In simple terms
A friendly intro before the formal notes — no formulas yet.
The Population Bathtub
Population change is driven by the balance between births and deaths, along with migration. This balance determines whether a country's population grows, shrinks, or stays the same.
Imagine a country's population is the water level in a bathtub. The tap pouring water in represents births, and the drain letting water out represents deaths. The rate at which the water level changes is the natural increase or decrease. Migration is like someone adding or removing a bucket of water.
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First, identify the key inputs: the Crude Birth Rate (CBR) and Crude Death Rate (CDR). These are the fundamental drivers of natural change.
- 2
Next, calculate the Natural Increase Rate (NIR) by subtracting the CDR from the CBR. This tells you the percentage by which the population is growing or shrinking annually, excluding migration.
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Then, consider the population structure, often shown in a population pyramid. A wide base (many young people) indicates future growth, a concept known as population momentum.
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Finally, analyse the socio-economic factors influencing these rates. For example, how does female education affect fertility rates, or healthcare access impact mortality?
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
Reading a pyramid Each bar is an age group, males on the left and females on the right. The base shows the birth rate, the slope of the sides the death rate, and the height how long people live.
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Full topic notes
Formal explanation with the rigour you need for the exam.
The Core Components of Population Change
The most basic measure of population change is the balance between births and deaths. When births exceed deaths, a population experiences natural increase. When deaths exceed births, it experiences natural decrease. We measure these using 'crude' rates, which means they apply to the entire population without considering its age or sex structure.
Crude Birth Rate (CBR) =
Crude Death Rate (CDR) =
From these two rates, we can calculate the Natural Increase Rate (NIR). Note the division by 10 to convert the 'per 1000' rates into a percentage.
Natural Increase Rate (NIR) % =
Factors Influencing Fertility and Mortality
Crude rates are useful, but to truly understand population change, we must look at the factors that influence them. These factors are complex and interwoven, spanning social, economic, cultural, and political spheres. For example, fertility rates are not just a biological phenomenon; they are heavily influenced by the status of women, access to education, availability of contraception, and cultural norms regarding family size.
Factors reducing fertility: Increased female education and employment, access to family planning, lower infant mortality (less need for 'replacement' children), urbanisation, and rising costs of child-rearing.
Factors reducing mortality: Improvements in public health (sanitation, clean water), advances in medicine (vaccines, antibiotics), better nutrition, and increased safety standards.
Factors increasing mortality (in specific contexts): Pandemics (e.g., HIV/AIDS, COVID-19), conflict and war, lifestyle diseases in HICs (e.g., heart disease), and natural disasters.
In essays, avoid simply listing factors. Instead, explain the linkages between them. For example, explain how increased female literacy leads to lower fertility rates (e.g., delayed marriage, career aspirations, better knowledge of contraception). Use specific, located examples to support your points.
Population Structure and Dependency
The age and sex structure of a population, often visualised in a population pyramid, has significant implications for its future. A key concept here is the dependency ratio, which compares the economically active population with the 'dependent' population (the young and the elderly). A high dependency ratio can place a strain on the working population to support services like schools and pensions.
Dependency Ratio =
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
In 2023, Country A had a population of 25 million. There were 500,000 live births and 200,000 deaths. Calculate the CBR, CDR, and NIR.
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Calculate CBR:
A country's population is distributed as follows: 35% are aged 0-14, 60% are aged 15-64, and 5% are aged 65 and over. Calculate the dependency ratio.
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Identify the dependent and active populations:
How it all connects
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Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- Crude Birth Rate (CBR)
The total number of live births per 1,000 people in a population in a given year. Formula: (Total Births / Total Population) × 1000.
- Crude Death Rate (CDR)
The total number of deaths per 1,000 people in a population in a given year. Formula: (Total Deaths / Total Population) × 1000.
- Natural Increase Rate (NIR)
The percentage by which a population grows in a year, excluding migration. Formula: (CBR - CDR) / 10. Expressed as a percentage (%).
- Total Fertility Rate (TFR)
The average number of children a woman would have during her childbearing years (typically 15-49). It's a hypothetical measure.
- Infant Mortality Rate (IMR)
The number of deaths of children under one year of age per 1,000 live births in a given year. A key indicator of a country's health and development.
- Life Expectancy at Birth
The average number of years a newborn infant can expect to live if current mortality rates continue to apply.
- Dependency Ratio
The ratio of people not in the workforce (dependants: under 15 and over 64) to those in the workforce (economically active: 15-64). Formula: ((% under 15 + % over 64) / % 15-64) × 100.
- Population Momentum
The tendency for a population to continue to grow even after fertility rates have fallen to replacement level (TFR ≈ 2.1). This is due to a large proportion of people in their childbearing years.
- Replacement Level Fertility
The TFR at which a population exactly replaces itself from one generation to the next, without migration. This rate is roughly 2.1 children per woman in most developed countries.
- Doubling Time
The number of years needed for a population to double in size. A quick estimate is the 'Rule of 70': Doubling Time ≈ 70 / NIR (%).
Name it
Read the meaning, then pick which of this lesson’s terms it describes. Miss one and you see what your choice really means.
The TFR at which a population exactly replaces itself from one generation to the next, without migration. This rate is roughly 2.1 children per woman in most developed countries.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
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Teach it back
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Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
Factors reducing fertility: Increased female education and employment, access to family planning, lower infant mortality (less need for 'replacement' children), urbanisation, and rising costs of child-rearing.
Factors reducing mortality: Improvements in public health (sanitation, clean water), advances in medicine (vaccines, antibiotics), better nutrition, and increased safety standards.
Factors increasing mortality (in specific contexts): Pandemics (e.g., HIV/AIDS, COVID-19), conflict and war, lifestyle diseases in HICs (e.g., heart disease), and natural disasters.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Test your knowledge on Changing Populations
Test your knowledge on Changing Populations
Extra simulations & links
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Frequently asked
Checkpoint
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