Natural Increase
Defines natural increase as the numerical difference between a population's crude birth rate and crude death rate, distinguishing it from net migration as a separate driver of population change. The key insight is that natural increase can be positive, negative, or zero depending on the balance of births and deaths, and it varies systematically across countries at different stages of the demographic transition. Contains: text explanation, formula for the natural increase calculation, worked example comparing two countries, and a common-mistake callout on conflating natural increase with total population growth.
Natural increase is one of the two fundamental components of population change (the other being migration). It is defined as the difference between the birth rate (the number of live births per 1,000 people in a population per year) and the death rate (the number of deaths per 1,000 people in a population per year), both measured over the same time period, usually one year.
When the birth rate exceeds the death rate, a population experiences positive natural increase and grows, even if no migration occurs at all. When the death rate exceeds the birth rate, a population experiences natural decrease. Natural increase is usually expressed as a percentage, allowing easy comparison between countries of very different population sizes.
Crude birth and death rates are expressed per 1,000 population; dividing by 10 converts the per-mille difference into a percentage rate of natural increase.
Natural increase is closely linked to a country's position in the demographic transition model. Countries with high fertility and relatively low mortality, such as Nigeria, tend to have high rates of natural increase and youthful, fast-growing populations. Countries with low fertility and an ageing population, such as Japan, may have very low or even negative natural increase, contributing to population decline and a rising dependency ratio.
It is important to remember that natural increase says nothing about migration. A country can have high natural increase but still lose overall population if emigration is very large, and conversely a country with natural decrease can still grow in total population if immigration is high enough to compensate.
Comparing natural increase in two contrasting countries
- Country A has a birth rate of 35 per 1,000 and a death rate of 8 per 1,000. Country B has a birth rate of 8 per 1,000 and a death rate of 10 per 1,000.
- For Country A: Natural Increase = (35 − 8) / 10 = 2.7%. This is a high rate of natural increase, typical of a youthful, high-fertility population such as Nigeria.
- For Country B: Natural Increase = (8 − 10) / 10 = −0.2%. This negative value indicates natural decrease, typical of an ageing, low-fertility population such as Japan.
- Conclusion: Country A's population will grow rapidly from natural increase alone, while Country B's population will shrink from natural causes unless offset by net immigration.
Common mistake: Students often treat 'natural increase' and 'population growth' as identical terms. Natural increase only accounts for births minus deaths. Total population change also includes net migration (immigration minus emigration), so a country can have positive natural increase yet still lose total population if emigration is very high, or vice versa.
- Natural increase = birth rate − death rate, expressed as a percentage after dividing by 10
- Positive natural increase means births exceed deaths; natural decrease means deaths exceed births
- Natural increase excludes migration entirely — it is a purely demographic (biological) measure
- High-fertility countries like Nigeria show high natural increase; ageing, low-fertility countries like Japan show low or negative natural increase
- Total population change = natural increase + net migration
Japan's Ageing Population
Explains how decades of very low fertility combined with the world's highest life expectancy have produced an aged population structure and a rapidly rising dependency ratio in Japan. The key insight is that a shrinking working-age base must support a growing elderly population, straining pensions, healthcare, and labour supply, and that government responses (pro-natalist policy, immigration, automation) only partially offset the structural imbalance. Contains: text explanation, population pyramid image, worked example analysing the dependency ratio, and a common-mistake callout distinguishing ageing from population decline.
Japan is the world's clearest example of an aged population structure, resulting from a sustained fall in fertility below replacement level combined with one of the highest life expectancies on Earth (around 84-85 years). Japan's total fertility rate has remained below 1.4 births per woman for decades—well under the replacement level of 2.1—while improvements in healthcare, diet, and sanitation have extended life expectancy. The result is a population pyramid that has inverted: it narrows sharply at the base (fewer children) and bulges in the older age groups, rather than forming the classic broad-based triangle of a youthful, high-fertility country such as Nigeria.
This demographic shift raises Japan's dependency ratio—the number of dependents (children aged 0-14 plus elderly aged 65+) relative to the working-age population (15-64). As the elderly share of the population grows and the working-age cohort shrinks, fewer economically active people must support a proportionally larger dependent group, mostly through pensions, healthcare spending, and social care. Unlike a youthful country like Nigeria, where a high dependency ratio stems mainly from a large child population that will eventually enter the workforce, Japan's dependency burden is dominated by the elderly component, which will not convert into future labour supply.
Standard formula for calculating the total dependency ratio, expressed per 100 working-age people.
