An Introduction to Demography by Joel Cohen (Transcript)

Joel Cohen


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.

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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 wellbeing and the wellbeing 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.

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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.

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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 comes 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.

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