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# Demography

## From Wikipedia, the free encyclopedia

Demography (from prefix demo- from Ancient Greek δῆμος dēmos meaning "the people", and -graphy from γράφω graphō, implies "writing, description or measurement"[1]) is the statistical study of populations, especially human beings.

Demography encompasses the study of the size, structure, and distribution of these populations, and spatial or temporal changes in them in response to birth, migration, aging, and death. As a very general science, it can analyze any kind of dynamic living population, i.e., one that changes over time or space (see population dynamics). Demographics are quantifiable characteristics of a given population.

Demographic analysis can cover whole societies or groups defined by criteria such as education, nationality, religion, and ethnicity. Educational institutions[2] usually treat demography as a field of sociology, though there are a number of independent demography departments.[3] Based on the demographic research of the earth, earth's population up to the year 2050 and 2100 can be estimated by demographers.

Formal demography limits its object of study to the measurement of population processes, while the broader field of social demography or population studies also analyses the relationships between economic, social, cultural, and biological processes influencing a population.[4]

### YouTube Encyclopedic

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• ✪ Joel Cohen: An Introduction to Demography (Malthus Miffed: Are People the Problem?)
• ✪ 07 Demography (Births, Deaths, Fertility & the Ageing Population)
• ✪ What is Demography in Community Medicine / PSM
• ✪ Demography Introduction (Geography) - Binogi.com
• ✪ The Shape of Things to Come: Future Demography Around the World - Professor Chris Whitty

