## Learning Outcomes

- Understand how developing countries fit into the demographic transition model
- Identify the main characteristics of many developing countries, including high TFR, higher IMR, and youthful age structure
- Explain why death rates usually fall before birth rates
- Calculate natural increase rate and doubling time using demographic data
- Interpret broad-based population pyramids and population momentum
- Distinguish between closely related measures such as TFR, CBR, CDR, and IMR

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand developing country characteristics within demographic transition, with a focus on the following syllabus points:

- Know that many developing countries are commonly associated with **Stage 2** or **Stage 3** of the demographic transition model
- Identify common features such as higher **total fertility rate (TFR)**, higher **infant mortality rate (IMR)**, lower life expectancy, and youthful age structure
- Explain why **crude death rate** often falls before **crude birth rate**, causing rapid population growth
- Understand how urbanization, women’s education, access to contraception, and lower infant mortality can reduce fertility
- Interpret age-structure diagrams, especially broad-base pyramids that indicate future growth
- Calculate **natural increase rate (NIR)** from crude birth rate and crude death rate
- Apply the **Rule of 70** to connect growth rates with doubling time
- Understand **population momentum** and why population can keep growing even after fertility declines

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why do many developing countries grow rapidly even after death rates fall?
2. What is the difference between total fertility rate and crude birth rate?
3. Why does a broad-based population pyramid suggest future population growth?
4. How do women’s education and urbanization usually affect birth rates?
5. If a country has a crude birth rate of 36 and a crude death rate of 12, what is its natural increase rate?

## Introduction

Demographic transition explains how human populations change as countries develop economically and socially. For AP Environmental Science, you are not expected to memorize every country in the world, but you are expected to recognise patterns. Many developing countries share a set of linked characteristics: relatively high birth rates, falling death rates, a large proportion of young people, and continued population growth.

This topic matters because the exam often asks you to connect demographic data to environmental consequences. A rapidly growing population can increase pressure on food supply, schools, jobs, housing, freshwater, sanitation, and waste systems. To answer these questions well, you need to explain not just what the data show, but why those patterns happen.

> **Key Term: demographic transition model**
> A model used to predict population trends as a country develops, based on changes in birth rates and death rates.

### Where developing countries fit in the demographic transition model

Many students make the mistake of treating “developing country” as a fixed category. APES questions are more precise than that. A developing country is better understood through its position in the demographic transition model, not just through a label.

In **Stage 2**, the **crude birth rate** stays high, but the **crude death rate** drops quickly. This creates very rapid population growth. The key point is that improved survival comes first. Better sanitation, cleaner water, more reliable food supplies, vaccination, and basic medical care reduce deaths, especially infant and child deaths. But family size often stays large at first, so births remain high.

In **Stage 3**, death rates stay low, while birth rates begin to fall. Population still grows, but more slowly than in Stage 2. This is why many developing countries do not stop growing as soon as they become more economically developed. The population trend changes in stages, not all at once.

> **Key Term: crude birth rate (CBR)**
> The number of live births per 1,000 people in a population in one year.
>
> **Key Term: crude death rate (CDR)**
> The number of deaths per 1,000 people in a population in one year.

A common APES distinction is this: **developing countries do not always have high death rates**. In Stage 2 especially, death rates may already have fallen a lot. The rapid growth happens because birth rates stay high after death rates decline.

A useful example from current demographic patterns is **Nigeria**, which has a high birth rate, a very young population, and rapid growth. By contrast, countries such as **India** and **Mexico** are often used as examples of later developing or industrial-stage populations, where birth rates are falling but growth still continues.

> **Test Tip:** If a question asks why population is rising quickly in a developing country, the strongest answer is usually not “because more babies are being born than before.” A better answer is that death rates have fallen while birth rates remain high.

### Typical characteristics of developing countries and why they happen

Many developing countries are described by a cluster of related indicators. These include higher **TFR**, higher **IMR**, lower life expectancy, a large pre-reproductive population, and faster natural increase. These are not random facts. They link together through cause and effect.

**Higher total fertility rate** often occurs where children are economically useful or socially expected. In rural areas, children may help with farm labour, collecting water, caring for siblings, or adding to family income. Where there is little pension support, parents may also depend on children in old age. In these conditions, larger families can seem practical.

**Higher infant mortality rate** is another common feature. If access to healthcare, sanitation, nutrition, vaccines, and clean water is limited, more babies die before age one. Families may respond by having more children because survival is less certain. That helps explain why high IMR and high fertility often occur together.

> **Key Term: total fertility rate (TFR)**
> The average number of children a woman is expected to have during her lifetime.
>
> **Key Term: infant mortality rate (IMR)**
> The number of deaths of infants under age 1 per 1,000 live births.

**Lower life expectancy** usually reflects the same underlying conditions: greater disease burden, weaker healthcare systems, unsafe water, poor sanitation, and undernutrition. In APES, this matters because these conditions shape both demographics and environmental vulnerability.

Students also need to know that **replacement-level fertility** is not always 2.1. That number works mainly in low-mortality populations. In places with higher child and infant mortality, the replacement rate must be higher because more births are needed for enough children to survive to reproductive age.

> **Key Term: replacement-level fertility**
> The average number of children per woman needed to keep a population size stable, assuming no migration; about 2.1 in low-mortality populations, but higher where mortality is higher.

Developing countries may also have a high **youth dependency burden**, even if the exact term is not central to the course. A very young population means many people need food, schools, vaccinations, and later jobs, while relatively fewer adults are available to support them. That creates pressure on infrastructure and public services.

This is why population growth is an environmental science issue as well as a social one. More people can mean higher demand for land, timber, fisheries, water, energy, and waste disposal. If development is fast but infrastructure is weak, pollution and habitat loss may rise.

