Factors Affecting Human Population Size




The problems to be faced are vast
and complex, but come down to this:
6.2 billion people are breeding
exponentially. The process of fulfilling
their wants and needs is stripping
earth of its biotic capacity to produce
life; a climatic burst of consumption
by a single species is overwhelming
the skies, earth, waters and fauna.

-- Paul Hawken (http://www.paulhawken.com)


Can a country sharply reduce its population growth in only 15 years?

Thailand did.

In 1971, Thailand adopted a policy to reduce population growth.

The country's population was growing at 3.2% per year.

The average Thai family had 6.4 children.

In 1986 the country's growth rate was cut to 1.6%.

By 2002 it was 0.8% and the average number of children per family was 1.8.

Thailand's population is projected to grow from 63 million in 2002 to 72 million in 2025.


Several reasons account for this impressive feat:

This transition was catalyzed by a public relations genius and former government economist who launched the PCDA in 1974 to help make family planning a national goal.

PCDA workers handed out condoms at festivals, movie theaters and even traffic jams.

They developed ads and witty songs about contraceptive use.

Between 1971 and 2002 the percentage of married women using modern birth control rose from 15% to 70%.

A loan plan was developed to enable people participating in family planning programs to install toilets and drinking water systems.

Low-rate loans were offered to farmers practicing family planning.

All is not completely rosy.

Although population growth is slowing and per capita income is increasing, pollution and public health are still problems.



***How is Population Size Affected by Birth and Death Rates?***

Populations grow or decline through the interplay of three factors:

Population change is calculated by subtracting the number of people leaving a population
(through death and emigration) from the number entering it (through birth and immigration) during a specified time period (usually a year):

Population Change = (Births + Immigration) - (Deaths + Emigration)


Instead of using the total number of births and deaths per year, population geographers and demographers use:

Average crude birth and death rates for developed and developing countries.

Average crude birth and death rates for continents.


Birth rates and death rates are coming down worldwide, but death rates have fallen more sharply.

As a result, more births are occurring than deaths.

About 215,000 people are born each day (95% in developing countries).

The rate of world population change is usually expressed as a percentage:


            Annual rate of         Birth rate - Death rate
      natural population   =   ----------------------------
                 change (%)                          10


Exponential growth has not disappeared but is occurring at a slower rate.

The rate of the world's annual population growth (natural increase) dropped 42% between 1963 and 2002, from 2.2% to 1.3%.

This is good news, but during the same period the population base rose by 94%m from 3.2 billion to 6.2 billion.

The bad news is that this drop in the rate of population increase is somewhat like learning that a truck heading straight at you has slowed from 100 mph to 58 mph while its weight has almost doubled.

Average annual rate of population change in 2002.

An exponential growth rate of 1.3% may seem small but it adds about 79 million people per year to the world's population.

Despite the drop in the rate of population growth, the larger base of population means that 79 million people were added in 2002, compared to 69 million in 1963, when the world's population growth rate peaked.

Average annual increase in the world's population, 1950-2002, and projected increase 2002-2050.


In number of people:

Projected population growth in various regions between 2002 and 2025.



*** How have Global Fertility Rates Changed? ***

Two types of fertility rates affect a country's population size and growth rate:

It is slightly higher than two children per couple (2.1 in developed countries and as high as 2.5 in some developing countries), mostly because some female children die before reaching their reproductive years.

Reaching replacement-level fertility does not mean an immediate halt in population growth because many future parents are alive.

If each of today's couples had an average of 2.1 children and their children also had 2.1 children, the world's population would grow for 50 years or more (assuming death rates do not rise).

TFRs have dropped sharply since 1950.

Decline in total fertility rates, 1950-2002.

In 2002, the average global TFR was 2.8 children per woman.

It was 1.6 in developed countries (down from 2.5 in 1950).

It was 3.1 in developing countries (down from 6.5 in 1950).

World Total Fertility Rates - 2002

UN population projections to 2050 vary depending on the world's projected average TFR.

UN world population projection, assuming that by 2050 the world's TFR is 2.6 (high), 2.1 (medium) or 1.7 (low) children per woman.

More than 95% of this growth is projected to take place in developing countries where acute poverty is a way of life for 1.4 billion people.

By 2050, the world's population is expected to be 63% urban, more tropical and considerably older than today.


The population of the US has grown from 76 million in 1900 to 300 million in 2006 even though the country's TFR has fluctuated wildly.

Total fertility rates for the US between 1917 and 2002.

In 1957, the peak of the baby boom after WWII, the TFR reached 3.7 children per woman.

Since then it has generally declined, remaining at or below the replacement level since 1972.

The drop in the TFR has led to a decline in the rate of population growth in the US.

However, the country's population is still growing faster (1.2% per year, including immigration) than that of any other developed country and is not even close to leveling off.

In 2002 this growth added about 2.9 million people.

  • 1.7 million more births than deaths
  • about 900,000 legal immigrants and refugees
  • an estimated 300,000 illegal immigrants

US birth rates fell sharply between 1910 and 1930 as:

  • the country underwent industrialization and urbanization
  • more women got an education and began working outside the home

Birth rates in the US - 1910-2002

This shift from high birth rates to low birth rates during industrialization is called a demographic transition.

Birth rates remained low in the 1930s because of the Great Depression and then began rising in the 1940s during WWII.

A sharp rise in the birth rate occurred after WWII.

This period of high birth rates from 1946 to 1964 is known as the baby boom period.

Between 1956 and 1972, birth rates began to decline as more women began working outside the home and the desired family size dropped from 4 to 2 children.

Between 1977 and 2000, a small echo boom in the number of births per year occurred as the large number of people born during the baby boom began having having children.

Birth rates are are projected to rise again between 2002 and 2050.

US population growth, 1900 - 2000 and projected to 2100.

Because of higher per capita rate of resource use, each addition to the US population has enormous environmental impact.