#### Transcription

My name is Joel Cohen I’m Professor of Populations at the Rockefeller University and at Columbia University in New York City.   My background is partially in public health and partially in applied mathematics. WHY SHOULD YOU STUDY DEMOGRAPHY? Why should you consider taking a course in demography in college?   You will be growing up in the generation where the baby boomers are going into retirement and dying.  You will face problems in the aging of the population that have never been faced before.   You will hear more and more about migration into the United States and in some cases, out, into Europe and out between rural areas and cities.  You need to understand as a citizen and as a tax payer and as a voter what’s really behind the arguments.    INTRODUCTION TO PROBLEMS IN DEMOGRAPHY  I want to tell you about the past, present and future of the human population.  So let’s start with a few problems. Right now, a billion people are chronically hungry.  That means they wake up hungry, they’re hungry all day and they go to sleep hungry.  A billion people are living in slums, not the same billion people, but there is some overlap.  Living in slums means they don’t have tenure in their homes, they don’t have infrastructure to take the garbage away, they don’t have secure water supplies to drink.   Nearly a billion people are illiterate.  Try to imagine your life being illiterate.  You can’t read the labels on the bottles in the supermarket, if you can get to a supermarket.  Two-thirds of those people who are illiterate are women and about 200 to 215 million women don’t have access to the contraceptives they want so that they can control their own fertility.   This is not only a problem in developing countries; about half of all pregnancies are unintended. So those are examples of population problems.  ﻿ DEMOGRAPHY AS A TOOL FOR SOLUTIONS Demography gives you the tools to address and to understand these problems.  It’s the study of populations of humans and non-human species that includes viruses like influenza, the bacteria in your gut, plants that you eat, animals that you enjoy or that provide your domestic animals.  And it includes non-living objects like light bulbs, and taxi cabs and buildings because these are also populations.  And it includes the study of these populations in the past, present and future using quantitative data and mathematical models as tools of analysis.   I see demography as a central subject related to economics, to human wellbeing as in material terms; related to the environment, to the wellbeing of the other species with which we share the planet; and the wellbeing and culture which affects our values and how we interact with one another.   WORLD POPULATION: THE PAST The key fact you need to remember, is that since the inventions of agriculture between 6,000 and 14,000 years ago, the population of the earth, the human population, has grown 1,000 fold from approximately seven million to nearly seven billion this year.  Put three zeroes on the end of seven million, you get seven billion.   Over the same interval, the earth has not gotten any bigger.  The continents haven’t expanded 1,000 fold or at all.  The oceans are the same size as they were before.  The atmosphere is the same size as it was before.  So the question that concerns a lot of people and me is whether the impacts that seven billion people or more in the future will have on the earth will endanger, will threaten our own well being and the well being of other species on the earth.  We know that humans have already caused the extinction of many species.  The question is, is that going to come back and bite us, and if so, in what ways?   Demography provides us with a reliable way to imagine and to reimagine the future.  So let’s get down to some nitty-gritty details here.   About 2,000 years ago, there were roughly a quarter of a billion people on the planet.  Today, there are almost seven billion. More than six-seventh of the growth since the beginning of humans 50,000 years ago has occurred in the last 200 years.   To go from a quarter of a billion to half a billion took 16 centuries.  So we reached about half a billion humans about 1600, more or less.  The population of the earth, the human population, if it were growing exponentially would go from a quarter billion to half in 16 centuries and from half to one in another 16 centuries.   What actually happened was that the human population of the earth reached a billion around 1800.  Why?  Because of food stuffs that came from the New World to the old; notably potato and corn or maize.  And because many of the people who were overcrowded in Europe went to America where there were fertile and unoccupied lands to use.  So the East/West exchange, the Columbian exchange across the Atlantic liberated population growth in the European sector, there was a similar development in Japan, an acceleration of population growth around the same time.   In 1800, the Industrial Revolution began and the population doubled from one billion to two billion by 1930, 1927, we don’t know exactly.  Why don’t we know exactly?  Because we didn’t have censuses that covered the whole world at that time.  So it’s a retrospective guess.   So our doubling times went from 1,600 years to 200 years, 1600 to 1800, to 130 years, 1800 to 1930.  The next doubling from two billion to four billion took only 44 years, 1974.   So for the last 2,000 years at least, except for the Black Death in the 14th century, the population growth rate was going up, up, up, up and around 1965, it began to decline.   So in absolute terms and in percentage terms, the number of people we are adding to the planet has begun to slow.    FERTILITY IS THE KEY TO UNDERSTANDING HUMAN POPULATION  Since 1950, humans have made the swiftest, voluntary change in reproduction in human history.  Around 1950 the average number of children per woman, per lifetime was very close to five.  Today, the average number of children per woman is about 2.5 or 2.6.  In other words, billions of people have changed their reproductive behavior to lower the number of children born in a lifetime from five to two-and-a-half, but not everywhere.   In Sub-Saharan Africa, the decline has been much less.  From perhaps 6.6 children to 5.1 over the half… the second half of the 21st century.  To understand the consequences of this fall to two-and-a-half children per woman, you need to know what is meant by replacement level fertility.   So I am going to introduce that by telling you about the theory of bathtubs.  A regular bathtub with no stopper.  So two things happen with a bathtub with no stopper.  Water comes in and water goes out.  And you can see intuitively that if the amount of water coming in per minute exceeds the amount of water going out per minute, the level of water in the bathtub is going to go up, and if the amount of water come in per minute is less than the amount being drained out, the level in the bathtub is going to go down.  So the amount of water coming in that just matches the amount going out keeps the level of the bathtub steady.  Okay?  That’s replacement level bathtub water.    Now, water coming in corresponds to births to the earth.  And water going out corresponds to death.  And the level of the bathtub corresponds to the total population size.  So, if the number of births just matches the number of deaths, the population stays steady and that’s the replacement level of fertility.    Now, you’re asking yourself, what is the replacement level of fertility?  The answer is, it’s about 2.1 children per woman. There has been an amazing transformation in the distribution of fertility across the world.  In 2003, this was not in any newspaper anywhere, but it was a very important event.  In 2003, half of all the women in the world were having replacement level or less.   And now more than half of humanity lives in a country at or below replacement level fertility.  It’s the first time in human history that this has happened.  And it’s important.  But you remember that the total fertility rate, the average number of children per woman is at 2.5, not 2.1.  And that’s because on this curve, the green curve, the folks with high fertility are further to the right of the red line than most of the folks with low fertility are to the left.  So the average is skewed to the right.  So we still have a growing population.  