### Age structure, population momentum, and future growth

One of the most tested features of this topic is the **age-structure diagram**, also called a population pyramid. Developing countries often show a broad-base pyramid. That wide base means a large proportion of the population is **pre-reproductive**, usually ages 0–14.

> **Key Term: age-structure diagram**
> A graph showing the numbers or percentages of people in different age groups in a population.
>
> **Key Term: pre-reproductive**
> The age group from 0 to 14, below typical reproductive age.

A broad base does more than show that birth rates have been high. It also predicts future growth. Why? Because those large younger cohorts will age into reproductive years. Even if each woman later has fewer children than the previous generation, the total number of births may still remain high because there are so many potential parents.

That is **population momentum**.

> **Key Term: population momentum**
> Continued population growth caused by a large proportion of young people entering reproductive age, even after fertility rates decline.

This point is often missed in exam answers. Students may say, “If fertility falls to replacement level, population stops growing.” That is not always true in the short term. A country with many young people can continue to grow for decades because of its age structure.

Compare three common shapes:

- **Pyramid-shaped**: rapid growth, many young people, common in developing countries
- **Bell-shaped**: slower or stable growth
- **Urn-shaped**: aging and declining population

A country with a pyramid-shaped structure is likely to face future pressure on schools first, then jobs, housing, transport, and food systems later. So an age-structure question is often really asking you to infer future environmental and economic demands.

> **Test Tip:** On age-structure questions, do not just describe the shape. Explain what the shape means for future growth. A wide base means many future parents, so growth is likely to continue.

### Quantitative measures: natural increase rate and doubling time

APES expects you to use a few simple calculations. For developing-country questions, the two most useful are **natural increase rate** and **doubling time**.

Natural increase rate ignores migration and is calculated from birth and death rates:

$$
\text{NIR (\%)} = \frac{\text{CBR} - \text{CDR}}{10}
$$

Suppose a developing country has:

- CBR = 36 births per 1,000
- CDR = 12 deaths per 1,000

Step 1: subtract death rate from birth rate

$$
36 - 12 = 24
$$

Step 2: divide by 10

$$
24 \div 10 = 2.4\%
$$

So the **natural increase rate** is **2.4% per year**.

Now use the **Rule of 70** to estimate doubling time:

$$
\text{Doubling time} = \frac{70}{\text{growth rate (\%)}}
$$

Step 1: substitute the growth rate

$$
70 \div 2.4 = 29.2
$$

So the population would double in about **29 years** if that rate stayed constant.

> **Key Term: natural increase rate (NIR)**
> The percentage growth of a population based on birth rate minus death rate, excluding migration.
>
> **Key Term: Rule of 70**
> A rule used to estimate doubling time by dividing 70 by the annual growth rate as a percentage.

These calculations matter because they show how quickly resource demand can rise. A country growing at over 2% annually may double its population within a few decades. That can intensify pressure on land, freshwater, waste systems, food production, and biodiversity.

Common student errors include:

- forgetting to divide by 10 when rates are given per 1,000
- using TFR instead of CBR in the NIR formula
- confusing IMR with CDR
- using the birth rate rather than the growth rate in the Rule of 70

A related point: migration can affect total population change, but for many APES demographic questions, you are expected to focus on **natural increase** unless immigration and emigration are explicitly given.

### Why fertility falls in Stage 3 developing countries

A key APES skill is explaining why birth rates decline later in development. Several causes often work together.

**Urbanization** reduces the economic value of large families. In cities, children are less likely to work on farms and more likely to cost money through schooling, food, housing, and transport. The logic of family size changes.

**Education for women and girls** is especially important. More education usually links to later marriage, later first birth, greater workforce participation, better knowledge of family planning, and more personal choice over reproduction.

**Access to contraception** allows families to act on those changing preferences. A desire for fewer children only reduces birth rates if people can control fertility in practice.

**Lower infant mortality** also changes decisions. When parents are more confident that children will survive, they often choose smaller families.

Retirement systems matter too. If parents no longer depend mainly on children for support in old age, pressure for large families decreases.

These are exactly the kinds of factors you should use in an FRQ response about reducing population growth in a developing country. Strong answers are specific and causal. For example:

- education for girls lowers TFR by delaying childbirth and increasing access to employment
- urbanization lowers fertility because children become more expensive and less useful as labour
- healthcare reduces IMR, which can lower desired family size

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Many developing countries are commonly found in Stage 2 or Stage 3 of the demographic transition model
- In Stage 2, death rates fall before birth rates, causing rapid population growth
- In Stage 3, birth rates begin to fall while death rates stay low, so growth slows but continues
- Developing countries often have higher TFR, higher IMR, lower life expectancy, and youthful age structures
- Crude birth rate is births per 1,000 people, while total fertility rate is children per woman over a lifetime
- Infant mortality rate measures infant deaths under age 1 per 1,000 live births
- Replacement-level fertility is about 2.1 only in low-mortality populations and is higher where mortality is higher
- A broad-based population pyramid shows a large pre-reproductive population and future growth potential
- Population momentum means population can keep growing even after fertility declines
- Natural increase rate can be calculated using $(CBR - CDR) / 10$
- The Rule of 70 estimates doubling time from growth rate
- Women’s education, urbanization, lower infant mortality, and contraception are major causes of fertility decline

## Key Terms and Concepts

- demographic transition model
- crude birth rate (CBR)
- crude death rate (CDR)
- total fertility rate (TFR)
- infant mortality rate (IMR)
- replacement-level fertility
- age-structure diagram
- pre-reproductive
- population momentum
- natural increase rate (NIR)
- Rule of 70


Demographic Transition - Developing country characteristics