How a population changes
Demography is the study of the size, structure and change of populations.
- Crude birth rate (CBR) = births per 1,000 people in a year.
- Crude death rate (CDR) = deaths per 1,000 people in a year.
- Natural increase = CBR − CDR. Divide by 10 to get a percentage (e.g. 20 − 8 = 12 per 1,000 = 1.2%).
- Net migration = immigrants − emigrants.
- Total fertility rate (TFR) = average number of children a woman has. About 2.1 is the replacement level.
- Life expectancy = average number of years a newborn is expected to live.
The demographic transition
- Stage 1 (high stationary): high births and deaths, little growth.
- Stage 2 (early expanding): deaths fall due to clean water, food, vaccines; births stay high; fast growth.
- Stage 3 (late expanding): births fall due to schooling (especially of girls), city life, jobs for women, family planning.
- Stage 4 (low stationary): low births and deaths; slow growth.
- Stage 5 (declining): births below deaths; the population ages and may shrink unless migrants arrive.
The model is a pattern, not a law: countries move at different speeds, and migration, wars or pandemics can change the path.
Population pyramids
A population pyramid is two back-to-back bar charts: males on one side, females on the other, age bands from youngest (bottom) to oldest (top).
- Expanding (triangle): wide base, many children, high births.
- Stationary (column): similar numbers in most age bands.
- Contracting (narrow base, wide top): fewer births, many older people.
A bulge in one band can show a past baby boom; a dent can show a war, famine or many people leaving.
Ageing and care of older people
Population ageing means the share of older people (usually 65+) rises. Causes: lower fertility and longer life expectancy.
Dependency ratio = (people aged 0–14 + people 65+) ÷ people aged 15–64 × 100. The old-age dependency ratio uses only the 65+ group.
Effects
- More spending on pensions and health care, fewer workers paying taxes.
- Labour shortages in some jobs; later retirement ages.
- Older people also give a lot: experience, voluntary work, caring for grandchildren.
Care as a shared responsibility
Care for older people works best when shared: families (time, love, support), communities (day centres, neighbours, religious and voluntary groups), employers (flexible hours for carers), and the state (pensions, health insurance, home nursing, accessible transport and buildings, laws against neglect). Respect and dignity matter as much as money.
Migration
Migration is moving to live in a new place. Emigrants leave a place; immigrants arrive.
- Push factors: lack of jobs, poverty, conflict, disasters, poor services.
- Pull factors: work, higher wages, safety, education, family ties.
It can be internal (rural to urban) or international, voluntary or forced (refugees). Migrants are often young adults, so the origin area ages and loses workers but gains remittances (money sent home); the destination gains workers and younger residents but must provide housing and services.
Try it at home
Make a family pyramid: list everyone in your extended family by age band (0–14, 15–29, 30–44, 45–64, 65+) and sex, then draw bars on squared paper. Compare it with your grandparents' generation: were families bigger then?
Key formulas and definitions
- Population change = (births − deaths) + (immigrants − emigrants)
- Crude birth/death rate = events ÷ population × 1,000
- Natural increase (%) = (CBR − CDR) ÷ 10
- Dependency ratio = (0–14 + 65+) ÷ (15–64) × 100
- Replacement-level fertility ≈ 2.1 children per woman
Worked examples
1. A town of 50,000 has 900 births and 400 deaths in a year. Find the CBR, CDR and natural increase.
CBR = 900 ÷ 50,000 × 1,000 = 18 per 1,000. CDR = 400 ÷ 50,000 × 1,000 = 8 per 1,000. Natural increase = 18 − 8 = 10 per 1,000 = 1.0%.
2. A country has 30 million children (0–14), 60 million aged 15–64 and 15 million aged 65+. Find the dependency ratio and old-age dependency ratio.
Dependency ratio = (30 + 15) ÷ 60 × 100 = 75. Old-age ratio = 15 ÷ 60 × 100 = 25, so there are 4 working-age people per older person.
3. A region had 2,000 births, 1,500 deaths, 300 immigrants and 1,200 emigrants. Did its population grow?
Change = (2,000 − 1,500) + (300 − 1,200) = 500 − 900 = −400. It fell by 400, because emigration was larger than natural increase.
4. Birth rate 38, death rate 15 per 1,000. Which transition stage fits best and why?
Stage 2: deaths have fallen a lot but births are still high, so growth is fast (2.3% a year).
Common mistakes
- Thinking a falling birth rate means the population falls at once. It keeps growing for years while many young people have children (population momentum).
- Mixing up emigrant (leaves) and immigrant (comes in). "E" = exit, "I" = in.
- Reading a pyramid by total width only. Check the base vs top and both sides.
- Seeing older people only as a burden. They also work, volunteer and care for others.