But this change is continuing and how fast it continues is something that you as voters, as potential scientists, as citizens will influence by what you choose to do about the 215 million women who have an unmet need for contraception.   WORLD POPULATION: THE PRESENT So much for the past, let’s go on to the present. This is a population pyramid.  It is one of the basic descriptive tools of demography and you should understand what it is.  Let’s start with the left side of the picture.   The horizontal axis, the width of the bar tells you how many people there are and the vertical axis correspondence to age group.  So the lowest bar is for people aged zero to four with males on the left and females on the right.  The next bar is people age five to nine. The top bar is 95 to 100.  And what you see is that in the rich countries, there are about as many people aged, let’s say zero to four as there are aged 85 or 90, but it’s basically a slender column.   Now compare it with the age pyramid for the poor countries.  The base of the pyramid is enormous compared to the number of elderly.  So there are many more workers to support the elderly, per elderly person.  The width of the bar, again is the number of people, so in the ranges from five to 14 of five to 19, that’s the school age population.  It means that the challenge of educating those children is much greater in the developing countries than it is in the rich countries because those bars keep getting wider as the developing countries pump in more children at the bottom of the pyramid and the age groups move up with time as they get older. And so the larger school age population is followed 10 years later by a much larger military age population.   So if you look at the age groups 19 to 30 or 15 to 30, whatever the legal ages or illegal ages are for fighting, you can see that the potential military force in the developing countries vastly exceeds that in the rich countries.  It doesn’t mean it’s military power for them, it means they can afford a military engagement in a way that the human resources of the rich countries make very difficult, increasingly difficult.   So where is the growth going?  The demographic growth is happening in the countries that can least afford to deal with the additional population.   What’s the average income?  The reason we call a rich country as rich is that their average income is about $32,000 a year per person and in the poor countries it is about$5,000 a year.   What fraction of people are living on less than $2.00 a day? Nobody lives on less than$2.00 a day in the rich countries and 51 percent, just about half in the poor countries.  In other words, about 3.5 billion people on our planet are living on $2.00 a day or less. So you might ask yourself, if things are so bad there how is it that their population is growing so rapidly? And the fact is that the difference in death rates is much smaller than the difference in fertility rates. So even though a higher fraction of children die before they reproduce, the average number of children that people have when they do reproduce in the poor countries more than compensates for the increase in the death rate. So that’s why we have rapid population growth at the same time that we have high fertility, high mortality because we had even higher fertility. The global economic inequality means that the most rapid demographic growth is associated with the people who have the least means to take care of the children that are born and the people with the greatest need for reproductive healthcare and services have the least means to afford it. It’s an important general question, How does the rich world benefit from the prosperity and development of the poor world? There are lots of different answers you can give. One is, purely economic. Richer people in China and Africa will buy more American music CD’s and more movies and more software and more high tech engines from General Electric and more products because they have more means to buy. So that’s one kind of an answer. A second is public health. There are millions of flights in both directions from the poor countries to the rich countries every year. And the microbes don’t know about passports. And they cross from Bombay or Mumbai to New York just a fast as they go from New York to Mumbai. And when there are outbreaks of drug resistant tuberculosis, those can travel around the world and they pose a danger to me and to you guys. So we have an interest in the health and well being. A direct, personal interest in the health and well-being of people in poor countries. WORLD POPULATION: THE FUTURE So now we’ve talked about the demographic past, and the demographic present. And next we’re gonna talk about the demographic future. Woody Allen said, “Eternity is very long, especially near the end.” So, we’re not going to talk about eternity. We are only going to talk about the near term future. How much of the future is relevant to you? Well, according to the United States Life Tables, published by the National Center for Health Statistic. An 18-year-old in the United States in the year 2011 has a 91 percent chance of surviving to 2050, 91 percent. Based on survival rates in 2006. If you behave wisely and if economic and medial progress continues, you have at least that good a chance of making it to 2050. So we’re going to talk about the world from now to 2050. I can say with confidence that four things will happen over the next 40 years or so. The world’s population will get bigger. It will grow more slowly. It will be older in the sense that the fraction of older people will increase dramatically and it will be more urban. And I’m going to go through each of those four to explain some of the details. What we don’t know too much about is what will be the future of migration, the future of household structures, and the future of families. We have some ideas about that, but that’s relatively less certain. THE FUTURE OF THE WORLD’S POPULATION : BIGGER This graph shows four different curves of the history and future population out to 2050. At the top, the curve shows the anticipated population if fertility remains at the level it is today and there’s no further decline in fertility. That’s called the Constant Fertility Assumption. And it shows that the world would go to about 11 billion people by 2050. However, if fertility drops as it has dropped in the past, the medium projection of the U.N. Population Division is that population would rise to about 9.1 billion by 2050. So that’s a difference of two billion. In other words, we’re counting on a continuing decline in fertility to lower the population size by about two billion by 2050. What we do between now and 2050 will have a huge impact on how difficult it is to feed, house, shelter, educate, and provide health for the billions of people on the planet in 2050. It will affect an enormous range of human problems. It’s possible that population growth would end before 2100 depending on the choices we make now. What choices am I talking about? Choices like, educating women, providing credit to women in countries where women are not now allowed to have credit. Providing reproductive healthcare so that women are not forced as they are in some countries to have children when they don’t want to. Raising the age of marriage so that 12-year-olds, 13-year-olds and 14-year-olds aren’t put into marriage. There’s a lot of things we can do to raise people, including even women, raise people’s control over their own lives. And we should be doing those things. I am not trying to persuade people not to have children. I think that is nuts, but I am trying to persuade people to have children that they can take care of and do well for and to focus on quality of children rather than numbers. And, I view demography rather broadly so I think you also need to know how your body works, how contraception works, what’s more reliable, what’s less reliable, what are the factors that affect contraception and how to take care of your own reproductive health and to help your children take care of their reproductive health and your friends and your family. THE FUTURE OF THE WORLD’S POPULATION : SLOWER GROWTH So the first fact about the future is that the population is going to get bigger and the second fact is that the population is going to grow more slowly depending… the slowing will depend