Interpreting Japan's rising dependency ratio
- Identify the driver: fertility has stayed below replacement level (roughly 1.3-1.4) for several decades, reducing the inflow of young people into the 0-14 and eventually 15-64 age bands.
- Identify the second driver: life expectancy has risen to among the highest globally, meaning people remain in the 65+ 'dependent' category for many more years than in the past.
- Recognise the compounding effect: as the working-age population (15-64) shrinks in absolute and relative terms, the denominator of the dependency ratio falls while the elderly numerator rises, pushing the ratio upward.
- Explain the consequence: a smaller workforce must fund pensions and healthcare for a larger elderly population, increasing fiscal pressure and prompting policy responses such as raising the retirement age, encouraging female labour-force participation, promoting automation/robotics, and cautiously expanding immigration.
Common mistake: Students often equate an 'ageing population' with a 'declining population' and treat them as the same process. Ageing refers to a rising median age and growing elderly share of the population structure; decline refers to the total population size falling. Japan is experiencing both simultaneously, but a country can age (elderly share rising) while its total population is still growing, if fertility is only moderately below replacement and past momentum sustains numbers for a time.
Exam tip: When examining Japan's ageing society, always link the demographic cause (sustained sub-replacement fertility + rising life expectancy) to a specific consequence (e.g. pension strain, labour shortages, rural depopulation, or policy responses like immigration reform). Simply describing the shape of the pyramid without this cause-effect chain will not earn full marks at AO2/AO3.
- Japan's total fertility rate has stayed well below the 2.1 replacement level for decades, driving long-term population ageing.
- Japan has one of the world's highest life expectancies (~84-85 years), extending time spent in the elderly dependent age bracket.
- Japan's population pyramid is top-heavy: narrow base, bulging upper/older bands, unlike youthful countries such as Nigeria.
- Dependency ratio = (population 0-14 + population 65+) ÷ population 15-64 × 100.
- Japan's dependency burden is driven mainly by the elderly, not children, straining pensions and healthcare with a shrinking workforce.
- Ageing (rising elderly share) is distinct from population decline (falling total numbers) — Japan shows both, but they are not the same concept.
Earthquakes as a Push Factor
Explains how earthquakes act as an environmental push factor by destroying housing, infrastructure, and livelihoods almost instantaneously, forcing sudden and often unplanned displacement of populations. The key insight is that unlike slow-onset hazards, seismic events give people no time to adapt in place, so displacement is immediate, large-scale, and initially internal before sometimes becoming international. Contains: text explanation, worked example, key-concept callout, and a common-mistake callout distinguishing sudden-onset from slow-onset push factors.
Earthquakes are classified as sudden-onset environmental disasters, meaning they strike with little or no warning and cause damage within seconds to minutes rather than over months or years. This immediacy is what makes them such a powerful push factor in forced migration: households cannot gradually adjust their livelihoods or housing before the hazard hits, as they might with slow-onset processes such as drought or desertification. Instead, an earthquake can instantly render homes uninhabitable, destroy roads, bridges, water supplies, and power grids, and disable hospitals and schools -- collapsing the physical infrastructure that a community depends on to remain in place.
When infrastructure destruction is severe, displacement often occurs in stages. Immediately after the event, survivors move short distances to emergency shelters or camps -- this is internal displacement. If reconstruction is slow, if aftershocks continue, or if livelihoods (farms, factories, markets) remain destroyed for an extended period, some of the displaced may undertake longer-distance internal migration toward cities offering aid, jobs, and services, and in some cases cross international borders in search of stability. The scale of displacement depends not only on the earthquake's magnitude but on a country's resilience: building codes, emergency response capacity, and wealth all shape how quickly infrastructure is restored and how many people are forced to leave permanently rather than return home.
Earthquakes push people from places primarily by destroying the built environment -- housing, transport networks, utilities, and economic infrastructure -- rather than by directly altering the natural resource base (as drought does to farmland). This distinction matters when comparing environmental push factors: seismic displacement is driven by damage to human systems, while displacement from drought or desertification is driven by degradation of natural systems.
Tracing displacement after a major earthquake
- Step 1 -- Immediate impact: strong shaking collapses poorly built housing and ruptures water and power lines within the affected zone.
- Step 2 -- Short-term displacement: residents whose homes are destroyed or structurally unsafe move to temporary shelters, relatives' homes, or camps nearby -- this is internal displacement, not international migration.
- Step 3 -- Infrastructure collapse prolongs the crisis: with roads, hospitals, and utilities disabled, aid delivery and reconstruction slow down, keeping people out of their original homes for months or years.
