lecture 2: population growth

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Population Growth

Nan Astone, PhDJohns Hopkins University

3

Objectives of the Lecture

At the end of this lecture and after reading the texts that accompany it, students will be able to:−

Explain the demographic balancing equation

Define natural increase and net migration−

Define population momentum

Interpret a population pyramid

Section A

The Demographic Balancing Equation

5

Demographic Balancing Equation

Pt2 = Pt1 + (B – D) + (I – O)−

Where …

Pt2 = population at time 2Pt1 = population at time 1B = birthsD = deathsI = in migrantsO = out migrants

6

Demographic Balancing Equation

Pt2 = Pt1 + (B – D) + (I – O)−

(B – D) is referred to as natural increase

7

For Most of History, Natural Increase Was 0

Births = deaths until about 1000 CEThen, gradually as the ability to store food and trade increased, births began to exceed deaths−

Famines and epidemics were periodic setbacks

8Sources: Population Reference Bureau; United Nations. (1998). World population projections to 2100.

World Population Growth through History

9Sources: First and second billion: Population Reference Bureau; Third through ninth billion: United Nations. (2005). World population prospects: the 2004 revision (medium scenario).

Number of Years to Add Each Billion

10Source: United Nations. (2005). World population prospects: the 2004 revision.

Natural increase

Rates of Birth, Death, and Natural Increase

Rates of birth, death, and natural increase per 1,000 population

11

Time

Stage 1 Stage 2 Stage 3 Stage 4

Naturalincrease

Birth rate

Death rate

Note: Natural increase is produced from the excess of births over deaths

The Classic Stages of “Demographic Transition”

12

Brief Digression

“The” demographic transition—nonsenseMany demographic transitions−

Fertility decline

−

Mortality decline−

Hurricane Katrina

−

The occupation of the Americas−

The Great Influenza

13

Demographic Balancing Equation

Pt2 = Pt1 + (B – D) + (I – O)−

(I – O) is referred to as net migration

14

Globally, Net Migration Is Always 0

Globally, net migration is always 0At least, most of us think so“The truth is out there!”

15

In Sum

For most of history, natural increase was 0Gradually, between 1000 CE and 1800 (or so), natural increase slowly increased as death rates went down and births began to gradually exceed deathsFrom 1800 onward, escalating sharply after 1950, births began toexceed deaths by a great deal, resulting in dramatic population growthAnd then, fertility began to decline

Section B

Population Momentum

17

Objectives of the Lecture

At the end of this lecture and after reading the texts that accompany it, students will be able to:−

Explain the demographic balancing equation

Define natural increase and net migration−

Define population momentum

Interpret a population pyramid

18Source: United Nations. (2005). World population prospects: The 2004 revision.

Average Number of Children per Woman

19

Once Fertility Reaches Replacement

So, will population be stable once fertility falls (or rises) toreplacement?Population momentum−

Population can continue to grow (or decline) even after natural increase has reached 0, because the number of women of childbearing age is very large (or small) because of past fertility rates

20

Number of Births and Fertility Rate for the U.S. 1960 to 2001

0

500000

1000000

1500000

2000000

2500000

3000000

3500000

4000000

4500000

20011998199519921989198619831980197719741971196819651962

Year

Number of Births

0

20

40

60

80

100

120

140

Births Per 1000 Women 15-49

number of births fertility rates

21

Population of Ireland, 1986 to 2016

22

Population of Ireland, 1986

23

Population of Ireland, 1996

24

Population of Ireland, 2006

25

Population of Ireland, 2016

26

Population of Ireland, 2026

27

Population of Ireland, 2036

28

Population of Ireland, 2046

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Why Is This Important?

Just because fertility rates have fallen for any given population doesn’t necessarily mean that the distribution of the population by age is going to remain the same