IB Geography 1.3.1 Trends in Family Size, Sex Ratios and Ageing Notes

Family Size, Sex Ratios and Ageing: Three Readings of One Age-Sex Structure

1. The three trends in this statement are three readings of one thing, namely how many people a population holds at each age and of each sex.
2. Family size sets how many people enter that structure at the bottom.

1. The sex ratio sets the balance between its two sides.
2. Ageing, or greying, describes what happens at the top once the entries at the bottom slow down.
3. Each trend is shaped by decisions over births, care, migration and retirement, so demographic change also redistributes power.
   1. Household decisions about childbearing rest partly with women and partly with husbands, parents-in-law and wider kin.
   2. State decisions set what is legal, what is funded and what is counted.
   3. Employer and housing markets set the cost of a child in money and in time out of paid work.
4. A single global average hides the regional divergence this statement asks about.
   1. World fertility stood at 2.25 births per woman in 2024 (UN World Population Prospects 2024), which UN DESA's World Fertility 2024 reports rounded to 2.2.
   2. That average spans a range from 0.75 in the Republic of Korea to 5.9 in Niger in the same year (Statistics Korea; World Bank).

Family Size: Households Are Shrinking on Every Continent, at Different Speeds

1. Average household size has fallen in most countries since 1970, at roughly half a person per household per decade worldwide (Esteve and colleagues, Genus, 2024, using the CORESIDENCE database of 156 countries).
2. The speed of that fall is what separates the continents.
   1. Households in the Americas lost about one person every two decades.
   2. Households in Europe lost about one person every five decades, because they were already small when the series begins.
   3. Africa shows the widest spread, with some countries recording almost no change across three decades.

1. The gap between countries is far wider than the gap between decades.
   1. Denmark averaged 1.83 people per household, the smallest value in that database.
   2. Senegal averaged 8.42, the largest.
2. Fertility sets the floor under household size, which is where the regional gap is widest.
   1. Sub-Saharan Africa recorded 4.3 births per woman in 2024, against a world figure of 2.25 (UN World Population Prospects 2024, reported by INED).
   2. Oceania excluding Australia and New Zealand recorded 3.0 in the same year.
   3. Northern Africa and Western Asia recorded 2.7.
   4. Central and Southern Asia recorded 2.2, the closest of the regions to the world figure.
3. Fertility now sits below the replacement level of 2.1 in 131 of 237 countries and areas, holding more than two thirds of the world's people (UN DESA, World Fertility 2024).
   1. More than one country in ten now records fewer than 1.4 births per woman (UN DESA, World Fertility 2024).
   2. China, the Republic of Korea, Singapore and Ukraine record fewer than 1.0.

Household Size Falls Faster Than Births Alone Would Explain

1. A household is a co-residence unit, so its size moves with living arrangements as well as with the number of births.
2. Later Marriage And Later First Births
   1. By 2024 the mean age at childbearing had passed 30 years in 38 of the countries and areas with a long history of low fertility, over two thirds of that group (UN DESA, World Fertility 2024).
   2. A later start compresses the years left in which a second or third birth can happen.
3. Nuclear Rather Than Extended Co-Residence
   1. Where adult children set up separate homes, one large household becomes two or three small ones with no change in the number of births.
   2. Urbanization pushes the same way, since urban dwellings are smaller and cost more per square metre than rural ones.
4. The Rise Of The One-Person Household
   1. One-person households headed by adults under 50 have grown in Europe, the Republic of Korea, Australia and Botswana (Esteve and colleagues, Genus, 2024).
   2. They remain rare across Africa and most of Asia.
   3. One-person households headed by people over 50 make up about a quarter of all households in Hungary, Germany and the Netherlands, against under a tenth across Africa and most of Asia.

Sex Ratios: A Narrow Natural Range, Skewed in a Named Set of Countries

1. The sex ratio at birth is the number of boys born per 100 girls, a quantity that varies far less between places than any other demographic rate.
2. The World Health Organization treats about 105 boys per 100 girls as expected, inside a natural range of roughly 103 to 107.
3. More boys are born than girls while male mortality runs higher at nearly every age, so the ratio falls across the life course.
   1. By age 50 it is close to 100 males per 100 females.
   2. By age 70 it is about 86 males per 100 females.
   3. Among people aged 100 and over it is about 24 (Our World in Data, 2021).

The Natural Level Is Regional, Not One Universal Number

1. Chao, Gerland, Cook and Alkema estimated a separate reference level for each region from 10,835 observations across 202 countries (PNAS, 2019).
   1. Sub-Saharan Africa has the lowest reference level, at 103.1 males per 100 female births.
   2. South-Eastern and Eastern Asia sit at 106.3.
   3. Oceania sits at 106.7, the highest of the regional baselines.
2. The spread between those baselines changes what counts as a skew.
   1. A ratio of 105 is unremarkable in Eastern Asia while it sits above the natural level for sub-Saharan Africa.

