AP® Human Geography review sheet from Aim for Five (aimforfive.com/geo/units/2/2-3)
Unit 2 · Topic 2.3
2.3 Population Composition
Population composition describes who makes up a population, especially by age and sex. Population pyramids let you read a country's past, see its present needs and predict its future.
Key terms
- population pyramid
- age structure
- age cohort
- sex ratio
Reading a population pyramid
A population pyramid is a pair of horizontal bar graphs. Males are usually on the left and females on the right. Each bar is an age cohort, a group born in the same span of years, usually 5 years, with the youngest at the bottom. The length of each bar is the number or percentage of people in that group.
To read one, look at the base (recent births), the middle (working-age adults) and the top (older people). Then look for bulges and notches, which point to past events like baby booms, wars, famines or migration.
| Shape | What it means | Typical example |
|---|---|---|
| Wide base, narrow top (true pyramid) | High birth rates, short life expectancy, fast growth, young population | Niger, Chad |
| Fairly straight sides | Birth and death rates both low; slow growth | United States, France |
| Narrow base, wide middle or top | Low birth rates, long life expectancy; aging and possible decline | Japan, Italy, South Korea |
Bulges and notches tell stories
- The U.S. pyramid shows a bulge for the baby boom generation, born from 1946 to 1964, now roughly 60 to 80 years old.
- China's pyramid shows smaller cohorts born after the one-child policy began in 1980.
- Gulf states like Qatar and the United Arab Emirates have a huge bulge of men aged about 25 to 50, because most residents are male migrant workers.
- At the local scale, a college town has a big 18–24 bulge, and a retirement community has a top-heavy pyramid.
Sex ratio
The sex ratio is the number of males per 100 females. About 105 boys are born for every 100 girls naturally, but women tend to live longer, so the ratio falls below 100 in older age groups. Unusual ratios point to human causes. Male-dominated labor migration skews ratios in the Gulf. Son preference combined with sex-selective abortion pushed China's and India's sex ratios at birth well above the natural level in recent decades.
Using pyramids to predict needs
Pyramids help governments and businesses plan. A wide base means the country will need more schools and, in about 20 years, many new jobs and homes. A big older population means more demand for health care, pensions and elder care, and fewer workers paying taxes. Businesses use the same logic: a growing young population is a market for baby products and phones, while an aging one is a market for medicine and retirement housing.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Interpreting a pyramid
A country's pyramid has a very wide base, and each bar is narrower than the one below it. Describe what the shape shows and explain one challenge it creates for the government.
Show the solutionHide the solution
- Step 1: Describe: a wide base means many young children; quickly shrinking bars higher up mean fewer older people.
- Step 2: Interpret: birth rates are high and life expectancy is relatively short, so the population is young and growing fast.
- Step 3: Explain a challenge: the government must build schools and train teachers now, and create enough jobs for a large cohort entering the workforce in 15 to 20 years.
Answer: The pyramid shows high birth rates and a young, fast-growing population. The government faces heavy demand for schools now and must create many jobs as the large young cohort reaches working age.
Common mistakes
- Reading the pyramid as a timeline. Each bar is an age group alive today, not a year in history.
- Ignoring migration. A bulge in working ages may come from immigrants, not past births.
- Assuming the sex ratio should be exactly 100. It is naturally about 105 at birth and below 100 among older people.
On the exam
- Stimulus questions often show a pyramid and ask you to describe its shape, identify a likely stage of the demographic transition, or predict future needs.
- When you describe a pyramid, point to a specific part, such as “the narrow base” or “the bulge at ages 60–74,” and then interpret it.
Connected topics
Videos
Check yourself
5 questions on 2.3 Population Composition. Pick an answer to see if you got it, and why.
A company that makes baby formula and diapers is deciding where to expand. Which population pyramid shape would make a country most attractive?
| Age group | Country X: male (% of total) | Country X: female (% of total) | Country Y: male (% of total) | Country Y: female (% of total) |
|---|---|---|---|---|
| 0–14 | 21 | 20 | 6 | 6 |
| 15–29 | 14 | 14 | 8 | 8 |
| 30–44 | 8 | 8 | 10 | 10 |
| 45–64 | 5 | 6 | 13 | 14 |
| 65 and over | 2 | 2 | 11 | 14 |
Hypothetical population data
Which statement best describes Country X's population compared with Country Y's?
Which country has the higher dependency ratio, and why?
Over the next 20 years, which of the following is Country Y most likely to need more of?
Country Y's pyramid most closely matches a country in which stage of the demographic transition model?
0 of 5 answered