In simple terms
A friendly intro before the formal notes — no formulas yet.
Population
A population grows when births and immigration outnumber deaths and emigration. As countries develop, births fall and people live longer, so the population ages — and fewer workers must support more pensioners.
Think of a country as a household budget: workers are the earners, children and pensioners the dependants. A household with four earners and two dependants manages easily; one with three earners and five dependants feels the strain even if everyone is well.
- 1
Growth = birth rate − death rate + net migration, measured per 1 000 people.
- 2
Birth rates fall as countries develop; death rates fall with health and sanitation.
- 3
The dependency ratio compares the young and old with the working-age population.
- 4
An ageing population raises pension and health costs and shrinks the workforce.
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
At a glance — side by side
Compare key properties side by side — ideal for exam contrasts.
Population patterns in developing and developed countries
| Feature | Typical developing country | Typical developed country |
|---|---|---|
| Birth rate | High: children as workers and old-age support, high infant mortality, limited family planning | Low: education, women in paid work, contraception, pensions |
| Death rate | Falling as sanitation and healthcare improve; can still be high | Low, but rising as the population ages |
| Age structure | Young: wide-based pyramid, many under 15 | Old: narrow base, many over 64 |
| Main dependency burden | Children: education and housing | Pensioners: pensions and healthcare |
| Occupational distribution | Large primary sector | Large tertiary sector |
| Policy concern | Slowing growth and providing services | Funding an ageing population and keeping the workforce large enough |
Birth rate
Typical developing country
Typical developed country
Death rate
Typical developing country
Typical developed country
Age structure
Typical developing country
Typical developed country
Main dependency burden
Typical developing country
Typical developed country
Occupational distribution
Typical developing country
Typical developed country
Policy concern
Typical developing country
Typical developed country
Full topic notes
Formal explanation with the rigour you need for the exam.
What makes a population grow
A country's population changes through three flows. The birth rate is the number of live births per 1 000 people per year; the death rate is the number of deaths per 1 000 per year. The difference is the natural increase (or decrease). Net migration — immigrants minus emigrants — adds or subtracts the third flow.
Population growth rate = birth rate − death rate + net migration rate. A country with a birth rate of 20, a death rate of 8 and net migration of +2 per 1 000 grows by 14 per 1 000, or 1.4% a year. At that rate a population doubles in roughly 50 years (70 ÷ 1.4). The fertility rate — average births per woman — is the other common measure; a rate of about 2.1 keeps a population stable without migration.
Birth rate and death rate are measured per 1 000 people per year.
Natural increase = birth rate − death rate; add net migration for total growth.
Growth rate of 1.4% a year doubles a population in about 50 years (rule of 70).
Fertility rate of about 2.1 births per woman is the replacement level.
Why birth and death rates differ
Birth rates are high where children are an economic asset (farm labour, support in old age), where infant mortality is high so families have more children to be sure some survive, where women marry young and have fewer opportunities for education and paid work, where contraception is scarce or culturally discouraged, and where religion favours large families. As countries develop, all of these change and birth rates fall — the demographic transition.
Death rates fall with clean water, sanitation, vaccination and better healthcare, with improved diets, and with safer work. Wars, famine and epidemics raise them. A developed country can have a higher death rate than a developing one simply because its population is older — so the death rate alone is a poor guide to health.
High birth rates: children as workers and old-age support, high infant mortality, early marriage, limited female education, little family planning, religious influence.
Falling birth rates with development: education, women's employment, urban living, contraception, pensions.
Falling death rates: clean water, sanitation, vaccines, healthcare, nutrition, safer work.
An old population raises the death rate even in a healthy country.
If asked why a developing country's birth rate is high, give three distinct reasons and link each to development — 'infant mortality is high, so…', 'few women are in paid work, so…'. Lists without links score only half the marks.
The structure of a population
Beyond its size, a population has a structure. The age structure — shown in a population pyramid — determines the share of the population that works. The gender distribution affects the size of the labour force where women's participation is low. The geographical distribution — rural or urban, which regions — shapes housing, transport and regional policy. The occupational distribution — primary, secondary, tertiary — reflects the stage of development and changes as economies grow.
The dependency ratio compares the dependent population (under 15 and over 64) with the working-age population (15–64). A ratio of 0.5 means two workers support every dependant; a ratio of 0.8 means only 1.25 workers per dependant. The higher the ratio, the heavier the burden of taxes and savings on each worker.
Age structure decides the share of the population that is of working age.
Gender, geographical and occupational distribution affect the labour force and where investment is needed.