on what we do now. By 2050 in the medium projection from the United Nations Population Division, the world will be growing by 31 million people a year. What’s it doing now? It’s growing by 76 million people a year. In the poor countries, the population will be growing by 32 million a year while in the rich countries; the population will actually be declining by a million people a year. Already today in 2011, population is declining in more than 50 countries. Not well-known. What’s happening here is a shift in childbearing desires and action from quantity to quality as people urbanize, as people get educated, as wealth improves, people are making greater investments in a smaller number of children. THE FUTURE OF THE WORLD’S POPULATION : OLDER So we’ve talked about population size, growth rate, slowing. The next is aging. So by 2050, there will be about three times as many elderly as children. This is the first time in human history that the elder population has outnumbered the young population. So what? Well, aging affects energy demand. So even if you are interested in the environment, you need to know about the age structure of populations. Older householders spend, at least in the United States, India and China, the three countries where it’s been studied in detail, spend more than younger households measured by the age of the head on utilities services and healthcare. Utilities are the most energy intensive part of the household budget. That’s not the only reason to care. The rise in the fraction of elderly poses an increasing challenge to a relatively reduced number of workers. And it’s possible that the well-being of elderly people could improve, if they’re educated or could get worse if they are warehoused in old people’s homes. We know for example, that people who are educated in their youth have much lower disability when they get older. And in fact, disability rates at any given age in the United States have been dropping by about one-and-a-half percent per year for the last 25 years. So there are far fewer disabled elderly now than there used to be. That’s the meaning of 60 is the new 40, 50 is the new 30. People are healthier at older ages. That’s the result of investment in education in youth. So, there are policy implications for a rising aged population, we better invest in educating people when young. THE FUTURE OF THE WORLD’S POPULATION : URBAN And the last of the four topics is cities. And I’m going to give you a very simplified and boiled down demographic history of the first half of the 21st century. In 2000, you could divide the world into two equal parts; half rural and half urban, about three billion rural and three billion urban. In 2050 the rural population will still be at three billion and the urban population will have doubled from three billion to six billion. That’s a simplified history. All of those additional three billion urban people will be in poor countries. The equivalent of a city of one million will have to be built every five days from now to 2050 and all of them in poor countries. So what’s the consequence? What is this massive shift towards urbanization mean? If we under invest in cities, we can go from a billion people living in slums today to four billion people living in slums. And if we invest in the cities, if the real estate companies realize the opportunities, the incredible demand that people have to live in decent housing, we could reduce the size of the slums. So, I cannot give you a deterministic picture. I can tell you what would be awful. We could have infectious diseases rampant. We could have warfare in the cities. We could have disorder, but that’s not necessary. We could have clean water supplies; we could have security for people in their houses. That doesn’t get as much news attention, but the point I want to make is, we have choices about the future of cities and many things about urbanization are positive. And I want to tell you a few of the things that are positive. Not because they’re automatic, but because they are positive in fact and because we can enhance the positive. Compared to rural areas, urban areas have lower fertility rates. Why? I’m a woman on the farm. You might not think so, but I am… okay. I’m a woman on the farm. The more children I have, the more help I can have in collecting the water, collecting the fire wood and tending to the goats and sheep. As soon as I move into a city, those children convert from an asset to a liability. I have to buy them clothing. I have to send them to school. There are school fees, the apartment is too small. The incentives completely shift direction. Urbanization makes people want to have fewer children. Cities also have higher usage of modern contraception and lower unmet needs for contraception. Why? I am a woman; you might not know it… I don’t have to walk 15 miles to the nearest health center and be exposed to the inspection of my husband’s friends when I go in for contraception. No. When I’m in the city, I just go around the corner and it’s anonymous. So, urbanization brings many features of liberation as well as changing the incentives. Cities concentrate economic productivity. Eighty percent of the world’s gross domestic product is produced in cities, although there are only 50 percent of the world’s people in cities. Cities generate cultural assets, educational resources, public health resources, medical care and they can promote energy efficiency. Let me give you an example. This graph shows the passenger transport, carbon dioxide per person. The denser the city, the lower the amount of carbon dioxide per person. New York City is reported, according to the Mayor’s office, to have less than one-third the carbon dioxide emissions per person of the U.S. average. People take the subway, people ride the bus. Cities also have hazards. Many cities are built along coastlines. Coastlines are where the continental plates of the oceans collide with the continents. That means they are prone to earthquakes. We just had the big example in Japan, but it’s true all around the world. The Ring of Fire around the Pacific Ocean coincides with where the cities are because cities are coastal. A lot of the world’s urban people live near the coastline and that’s where the subduction zones are in California. So cities are vulnerable to rising sea level, to coastal storms, cities concentrate people so they’re vulnerable to infectious diseases, water supply attacks, and cities are excellent targets for military and terrorist attacks, as we know in New York City and many other cities. It used to be, battles were fought on battlegrounds. No more. They’re fought in cities. And that will increasingly be the case as cities concentrate assets. This is New York City as it is now. The red zone will be underwater if the water level rises by one meter on the average. One meter is probably more than we’ll have in the next 50 years, but could easily happen by the end of this century at current rates. Now, a six meter rise would happen if the Greenland and Antarctic ice masses melted. Six meters is about 20 feet. And that would be a catastrophe in many respects including for me, there’s a little place over here in New Jersey which is my favorite nudist colony. And it would be completely underwater. So that would be a terrible thing to happen. Urban growth could affect the food supply. Right now, cities occupy three percent of the land surface of the earth. The land, the arable land, the land where we can grow food well is about 10 or 11 percent of the land surface of the earth. It’s not surprising that many cities are smack in the middle of the best arable land because that was where a food surplus could be easily produced without having to ship the food. Now, if cities are going to double, we have another choice. Do we double the area from three percent to six percent and eat up our arable land, literally or do we double the density and keep the areas of the cities constant at three percent. This is a choice for the future and it depends on zoning and culture and real estate developers and economics and choices that we make as citizens. CONCLUSION - FOOD: THE WORLD IS RUNNING OUT OF RESOURCES TO SUPPORT ITS GROWING POPULATION How do we address the problems that we have. There are three kinds of solutions that people have put forward; bigger pie, fewer forks, better manners. The bigger pie people say we