Where the Ratio at Birth Is Measurably Skewed

1. Chao and colleagues found strong statistical evidence of a skewed sex ratio at birth in 12 countries and territories (Chao, Gerland, Cook and Alkema, PNAS, 2019).
2. Those 12 cluster in three regions rather than scattering across the world.
   1. East and South-East Asia contributes China, Vietnam, Taiwan and Hong Kong.
   2. South Asia contributes India.
   3. The South Caucasus and the Western Balkans contribute Armenia, Azerbaijan, Georgia, Albania and Montenegro.
   4. Tunisia and the Republic of Korea complete the list, with the Korean ratio having since returned to the natural range.

India: A National Average That Hides Its Own Range

1. India's ratio measured by the National Family Health Survey improved from 919 girls per 1,000 boys in NFHS-4 to 929 in NFHS-5 (UNFPA India).
   1. Turned the other way round, that is a fall from about 109 to about 108 boys per 100 girls.
   2. The improvement came mainly from urban areas, with no significant change measured in rural ones.
   3. For third births following two daughters the ratio was 764 girls per 1,000 boys in 2015 to 2018, far more skewed than the national figure.
2. The national figure averages over a wide internal range, so it hides where the skew actually sits.
   1. Himachal Pradesh recorded 875 girls per 1,000 boys at birth and Rajasthan 891 (UNFPA India, NFHS-5).
   2. Chhattisgarh recorded 960 and Karnataka 978, close to or inside the natural range.
   3. The gap between those state figures is wider than the gap between India's national figure and the natural level.
3. The conditions behind son preference vary by state as well, which is what a gender parity index measures.
   1. Bihar scored 0.42 and Mizoram 0.70 on the Femdex index, where 1.00 is parity (McKinsey Global Institute, Power of Parity: India, November 2015).
   2. Femdex is a composite of labour, education and legal indicators, so it describes women's position rather than the sex ratio itself.

The Three Conditions That Produce a Skew

1. A skewed ratio at birth needs three measurable conditions to hold at the same time.
2. Son Preference
   1. Where inheritance passes through sons, where married daughters move to the husband's household, and where old-age support comes from sons, a son carries a larger expected return to parents than a daughter.
   2. The preference is about that expected economic and social return rather than about feeling toward individual children.
3. Access To Prenatal Sex Determination
   1. Ultrasound made the sex of a fetus cheaply knowable from the 1980s onward.
   2. Sex-selective abortion turns a preference into a birth statistic, which is why measured skews appear only after the technology spreads.
4. Falling Fertility
   1. A couple intending six children will very probably have a son without doing anything.
   2. A couple intending two faces a one in four chance of having no son, a pressure demographers call the fertility squeeze.
   3. This is why the skew is sharpest at third and higher birth orders in both China and India.

Missing Women: The Cumulative Count

1. Postnatal neglect adds to the prenatal effect, so the two are counted separately.
   1. UNFPA put the cumulative number of missing women at 142.6 million in 2020, up from 61 million in 1970 (State of World Population 2020).
   2. About two thirds of that total came from prenatal sex selection and about one third from excess female mortality after birth.
   3. China accounted for 72.3 million and India for 45.8 million of the 2020 total.
   4. That count is a stock of women absent from the living population, which is a different quantity from the 23.1 million missing female births estimated for 1970 to 2017.

Migration Skews Sex Ratios Without Touching Births

1. The world's total population sex ratio was about 101 males per 100 females in 2024 (UN World Population Prospects 2024).
2. A national ratio also moves when adults arrive or leave in unequal numbers by sex.
3. Male-dominated labour immigration produces the world's most male-skewed national populations.
   1. Qatar's population was 71% male in 2024, the highest share of any country (World Bank).
   2. The United Arab Emirates, Oman, Bahrain, Kuwait and Saudi Arabia follow, all of them recruiting construction and service labour largely from South and South-East Asia.
   3. The skew sits in the working ages, so it shows as a bulge on the male side of the pyramid rather than at its base.
4. Female-skewed national ratios come from the opposite movement together with longer female survival.
   1. Hong Kong, Moldova, Guadeloupe and Martinique record the world's lowest sex ratios (UN World Population Prospects 2024).
   2. Sustained emigration of working-age men and a large older population pull the ratio the same way.
5. The decision here sits with the state rather than the household, since visa rules and recruitment agreements set who may enter and on what terms.