Dependency ratio = (under 15 + over 64) ÷ working-age population (15–64).
A higher dependency ratio means more dependants per worker and a greater tax burden.
Effects of changes in size and structure
A rapidly growing population brings a larger future labour force and market but strains schools, housing, healthcare and food supply, and can push GDP per head down if output grows more slowly than people. An ageing population — common in developed countries with low birth rates and long life expectancy — raises pension and healthcare costs, shrinks the workforce, raises the dependency ratio, and can slow growth and innovation, though older people also have skills and savings.
A falling population reduces demand and the labour force and can leave infrastructure underused. The optimum population is the size at which output per head is highest given the country's resources and technology; a country is under-populated if more people would raise output per head and over-populated if fewer would. Migration changes structure quickly: immigrants are mostly of working age, which lowers the dependency ratio of the receiving country and raises that of the country they leave.
Rapid growth: bigger future labour force and market, but pressure on services and possibly lower GDP per head.
Ageing: higher pension and health costs, smaller workforce, higher dependency ratio; responses include later retirement and immigration.
Optimum population: the size that maximises output per head; under- and over-population are relative to resources.
Migration shifts working-age people between countries, changing both dependency ratios.
Any 'effects of an ageing population' question should include a government response — raising the retirement age, encouraging immigration, raising taxes, or encouraging private pensions — and a comment on which groups each response hurts.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Country X has a population of 50 million. In one year there are 20 births and 8 deaths per 1 000 people, and net migration adds 2 people per 1 000.
(a) Calculate the rate of natural increase and the total population growth rate. (b) Calculate the population at the end of the year. (c) Estimate how many years it would take the population to double at this growth rate, and explain one problem rapid growth might cause.
- 1
Step 1 — Natural increase (a): birth rate − death rate = 20 − 8 = 12 per 1 000, or 1.2% a year. Adding net migration of 2 per 1 000 gives a total growth rate of 14 per 1 000 = 1.4% a year.
Country Y has 12 million people under 15, 30 million aged 15–64 and 8 million aged 65 and over. Forecasts suggest that in twenty years the under-15 group will be 10 million, the 15–64 group 30 million and the over-64 group 15 million.
(a) Calculate the dependency ratio today. (b) Calculate the forecast dependency ratio. (c) Analyse two economic effects of the change and suggest one policy response.
- 1
Step 1 — Dependency ratio today (a): dependants = 12 + 8 = 20 million; working age = 30 million. Ratio = 20 ÷ 30 = 0.67 — about 67 dependants per 100 workers.
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
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.
- birth rate
The birth rate is the number of live births per 1 000 people per year; the death rate is the number of deaths per 1 000 per year.
- natural increase
The difference is the natural increase (or decrease). Net migration — immigrants minus emigrants — adds or subtracts the third flow.
- Net migration
Immigrants minus emigrants — adds or subtracts the third flow.
- fertility rate
Average births per woman — is the other common measure; a rate of about 2.1 keeps a population stable without migration.
- Birth rates
Birth rates are high where children are an economic asset (farm labour, support in old age), where infant mortality is high so families have more children to be sure some survive, where women marry young and have fewer opportunities for education and paid work, where contraception is scarce or culturally discouraged, and where religion favours large families.
- age structure
Shown in a population pyramid — determines the share of the population that works.
- geographical distribution
Rural or urban, which regions — shapes housing, transport and regional policy.
- occupational distribution
Primary, secondary, tertiary — reflects the stage of development and changes as economies grow.
- optimum population
The optimum population is the size at which output per head is highest given the country's resources and technology; a country is under-populated if more people would raise output per head and over-populated if fewer would.
- Death rate
The number of deaths per 1 000 people in a year.
- Dependency ratio
The number of people under 15 and over 64 divided by the number of working age (15–64).
- Ageing population
A population in which the average age is rising because birth rates are low and people live longer.
- Population pyramid
A diagram showing the number of people in each age group, split by gender.
- Demographic transition
The pattern in which death rates fall first and birth rates fall later as a country develops, so population growth speeds up and then slows.
Sort it out
One statement at a time, say which heading it belongs under. Every statement comes from this lesson’s side-by-side comparison.
Pensioners: pensions and healthcare
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
Birth rate and death rate are measured per 1 000 people per year.
Natural increase = birth rate − death rate; add net migration for total growth.
Growth rate of 1.4% a year doubles a population in about 50 years (rule of 70).
Fertility rate of about 2.1 births per woman is the replacement level.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Mark a population question
Mark a population question
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Mark a population question on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
Discuss Population
Ask, share and discuss with other Economics students