AP® Human Geography review sheet from Aim for Five (aimforfive.com/geo/units/2/2-4)
Unit 2 · Topic 2.4
2.4 Population Dynamics
A population changes in only three ways: births, deaths and migration. This topic gives you the rates geographers use to measure that change, including the rate of natural increase and doubling time, and the factors that push those rates up or down.
Key terms
- crude birth rate
- crude death rate
- rate of natural increase
- doubling time
- total fertility rate
- replacement-level fertility
The three drivers
Fertility is how many children are born. Mortality is how many people die. Migration is people moving in or out. The basic equation is: population change = births − deaths + immigrants − emigrants. Births minus deaths is natural increase; immigrants minus emigrants is net migration.
Key rates
| Measure | How it's calculated | Notes |
|---|---|---|
| Crude birth rate (CBR) | Births per 1,000 people per year | “Crude” because it doesn't adjust for age |
| Crude death rate (CDR) | Deaths per 1,000 people per year | Can be high in old populations even with good health care |
| Rate of natural increase (RNI) | (CBR − CDR) ÷ 10, as a percent | Leaves out migration |
| Doubling time | About 70 ÷ RNI (in percent), in years | Assumes the growth rate stays the same |
| Total fertility rate (TFR) | Average children per woman over her lifetime | About 2.1 is replacement level |
| Infant mortality rate (IMR) | Deaths of babies under age 1 per 1,000 live births | A strong sign of health care and living conditions |
Replacement level and real-world numbers
Replacement-level fertility is a TFR of about 2.1. That's enough children to replace both parents, plus a little extra because some children die before adulthood. It is higher where child mortality is higher. A country with a TFR below 2.1 will eventually shrink unless people live longer or immigrants arrive.
The UN estimated the world's TFR at about 2.2 in 2024, down from about 5 in the 1960s. Rates vary widely: many countries in Sub-Saharan Africa are above 4, while South Korea's fell below 0.8 in 2023 and 2024, among the lowest ever recorded for any country.
What pushes rates up or down
- Social: more education, especially for women, usually means later marriage and fewer children. Better health care lowers death rates and infant mortality.
- Cultural: religious teachings and family traditions can encourage larger or smaller families.
- Economic: on farms, children can help with work, so large families make economic sense; in cities, children are expensive to raise and educate. Pensions reduce the need to rely on children in old age.
- Political: governments can encourage or discourage births (topic 2.7) and control migration.
- Migration can change a country's numbers even when natural increase is near zero. Germany has had more deaths than births nearly every year since the early 1970s, but immigration has kept its population growing in many years.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
RNI and doubling time
A country has a crude birth rate of 30 per 1,000 and a crude death rate of 10 per 1,000. Find its rate of natural increase and its approximate doubling time.
Show the solutionHide the solution
- Step 1: Subtract: 30 − 10 = 20 more births than deaths per 1,000 people.
- Step 2: Convert to a percent: 20 per 1,000 is 2.0%.
- Step 3: Use the rule of 70: 70 ÷ 2.0 = 35 years.
- Step 4: Add the condition: this assumes the 2.0% rate stays steady and ignores migration.
Answer: RNI = 2.0%; doubling time ≈ 35 years.
- Example 2
The migration trap
Country Y has a CBR of 8 and a CDR of 11 per 1,000, yet its population grew last year. Explain how.
Show the solutionHide the solution
- Step 1: Compute RNI: (8 − 11) ÷ 10 = −0.3%, so natural increase is negative.
- Step 2: Recall what RNI leaves out: migration.
- Step 3: Conclude: net immigration must have been large enough to outweigh the natural decrease.
Answer: Natural increase is −0.3%, so the growth must come from immigration exceeding the natural decrease.
Common mistakes
- Forgetting to divide by 10 to turn a rate per 1,000 into a percent. A gap of 20 per 1,000 is 2%, not 20%.
- Saying a high crude death rate means poor health care. Aging countries like Japan have high CDRs because so many residents are old.
- Treating RNI as total growth. It leaves out migration.
On the exam
- Calculations on this exam are simple and done without a calculator; know the RNI and rule-of-70 shortcuts.
- Questions often ask you to explain one social, cultural, economic or political factor that lowers fertility; name the factor and link it to the decision to have fewer children.
Connected topics
Videos
Check yourself
4 questions on 2.4 Population Dynamics. Pick an answer to see if you got it, and why.
| Country | Crude birth rate (per 1,000) | Crude death rate (per 1,000) |
|---|---|---|
| J | 38 | 8 |
| K | 18 | 6 |
| L | 10 | 12 |
| M | 11 | 9 |
Hypothetical data for one year
What is Country K's rate of natural increase?
If Country J's rate of natural increase stays the same, about how long would its population take to double?
Country L's total population grew last year. Which of the following best explains this?
A country's infant mortality rate falls from 90 to 20 per 1,000 live births over 30 years. Which of the following most likely contributed to this decline?
0 of 4 answered