Ageing: Every Region Is Greying, from Very Different Starting Points

1. Ageing, or greying, means a rising share of a population at older ages, which two changes produce together.
   1. Fewer births narrow the base of the population pyramid.
   2. Longer survival widens its top.

1. Falling fertility does more of the work than rising life expectancy, because it changes the share at every age rather than adding years only at the end.
2. There were 857 million people aged 65 or over in 2025, about 10% of the world (Pew Research Center analysis of UN World Population Prospects 2024).
   1. The UN projects that number to reach 2.2 billion by the late 2070s, when it would pass the number of children under 18 for the first time.
   2. Pew reads the same projections as putting the 65 and over share at 24% by 2100.
3. The regional spread in age structure is very wide, with Europe and Africa at its two ends.
   1. Africa had 40% of its people under 15 and 3% aged over 65 in 2022 (Population Reference Bureau).
   2. Europe had 16% under 15 and 19% over 65 in the same year, the only region where the older group is the larger of the two.
   3. Asia had 24% under 15 and 10% over 65.
   4. Latin America and the Caribbean was close to Asia at 23% and 10%.
   5. North America had 18% and 17%, the narrowest gap outside Europe.
   6. The world figures were 25% and 10%.
4. Median age tells the same story in one number.
   1. The world median age was 31 years in 2025, against 19 in Africa and 43 in Europe (Pew Research Center, using UN World Population Prospects 2024).
   2. Latin America and the Caribbean sat at 32 years and the United States at 39.
   3. Africa is projected to stay the youngest region in 2100 at a median age of 35, by which point the world figure would be 42.

Europe Holds the Oldest Regional Structure, Japan the Oldest National One

1. A little over a fifth of the European Union, 22.0%, was aged 65 or over on 1 January 2025 (Eurostat).
   1. Italy at 24.7%, Portugal at 24.3% and Bulgaria at 24.0% held the highest shares.
   2. Luxembourg at 15.2% and Ireland at 15.7% held the lowest.
   3. The EU median age reached 44.9 years on the same date.
2. East Asia is ageing quickly, though its regional aggregate is still younger than Europe's.
   1. The World Bank's East Asia and Pacific region stood at 13.9% aged 65 or over in 2025, where the world figure was 10.2% in 2024.
   2. That aggregate pools Japan and China with much younger Indonesia and the Philippines, so it runs below the oldest countries inside it.
3. Japan is the oldest single country in that region, with 29.8% of its population aged 65 or over in 2024 (World Bank).
4. The 80 and over group is growing faster than the 65 and over group.
   1. People aged 80 or over made up 6.2% of the EU on 1 January 2025, a share Eurostat projects to reach 15.3% by 2100.
   2. That group needs a different mix of services from people in their late sixties, so a single 65 and over figure flattens the difference.

1. The population pyramid shows the sex dimension of ageing that a single percentage hides.
   1. The female surplus in Japan's pyramid begins above roughly age 70 and widens with age.
   2. Decisions on pensions, long-term care and housing therefore land on a group that is disproportionately female.

Speed of Ageing Separates Regions More Than Level Does

1. Europe's populations aged over a long period, which left time to build pension and health systems alongside the change.
   1. France took almost 150 years for the share of its population aged over 60 to move from 10% to 20% (World Health Organization).
   2. Brazil, China and India will make the same move in a little over 20 years.
2. A country that ages that fast reaches a high older share at a lower income per head than European countries did.
   1. The institutions that pay for pensions and long-term care have less time to be built and funded.
   2. Where the change is compressed, families absorb more of the care, which falls unevenly on women.
3. Countries whose populations have already peaked expect the share aged 65 or over to move from 17% in 2024 to 33% by 2054 (UN World Population Prospects 2024).
   1. The same group's population aged 80 or over is projected to rise from 85 million in 2024 to around 267 million by 2100.

How the Three Trends Interact: Each One Changes the Speed of the Others

1. The three trends feed each other, with the direction of that feedback differing by region.
2. Smaller families age a population from the base, which is why Europe is the only region whose over-65 share exceeds its under-15 share.
3. A skewed sex ratio at birth removes future mothers, so it cuts births roughly two decades later and speeds ageing again.
   1. The Chinese cohorts born around the 2004 peak are the ones now reaching childbearing age.
4. Africa's wide base in 2022 followed decades of high fertility, which sub-Saharan rates of 4.3 births per woman in 2024 show has not yet ended.
   1. Ageing is still under way there, because the number of older people is rising quickly even while their share of the total is not.
5. A regional figure and a national figure inside it can point in opposite directions.
   1. Africa's median age of 19 makes it the world's youngest region, while Tunisia sits among the 12 countries with a measured skew in the sex ratio at birth.
   2. No sub-Saharan country appears on that list, which records where a skew has been measured rather than proving that none exists.