AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/3/3-6)
Unit 3 · Topic 3.6
3.6 Age Structure Diagrams
An age structure diagram, or population pyramid, shows how many males and females are in each age group. Its shape reveals whether a population will grow quickly, stay stable or shrink, because the size of the youngest groups shapes future births.
Key terms
- age structure diagram
- population pyramid
- pre-reproductive age
- reproductive age
- post-reproductive age
How to read the diagram
An age structure diagram is a set of horizontal bars stacked by age group, usually in five-year groups, with the youngest at the bottom. Males are on one side of a center line and females on the other. The length of each bar shows the number or percent of the population in that group.
The groups are often lumped into three stages: pre-reproductive (0–14), reproductive (15–44) and post-reproductive (45 and up). The size of the pre-reproductive group is the best clue to future growth, because those children will soon become parents.
Always check whether the bars show actual numbers of people or percentages of the population. Percentages let you compare the shapes of a large country and a small one directly.
The three main shapes
| Shape | What it looks like | What it means | Typical setting |
|---|---|---|---|
| Rapid growth | True pyramid: wide base, narrow top | Many children, high birth rate, shorter life expectancy | Many lower-income countries, such as Niger |
| Slow or stable growth | Column or beehive: fairly even widths up to older ages | Birth rate near replacement | Many higher-income countries, such as the U.S. |
| Decline | Narrower base than the middle | Fewer children than parents; aging population | Countries with very low fertility, such as Japan |
Population momentum
A country with a wide base will keep growing for decades even if families start having fewer children right away. That's because a large group of children is moving into its reproductive years. Even if each woman has just two children, so many women are having them that the total number of births stays high. This is called population momentum.
The opposite is also true. Japan's population has been shrinking since around 2010, and with a narrow base, there are fewer potential parents each year, so the decline is hard to reverse quickly.
Population momentum explains why world population is still growing even though global fertility is now close to replacement level: the many young people born in recent decades are now having children of their own.
What the shape means for a society
- Wide base: high demand for schools, food and jobs in coming years; often linked to rapid population growth.
- Top-heavy (narrow base): more older people relative to workers, so more pressure on pensions and health care, and possible worker shortages.
- Bulges and dents: a bulge can show a baby boom; a dent can show a war, famine or a sharp drop in birth rates in the past.
- Unequal sides: more males or females in an age group can reflect migration for work, war deaths or a preference for sons.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Predicting growth from shape
Country A's diagram has 44% of its population aged 0–14 and only 3% over 65. Country B's diagram has 12% aged 0–14 and 29% over 65. Predict each country's growth over the next 20 years and explain.
Show the solutionHide the solution
- Step 1: Country A has a very wide base. Those children will soon reach reproductive age, so births will stay high: rapid growth.
- Step 2: Country B has a narrow base and a large older population. Fewer future parents and many older people means deaths may exceed births: slow decline.
- Step 3: Use population momentum in your reasoning for A: even if fertility falls, the large young cohort keeps the population growing.
Answer: Country A will grow rapidly; Country B will likely shrink slowly.
- Example 2
Explaining a dent
In a country's age structure diagram, the bars for people now aged 30–34 are noticeably shorter than the groups above and below them. Suggest an explanation.
Show the solutionHide the solution
- Step 1: People now aged 30–34 were born 30 to 34 years ago.
- Step 2: A short bar means fewer births in those years, or more deaths or emigration of that group.
- Step 3: Possible causes: a temporary drop in births during an economic crisis, war or famine around that time, or a government policy limiting births.
Answer: Fewer people were born (or survived) about 30–34 years ago, possibly because of a crisis such as war, famine or economic hardship.
Common mistakes
- Focusing on the top of the diagram. The base (ages 0–14) tells you most about future growth.
- Assuming a population stops growing as soon as fertility drops to replacement. Population momentum keeps it growing for decades if the base is wide.
- Calling a column-shaped diagram ‘declining’. A column means slow or no growth; a narrower base than middle means decline.
On the exam
- Expect described diagrams for two countries and a question about which will grow faster. Justify with the size of the pre-reproductive group.
- Connect shapes to demographic transition stages and total fertility rate when asked to explain them.
Connected topics
Videos
Check yourself
4 questions on 3.6 Age Structure Diagrams. Pick an answer to see if you got it, and why.
A country's age structure diagram shows a very large bulge of people aged 25–35, with narrower bars for younger ages. Assuming birth rates stay low, which change is most likely 40 years from now?
A country's age structure diagram looks normal except for a deep notch among males aged 20–35; the female bars for the same ages are full. Which is the most likely cause?
| Age group | Population (millions) |
|---|---|
| 0–14 | 4.2 |
| 15–64 | 9.1 |
| 65 and older | 0.7 |
Hypothetical census data
What percentage of the country's population is under age 15?
Which description best fits this country's age structure diagram?
0 of 4 answered