should use technology to increase production. The fewer forks people say, we should use contraception to reduce population growth and we should consume less material products. And the better manners people say, we should eliminate violence, inequities between men and women, inequities between rich and poor, inequities between young and old. We should eliminate rational subsidies and just make things work more efficiently. Get rid of corruption. We need all of those and I took a few years to try to figure out what’s my best way to support those three strategies, all of them. And I came to the conclusion that the best response would be to educate all children, boys and girls, well for 10 to 12 years, high quality, primary and secondary education. I realized there is no chance of educating people if their brains haven’t been fed adequately, in utero and after birth, especially for the first three years. So I am now moving around to working on the problem of getting food, adequate, good food, to pregnant women, lactating women, and infants up to the age of three because there are many countries where by the time a child gets to school, it’s too late the brain has lost its capacity to learn. It comes back to my ecological interests in food. You can’t educate without a brain that works. So now we’re talking about food. I started this conversation saying that there are a billion hungry people, chronically hungry. I want to come back to this. We depend on other species. Here is a list of what other species provide to humans. And I’m going to read the list because it’s important. Food for people, feed for our domestic animals, fuel, biofuel for example, and wood to burn, biomass. And in many countries people burn dung. The waste products of animals. That’s an important source of fuel. Fiber, so we depend on trees for many paper and other products. Fascination, we love to go to the zoo and see animals. We love to see wildlife. When people go out in nature, they’re thrilled if they see a deer or some other kind of wildlife. In fact, in Central Park, the German tourists are thrilled to see squirrels. We find animals fascinating. Pharmaceuticals, most of our drugs are natural products tuned up to serve human needs. Animals provide transport, they carry people places. They provide traction, they pull plows, they pull carts. Other species provide symbioses. I’ve talked about the animals, not the animals, the bacteria that live in our guts. And they provide infection. They can cause disease. So the question I want to address now is can we grow enough food to bring us to 2050 without catastrophe? These are data from the Food and Agricultural Organization in Rome. They are estimates of the number of people in the world who are chronically under nourished day after day. The current estimate is about 925 million people. That’s nearly a billion and it is higher than the number has been in the last 40 years. Ninety-eight percent of these people live in poor countries. Not only poor countries. Here is something that shocked me and I hope it will shock you. At some point during 2009, 17.4 million U.S. households, one household in seven in the United States, lacked enough money and other resources to provide food for all members of the household. The current level of food insecurity in the United States is higher than it has been since the USDA started collecting the statistics by sample surveys in 1995. We are at an all time high of hunger in this country. So the question you should be asking yourself is, well aren’t we growing enough food? What’s the problem? We’ve got seven billion people and there are a billion of them hungry. The answer is, less than half of the grain that we grow goes into people’s mouths. Divide the world’s grain into six equal pieces. One piece we use to make biofuels, starches, for seed and other industrial uses; plastics. Two-sixths, we feed to our domestic animals of the rich people, those who have the means to afford those animals and meet products. Less than half, the other three-sixths goes directly into human mouths. We could be feeding 11 billion, but we only feed half of that amount, 5 ½ billion into human mouths. We put machines and animals in line before people who don’t have money to express demand in markets. Hunger does not fit into our economic theories. It’s economically invisible because people who are very poor, remember, half a billion are only living on$2.00 a day or less.  People who are very poor can’t enter the market and plunk down their cash and say, “I want that.”  They are invisible economically.  So my hunger does not affect your costs for grain.  This is a problem with our economics.  And it’s a reflection of our values.   CONCLUSION - WHAT’S THE SOLUTION?   So, here’s Joel’s formula for how to solve the world’s problems.   Well, population; let’s go at it with all hammer and tongs.  Eliminate all unintended pregnancies and educate all children to give people control over their bodies and over their own lives.   Economics, open credits and markets to small farmers.  A majority of the world’s farmers are women.  They are the ones out in the field actually doing the work.  Eliminate perverse subsidies in rich countries make it very difficult for poor farmers to enter markets because they lower the price in artificial ways.  And let’s raise the incomes of the poor.   Environment.  Use the best farm lands for farms and internalize the external costs of agriculture.  Get rid of the pollution and use chemicals in a way that doesn’t damage the environment.   And fourth, promote healthy diets and value adequate nutrition for every person.  I would add one other thing under this culture question and that is, we need to fund more research in agricultural productivity for the crops that matter in poor countries.  Not only for the industrial craps that fund our biofuel habit, but for the crops that provide food to the poor.   CONCLUSION - SUMMATION  When you walk away from this conversation, I hope that you’ll remember that population interacts with economics, the environment and culture so that you immunize yourself against people who will try to sell you an overly simple bill of goods.  And there are a lot of people.  There are people who say, “Demography is destiny,” and all we have to do is get a contraceptive in every pot and we’ll solve the world’s problems.  That’s wrong.   And there are people who say all we have to do is get the market right.  Let the market take care of all the prices.  In my view, that is equally wrong and much more dangerous.  There are people who say, “It’s only a matter of law and getting the laws right.”  Yeah, but it’s also a matter of technology and contraception and economics.  And there are people who say, “Forget about the people, let’s just save the environment.”  I don’t believe that because I’m a human being and I value other human beings.  We’ve got to get all of these things working together and the environment can be on the side of human well being because poor rural people depend directly on the environment for their sustenance.  If they want to have a sustainable sustenance, they have to have a sustainable environment.    Demography makes it possible to imagine and to re-imagine the future.   I’d encourage any freshman, sophomore, junior, senior, adult, high school student—I’m not age prejudiced—anybody who wants to do three things to consider demography.  It’s not the only field that offers these attractions but it does offer them in spades.  It’s really very attractive.  First of all, demography gives you tools and analytical perspectives to understand better the world around you.  That’s understanding.    Secondly, it gives you equipment to solve problems mentally.  It’s mentally exciting; you really have to use your noggin, and if you’ve got one use it or lose it.  So it’s use it.  And third, it is the means to intervene more wisely and more effectively in the real world to improve the wellbeing, not only of yourself—important as that may be—but of people around you and of other species with whom we share the planet.    So it prepares you to go out and do something that’s worth doing for a larger good than only yourself.  So there’s an old saying, “If I am not for myself who will be; but if I am only for myself what am I; and if not now, when”?  So now is the time.  Pull up your pants and get to work.  Thank you. ﻿

