Name: 人口統計學介紹－人口真的是問題的根源嗎？（Joel Cohen: An Introduction to Demography (Malthus Miffed: Are People the Problem?)）
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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.  

情態副詞

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 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