IB Environmental systems and societies 8.1C Age-sex Pyramids and Demographic Transition Model Notes

Age-Sex Pyramids: Reading a Population's Past and Future

How to Read an Age-Sex Pyramid

1. The x-axis shows population size, as an absolute number or a percentage.
2. The y-axis shows age groups, usually in 5-year cohorts.
3. The left side shows males and the right side shows females.
4. The overall shape reveals birth rate, death rate, and life expectancy at a glance, which is why pyramids let you compare societies and predict future trends.

The Three Pyramid Shapes and What They Mean

1. Expanding (youthful population)
   1. A broad base and narrow top signal a high birth rate and lower life expectancy.
   2. Common in lower-income countries such as Nigeria and Afghanistan.
   3. It strains schools, healthcare, and food supply.
2. Stationary (stable population)
   1. Straighter sides with fairly even cohorts until old age.
   2. Birth and death rates are moderate and near replacement level.
   3. Seen in high-income countries such as the USA and France.
3. Contracting (ageing population)
   1. A narrow base and wide top signal a low birth rate and high life expectancy.
   2. Common where populations are shrinking, such as Japan and Germany.
   3. It strains pensions and elderly care as the workforce shrinks.

Why Governments Use Age-Sex Pyramids

1. Planning: allocating resources for healthcare, schools, and jobs.
2. Forecasting: predicting workforce trends and dependency ratios.
3. Policy: shaping decisions on immigration, retirement age, and family benefits.

The Demographic Transition Model: How Rates Shift as Countries Develop

1. The Demographic Transition Model (DTM) shows how birth and death rates change as a society develops, and what that does to population growth.
2. It runs through five stages, from high birth and death rates to low rates and eventual decline.

The Five Stages of the DTM

Stage 1: High Stationary

1. Birth rate: high, from no contraception, high infant mortality, and a cultural preference for large families.
2. Death rate: high, from disease, famine, poor sanitation, and little medical care.
3. Growth: near zero, because high births are cancelled by high deaths.

Stage 2: Early Expanding

1. Death rate: falls sharply as sanitation, food supply, and medicine improve.
2. Birth rate: stays high, held up by cultural habits and little access to contraception.
3. Growth: rapid, as the gap between births and deaths widens.

Stage 3: Late Expanding

1. Birth rate: falls as contraception, healthcare, and women's education spread.
2. Death rate: low and stable.
3. Growth: slows as birth rates approach death rates.

Stage 4: Low Stationary

1. Birth and death rates: both low and stable.
2. Life expectancy: high, from advanced healthcare and living standards.
3. Growth: minimal or stable.

Stage 5: Declining

1. Birth rate: very low, often below the replacement level of 2.1 children per woman.
2. Death rate: stable or rising, because the population is ageing.
3. Growth: negative, so the population shrinks.

The DTM Is a Model, Not a Law

1. Not every country passes through all five stages; some skip or stall.
2. It was based on Europe's history, so it fits other regions imperfectly and ignores migration.