## History

Demographic thoughts traced back to antiquity, and were present in many civilisations and cultures, like Ancient Greece, Ancient Rome, China and India.[5] Demography is made up of two word Demos and Graphy . The term Demography refers to the overall study of population.

In ancient Greece, this can be found in the writings of Herodotus, Thucidides, Hippocrates, Epicurus, Protagoras, Polus, Plato and Aristotle.[5] In Rome, writers and philosophers like Cicero, Seneca, Pliny the elder, Marcus Aurelius, Epictetus, Cato, and Columella also expressed important ideas on this ground.[5]

In the Middle ages, Christian thinkers devoted much time in refuting the Classical ideas on demography. Important contributors to the field were William of Conches,[6] Bartholomew of Lucca,[6] William of Auvergne,[6] William of Pagula,[6] and Ibn Khaldun.[7]

One of the earliest demographic studies in the modern period was Natural and Political Observations Made upon the Bills of Mortality (1662) by John Graunt, which contains a primitive form of life table. Among the study's findings were that one third of the children in London died before their sixteenth birthday. Mathematicians, such as Edmond Halley, developed the life table as the basis for life insurance mathematics. Richard Price was credited with the first textbook on life contingencies published in 1771,[8] followed later by Augustus de Morgan, ‘On the Application of Probabilities to Life Contingencies’ (1838).[9]

In 1755, Benjamin Franklin published his essay Observations Concerning the Increase of Mankind, Peopling of Countries, etc., projecting exponential growth in British colonies.[10] His work influenced Thomas Robert Malthus,[11] who, writing at the end of the 18th century, feared that, if unchecked, population growth would tend to outstrip growth in food production, leading to ever-increasing famine and poverty (see Malthusian catastrophe). Malthus is seen as the intellectual father of ideas of overpopulation and the limits to growth. Later, more sophisticated and realistic models were presented by Benjamin Gompertz and Verhulst.

The period 1860-1910 can be characterised as a period of transition wherein demography emerged from statistics as a separate field of interest. This period included a panoply of international ‘great demographers’ like Adolphe Quételet (1796–1874), William Farr (1807–1883), Louis-Adolphe Bertillon (1821–1883) and his son Jacques (1851–1922), Joseph Körösi (1844–1906), Anders Nicolas Kaier (1838–1919), Richard Böckh (1824–1907), Émile Durkheim (1858-1917), Wilhelm Lexis (1837–1914), and Luigi Bodio (1840–1920) contributed to the development of demography and to the toolkit of methods and techniques of demographic analysis.[12]

## Methods

There are two types of data collection—direct and indirect—with several different methods of each type.

### Direct methods

Direct data comes from vital statistics registries that track all births and deaths as well as certain changes in legal status such as marriage, divorce, and migration (registration of place of residence). In developed countries with good registration systems (such as the United States and much of Europe), registry statistics are the best method for estimating the number of births and deaths.

A census is the other common direct method of collecting demographic data. A census is usually conducted by a national government and attempts to enumerate every person in a country. In contrast to vital statistics data, which are typically collected continuously and summarized on an annual basis, censuses typically occur only every 10 years or so, and thus are not usually the best source of data on births and deaths. Analyses are conducted after a census to estimate how much over or undercounting took place. These compare the sex ratios from the census data to those estimated from natural values and mortality data.

Censuses do more than just count people. They typically collect information about families or households in addition to individual characteristics such as age, sex, marital status, literacy/education, employment status, and occupation, and geographical location. They may also collect data on migration (or place of birth or of previous residence), language, religion, nationality (or ethnicity or race), and citizenship. In countries in which the vital registration system may be incomplete, the censuses are also used as a direct source of information about fertility and mortality; for example the censuses of the People's Republic of China gather information on births and deaths that occurred in the 18 months immediately preceding the census.

