3.6: Age structure diagrams

Age structure diagram

A visual representation of the number of individuals within specific age groups for a country, typically expressed for males and females.

Population momentum

“Why Populations Keep Growing”

Population momentum occurs when a country’s fertility rate declines to or below replacement level (2.1 children per woman), yet the population size continues to grow due to the age structure of the population.

• The higher the percentage of young people in a population, the more growth the country will see

Types of Growth (Age structure diagram)

• Each bar represents 5 years

• The total area of the bars represents the size of the whole population

• Three broad categories: pyramid, column, inverted pyramid

Total fertility rate (TFR)

An estimate of the average number of children that each woman in a population will bear throughout her childbearing years.

• Average births per woman

• Not a measurement per 1,000 

• U.S. TFR ~ 1.64 (2020)

• World TFR ~ 2.30 (2020)

Replacement-level fertility

The total fertility rate required to offset the average number of deaths in a population in order to maintain the current population size.

• Compare replacement-level fertility with TFR to get a sense of a country’s level of development

• Ex. TFR of 2.1 – replace parents and account for children who die before reproductive age