Population Growth and the Demographic Transition

Ian RH Rockett, PhD, MPHProfessor and Associate Chair

Department of Community MedicineWest Virginia University

PO Box 9190 Morgantown, WV 26506-9190

USA

irockett@hsc.wvu.edu

Learning Objectives

1. To view population growth from a Malthusian perspective

2. To calculate crude death rates, birth rates, rates of natural increase, and population doubling times

3. To comprehend the concept of the Demographic Transition

Performance Objectives1. Examine patterns of natural

increase

2. Classify populations and sub-populations within the demographic transition framework

3. Predict growth trends in populations and sub-populations

Demography

a kindred population science

with epidemiology, it shares the Greek root demos (people) and the same founder, 17th century Englishman, John Graunt

Demography is the scientific study of the determinants and

consequences of human population trends

By the beginning of the 21st century, world population

reached 6 billion. Most of the growth has occurred in the

past 200 years.

Figure 1 World Population Growth

Source: Joseph A. McFalls, Jr. Population: A Lively Introduction. Third edition. Population Reference Bureau 53(3); 1998: 38

The unprecedented population growth of modern times

heightens interest in the notion of doubling time. Calculation of

population doubling time is facilitated by the Law of 70.

Law of 70

If a population is growing at a constant rate of 1% per year, it

can be expected to double approximately every 70 years

-- if the rate of growth is 2%, then the expected doubling time

is 70/2 or 35 years.

T.R. Malthus, 1766-1834

English clergyman, Thomas Robert Malthus, was the first person to draw widespread

attention to the two components of natural increase, births and deaths (fertility and mortality).

In his Essay on the Principle of Population

, initially published in 1798, Malthus postulated that

population tended to grow geometrically while the means of

subsistence (food) grew only arithmetically.

The Malthusian Trap

arithmetic growth (food): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10…

geometric growth (population):1, 2, 4, 8, 16, 32, 64, 128, 256, 512…

Malthus argued that the difference between geometric and arithmetic growth caused a tension between the growth of population and that

of the means of subsistence.-- this gap could not persist

indefinitely.

Owing to war, disease, hunger, and vice, mortality would serve

as a positive check on population growth.

Solution to the Malthusian Trap

Preventive checks: birth control through (1) later age at marriage.

(2) abstinence from sex outside marriage.

(Malthus opposed artificial methods of birth control on moral grounds. Viewed contraception as a vice)

Population Explosion

Contrary to Malthus’s prediction, mortality has not yet risen to curb

world population growth.

< 1 billion people in 1800

6 billion by the end of the

20th century

Population Explosion

Why was Malthus unable to foresee the population explosion (also known as the population bomb)?

He did not recognize the force of the Industrial Revolution, which produced exponential growth in the means of subsistence.

The Demographic Transition

During the first half of the 20th century,

demographers conceived the notion of the

demographic transition.

The Demographic Transition

The demographic transition framework illustrates

population growth in terms of discrepancies and changes in

two crude vital rates – mortality and fertility (ignores

migration)

CRUDE VITAL RATES

Crude Death Rate (CDR) =

# deaths in calendar year * k

mid–year population

CRUDE VITAL RATES

Crude Birth Rate (CBR) =

# deaths in calendar year * k

mid–year population

Rate of Natural Increase = CBR - CDR

Figure 2 The Demographic Transition

Source: Joseph A. McFalls, Jr. Population: A Lively Introduction. Third edition. Population Reference Bureau 53(3); 1998: 39

Four Perspectives on Demographic Transition

(1) Description

(2) Classification

(3) Explanation

(4) Prediction

Figure 3 Demographic/ Epidemiologic Transition Framework

Source: Ian R.H. Rockett. Population and Health: An Introduction to Epidemiology. Second edition. Population Reference Bureau 54(4); 1999: 9