Map of countries by population
Rate of human population growth showing projections for later this century

### Indirect methods

Indirect methods of collecting data are required in countries and periods where full data are not available, such as is the case in much of the developing world, and most of historical demography. One of these techniques in contemporary demography is the sister method, where survey researchers ask women how many of their sisters have died or had children and at what age. With these surveys, researchers can then indirectly estimate birth or death rates for the entire population. Other indirect methods in contemporary demography include asking people about siblings, parents, and children. Other indirect methods are necessary in historical demography.

There are a variety of demographic methods for modelling population processes. They include models of mortality (including the life table, Gompertz models, hazards models, Cox proportional hazards models, multiple decrement life tables, Brass relational logits), fertility (Hernes model, Coale-Trussell models, parity progression ratios), marriage (Singulate Mean at Marriage, Page model), disability (Sullivan's method, multistate life tables), population projections (Lee-Carter model, the Leslie Matrix), and population momentum (Keyfitz).

The United Kingdom has a series of four national birth cohort studies, the first three spaced apart by 12 years: the 1946 National Survey of Health and Development, the 1958 National Child Development Study,[13] the 1970 British Cohort Study,[14] and the Millennium Cohort Study, begun much more recently in 2000. These have followed the lives of samples of people (typically beginning with around 17,000 in each study) for many years, and are still continuing. As the samples have been drawn in a nationally representative way, inferences can be drawn from these studies about the differences between four distinct generations of British people in terms of their health, education, attitudes, childbearing and employment patterns.[15]

## Common rates and ratios

• The crude birth rate, the annual number of live births per 1,000 people.
• The general fertility rate, the annual number of live births per 1,000 women of childbearing age (often taken to be from 15 to 49 years old, but sometimes from 15 to 44).
• The age-specific fertility rates, the annual number of live births per 1,000 women in particular age groups (usually age 15-19, 20-24 etc.)
• The crude death rate, the annual number of deaths per 1,000 people.
• The infant mortality rate, the annual number of deaths of children less than 1 year old per 1,000 live births.
• The expectation of life (or life expectancy), the number of years that an individual at a given age could expect to live at present mortality levels.
• The total fertility rate, the number of live births per woman completing her reproductive life, if her childbearing at each age reflected current age-specific fertility rates.
• The replacement level fertility, the average number of children women must have in order to replace the population for the next generation. For example, the replacement level fertility in the US is 2.11.[16]
• The gross reproduction rate, the number of daughters who would be born to a woman completing her reproductive life at current age-specific fertility rates.
• The net reproduction ratio is the expected number of daughters, per newborn prospective mother, who may or may not survive to and through the ages of childbearing.
• A stable population, one that has had constant crude birth and death rates for such a long period of time that the percentage of people in every age class remains constant, or equivalently, the population pyramid has an unchanging structure.[16]
• A stationary population, one that is both stable and unchanging in size (the difference between crude birth rate and crude death rate is zero).[16]

A stable population does not necessarily remain fixed in size. It can be expanding or shrinking.[16]

Note that the crude death rate as defined above and applied to a whole population can give a misleading impression. For example, the number of deaths per 1,000 people can be higher for developed nations than in less-developed countries, despite standards of health being better in developed countries. This is because developed countries have proportionally more older people, who are more likely to die in a given year, so that the overall mortality rate can be higher even if the mortality rate at any given age is lower. A more complete picture of mortality is given by a life table, which summarizes mortality separately at each age. A life table is necessary to give a good estimate of life expectancy.

## Basic equation

Suppose that a country (or other entity) contains Populationt persons at time t. What is the size of the population at time t + 1 ?

${\displaystyle {\text{Population}}_{t+1}={\text{Population}}_{t}+{\text{Natural Increase}}_{t}+{\text{Net Migration}}_{t}}$

Natural increase from time t to t + 1:

${\displaystyle {\text{Natural Increase}}_{t}={\text{Births}}_{t}-{\text{Deaths}}_{t}}$

Net migration from time t to t + 1:

${\displaystyle {\text{Net Migration}}_{t}={\text{Immigration}}_{t}-{\text{Emigration}}_{t}}$

This basic equation can also be applied to subpopulations. For example, the population size of ethnic groups or nationalities within a given society or country is subject to the same sources of change. When dealing with ethnic groups, however, "net migration" might have to be subdivided into physical migration and ethnic reidentification (assimilation). Individuals who change their ethnic self-labels or whose ethnic classification in government statistics changes over time may be thought of as migrating or moving from one population subcategory to another.[17]

More generally, while the basic demographic equation holds true by definition, in practice the recording and counting of events (births, deaths, immigration, emigration) and the enumeration of the total population size are subject to error. So allowance needs to be made for error in the underlying statistics when any accounting of population size or change is made.

The figure in this section shows the latest (2004) UN projections of world population out to the year 2150 (red = high, orange = medium, green = low). The UN "medium" projection shows world population reaching an approximate equilibrium at 9 billion by 2075. Working independently, demographers at the International Institute for Applied Systems Analysis in Austria expect world population to peak at 9 billion by 2070.[18] Throughout the 21st century, the average age of the population is likely to continue to rise.

