population, partly keeping these trends in mind. You mentioned the examples of Romania and France as two countries that have tried to increase their birth rates, but they went about it in very different ways. What did they learn, and what can we learn from them?HARPER: I think the message from those countries that have tried to boost fertility - and, of course, in Romania, it was very draconian. They absolutely banned, basically, contraception and abortions, and they forced women to have children. And as soon as that particular regime fell, then their birthrate just plummeted. And Romania now has one of the lowest birthrate in Europe. France has done it very differently. France has tried to give financial incentives. But I think we have to step back. We know that there is huge pressure on our environment, and high-income countries consume far more than any other peoples on this planet. And therefore, I think we really have to ask ourselves, is it right for high-income countries to try and boost the number of children they have and therefore increase the consumption that they have on our planet?And the second thing is, demographically, it simply doesn't work. So Hungary, at the moment, for example, is a country which is trying to encourage women to have more children because they're worried about their declining population and the falling number of workers in their population. They don't want to use immigration, which is the natural way to support an aging population. If they're successful, all they're going to do is they're going to have 20 years of lots and lots of children because it takes 20 years before you have basically a baby able to go into the labor market.So they're going to have lots of older dependents, lots of younger dependents. They're actually going to make, if you like, the support ratio or the dependency ratio - that is, workers to dependents - worse, and all they're doing is just delaying the aging of their population. So it's better that we should understand that you don't need large numbers of people in the 21st century to drive an economy.VEDANTAM: There's a world with a shrinking population. Is that a good thing when it comes to climate change? I want to talk about a couple of the potentially positive implications of the changes we're seeing. And it seems to me, from what you just said, that a smaller world is potentially a world that has less impact on the environment.HARPER: Absolutely. I mean, nowadays we don't talk so much about population size as population consumption or carbon footprint. And I have to say, one of the worst countries - I'm really sorry about this - is the U.S. You consume, per person, four or five times the Earth's resources as most Africans do. Europeans, we're two to three times more. We all have a responsibility to say, look; it's unthinkable that people in the Southern Hemisphere are going to live in the kind of poverty that they're living across the coming decades. We've got to allow them and support them to increase their consumption of food and water and goods and raise their standard of living. And the only way we can do that is by reducing the consumption in the North. And one way we can do that is by not having large numbers of children that are going to contribute to our overall consumption.So I think a declining population is actually a good population. And we're scared of an aging population, but we shouldn't be scared of an aging population so long as we can maintain health in our population. And if you think of Sarah Harper, who obviously was - you know, seemed to have had a very good, healthy life and then died at a very old age, well, that's what we should be aiming for so that we have long-lived people who are contributing across their lives. They're healthy and fit. And we really sort of squash frailty and dependency just into the end of our lives, and then we don't have to be worried about an aging population.(SOUNDBITE OF MUSIC)VEDANTAM: Sarah says many of the demographic changes she sees around the world have also shown up in her own life.HARPER: So 15 years ago, my parents moved to my village. And they were fantastic, active grandparents for my family - I've got three children - absolutely wonderful. And then about six or seven years ago, they became a little bit frailer, and then four years ago, my father very, very suddenly died and left my mother, who was in her early 90s. And she moved in with us, and we cared for her for two years, and then she died.And so I would say that I, like nearly everyone I know, had this period of four or five years where I was completely focused on the well-being of my parents. And as I say - just everyone I know of my age is in that situation. But I have to say it was also fantastic for my children. So my children were in their late teens, just tipping into their 20s. They were at university, but they were coming home a lot. And I think being with my parents, their grandparents, at that stage of their lives was an incredibly rewarding thing. I mean, they were all with my mother who died at home. And I think they learned so much about the value of life.And in particular, you could say that my mother, when she was in her 90s, you could say she wasn't contributory, but she was incredibly contributory. She really contributed to our family by being an older person living with us and just being part of our family and helping. You know, we live in a very age-segregated society, and I think the more we can bring the generations together, and my children as teenagers really valued my mother's experiences. And so yes, it just - it has been a very personal sort of journey over the last few years.(SOUNDBITE OF MUSIC)VEDANTAM: Sarah Harper is a professor of gerontology at the University of Oxford. She's the author of "How Population Change Will Transform Our World." Sarah, thank you for joining me today on HIDDEN BRAIN.HARPER: Thank you.(SOUNDBITE OF MUSIC)VEDANTAM: This week's show was produced by Lushik Wahba and Laura Kwerel. It was edited by Rhaina Cohen and Tara Boyle. Our team includes Jenny Schmidt, Parth Shah, Thomas Lu and Cat Schuknecht. Our unsung hero this week is Alison Hofer. She's a support engineer with NPR's digital media team. That means she's one of the people who respond to our pleas for help when something goes wrong in publishing our podcast. Thank you, Alison, for working so hard to troubleshoot technical snafus and for doing so with calmness and collegiality.For more HIDDEN BRAIN, you can find us on Facebook and Twitter. If you like today's show, please remember to share it with one friend. I'm Shankar Vedantam, and this is NPR.(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.