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  • 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, theyre 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 billionMore

  • 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 halfthe 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, youre 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 weve talked about the demographic

  • past, and the demographic present.  And next were gonna talk about the demographic future.

  •   Woody Allen said, “Eternity is very long,