- Step 4 -- Secondary migration: if local livelihoods (agriculture, small business, factory jobs) cannot recover, some displaced people migrate further -- often to a capital city or another region with intact infrastructure and job opportunities.
- Step 5 -- Outcome: the earthquake has acted as a push factor by removing the physical and economic basis for staying, even though the hazard event itself lasted only seconds.
Common mistake: Students often lump earthquakes together with slow-onset hazards like drought when listing 'environmental push factors', without noting the difference in timing. In exam answers, always specify that earthquakes are sudden-onset -- this affects the speed of displacement (immediate, reactive) and the type of damage (infrastructure destruction) compared with slow-onset hazards, which cause gradual livelihood decline (e.g. loss of farmland) and allow more time for adaptive strategies before people are forced to move.
- Earthquakes are sudden-onset hazards: displacement can occur within minutes, with no time for households to adapt in place.
- The main push mechanism is infrastructure destruction -- collapsed housing, damaged roads, disabled utilities and hospitals.
- Displacement is typically staged: immediate internal displacement to shelters/camps, then possible secondary migration if recovery is slow.
- A country's building codes, wealth, and emergency response capacity (resilience) shape how many people are permanently displaced.
- Earthquakes differ from slow-onset environmental push factors (e.g. drought) which degrade natural resources gradually rather than destroying built infrastructure suddenly.
Fertility Rate
Defines fertility rate as the average number of births per woman during her reproductive years and explains how it drives population structure, from youthful growing populations to ageing, shrinking ones. The key insight is that fertility rate relative to a replacement-level threshold determines whether a population grows, stabilizes, or declines over time. Contains: text explanation, a formula-style definition, a comparative table of Japan and Nigeria, a worked example, and a common-mistake callout.
Fertility rate (more precisely, the total fertility rate, TFR) is the average number of children a woman would have during her childbearing years (typically ages 15-49), based on current age-specific birth rates. It is one of the most important demographic indicators because it directly determines the shape of a country's population pyramid and its long-term trajectory of growth or decline.
Total fertility rate is calculated by summing the age-specific birth rates across a woman's reproductive lifespan (usually ages 15-49).
A critical benchmark is the replacement level fertility rate, generally around 2.1 births per woman in high-income countries (slightly higher in countries with high infant mortality, since more children must be born to ensure two survive to adulthood). This figure accounts for two children replacing their parents, with the extra 0.1 offsetting deaths before reproductive age.
- If TFR exceeds 2.1, the population will grow across generations (assuming no offsetting migration).
- If TFR is at 2.1, the population will eventually stabilize.
- If TFR is below 2.1, the population will shrink over time, leading to an ageing structure.
| Country | Fertility rate pattern | Population structure consequence |
|---|---|---|
| Japan | Low fertility, well below replacement level | Ageing society, shrinking workforce, high dependency ratio |
| Nigeria | High fertility, well above replacement level | Youthful population structure, rapid population growth |
Fertility rate shapes the base of the population pyramid directly: high fertility produces a wide base (many young dependents), while low fertility produces a narrow base that, over decades, works its way up the pyramid to create a top-heavy, ageing structure.
Interpreting fertility rate change
- A country has a TFR of 1.3 births per woman, well below the replacement level of roughly 2.1.
- This means, on average, each generation of women is producing fewer children than needed to replace the current population size.
- Over successive generations, the number of people entering the workforce and reproductive age will shrink relative to older, retired generations.
- This produces an ageing population and a rising dependency ratio, since fewer working-age people must support a growing number of elderly dependents.
- Conclusion: a persistently sub-replacement TFR is a long-term driver of population decline and demographic ageing, as seen in countries such as Japan.
Common mistake: Students often confuse fertility rate with birth rate. Birth rate measures live births per 1,000 people in the total population per year, whereas fertility rate measures the average number of births per woman across her reproductive lifespan. Fertility rate is a rate specific to women of childbearing age, not the whole population, so the two figures can differ significantly, especially where age structures vary widely between countries.
- Fertility rate = average number of children a woman has during her reproductive years (roughly ages 15-49).
- Replacement level fertility is approximately 2.1 births per woman in developed countries.
- TFR above 2.1 → population growth; TFR at 2.1 → stabilization; TFR below 2.1 → population decline and ageing.
- Fertility rate directly shapes the base width of a population pyramid.
- Do not confuse fertility rate (per woman) with birth rate (per 1,000 total population).