### Science of population

Populations can change through three processes: fertility, mortality, and migration. Fertility involves the number of children that women have and is to be contrasted with fecundity (a woman's childbearing potential).[19] Mortality is the study of the causes, consequences, and measurement of processes affecting death to members of the population. Demographers most commonly study mortality using the Life Table, a statistical device that provides information about the mortality conditions (most notably the life expectancy) in the population.[20]

Migration refers to the movement of persons from a locality of origin to a destination place across some predefined, political boundary. Migration researchers do not designate movements 'migrations' unless they are somewhat permanent. Thus demographers do not consider tourists and travellers to be migrating. While demographers who study migration typically do so through census data on place of residence, indirect sources of data including tax forms and labour force surveys are also important.[21]

Demography is today widely taught in many universities across the world, attracting students with initial training in social sciences, statistics or health studies. Being at the crossroads of several disciplines such as sociology, economics, epidemiology, geography, anthropology and history, demography offers tools to approach a large range of population issues by combining a more technical quantitative approach that represents the core of the discipline with many other methods borrowed from social or other sciences. Demographic research is conducted in universities, in research institutes as well as in statistical departments and in several international agencies. Population institutions are part of the Cicred (International Committee for Coordination of Demographic Research) network while most individual scientists engaged in demographic research are members of the International Union for the Scientific Study of Population,[22] or a national association such as the Population Association of America in the United States,[23] or affiliates of the Federation of Canadian Demographers in Canada.[24]

## Notes

1. ^
2. ^ "The Science of Population". demographicpartitions.org. Archived from the original on 14 August 2015. Retrieved 4 August 2015.
3. ^
4. ^ Andrew Hinde Demographic Methods Ch. 1 ISBN 0-340-71892-7
5. ^ a b c S.C.Srivastava,Studies in Demography, p.39-41
6. ^ a b c d Peter Biller,The measure of multitude: Population in medieval thought[1].
7. ^ See, e.g., Andrey Korotayev, Artemy Malkov, & Daria Khaltourina (2006). Introduction to Social Macrodynamics: Compact Macromodels of the World System Growth. Moscow: URSS, ISBN 5-484-00414-4.
8. ^ “Our Yesterdays: the History of the Actuarial Profession in North America, 1809-1979,” by E.J. (Jack) Moorhead, FSA, ( 1/23/10 – 2/21/04), published by the Society of Actuaries as part of the profession’s centennial celebration in 1989.
9. ^ The History of Insurance, Vol 3, Edited by David Jenkins and Takau Yoneyama (1 85196 527 0): 8 Volume Set: ( 2000) Availability: Japan: Kinokuniya).
10. ^ von Valtier, William F. (June 2011). ""An Extravagant Assumption": The Demographic Numbers behind Benjamin Franklin's Twenty-Five-Year Doubling Period" (PDF). Proceedings of the American Philosophical Society. 155 (2): 158–188.
11. ^ Zirkle, Conway (25 April 1941). "Natural Selection before the 'Origin of Species'". Proceedings of the American Philosophical Society. Philadelphia, PA: American Philosophical Society. 84 (1): 71–123. ISSN 0003-049X. JSTOR 984852.
12. ^ de Gans, Henk and Frans van Poppel (2000) Contributions from the margins. Dutch statisticians, actuaries and medical doctors and the methods of demography in the time of Wilhelm Lexis. Workshop on ‘Lexis in Context: German and Eastern& Northern European Contributions to Demography 1860-1910’ at the Max Planck Institute for Demographic Research, Rostock, August 28 and 29, 2000.
13. ^ Power C and Elliott J (2006). "Cohort profile: 1958 British Cohort Study". International Journal of Epidemiology. 35 (1): 34–41. doi:10.1093/ije/dyi183. PMID 16155052.
14. ^ Elliott J and Shepherd P (2006). "Cohort profile: 1970 British Birth Cohort (BCS70)". International Journal of Epidemiology. 35 (4): 836–43. doi:10.1093/ije/dyl174. PMID 16931528.
15. ^ The last three are run by the Centre for Longitudinal Studies
16. ^ a b c d Introduction to environmental engineering and science by Masters and Ela, 2008, Pearson Education, chapter 3
17. ^ See, for example, Barbara A. Anderson and Brian D. Silver, "Estimating Russification of Ethnic Identity Among Non-Russians in the USSR," Demography, Vol. 20, No. 4 (Nov., 1983): 461-489.
18. ^ Lutz, Wolfgang; Sanderson, Warren; Scherbov, Sergei (19 June 1997). "Doubling of world population unlikely" (PDF). Nature. 387 (6635): 803–805. doi:10.1038/42935. PMID 9194559. Archived from the original (PDF) on 16 December 2008. Retrieved 2008-11-13.
19. ^ John Bongaarts. The Fertility-Inhibiting Effects of the Intermediate Fertility Variables. Studies in Family Planning, Vol. 13, No. 6/7. (Jun. - Jul., 1982), pp. 179-189.
20. ^ N C H S - Life Tables
21. ^ Donald T. Rowland Demographic Methods and Concepts Ch. 11 ISBN 0-19-875263-6
22. ^ International Union for the Scientific Study of Population
23. ^ Population Association of America
24. ^ Canadian Population Society Archived 26 June 2011 at the Wayback Machine

## Further reading

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