IB Geography G.2.1 Urbanization and Centripetal Population Movements Notes

Urbanization Is a Share of a Population and Urban Growth Is a Count

1. Urbanization is measured as the proportion of a population living in urban areas, so the figure moves only when that proportion moves.
2. Urban growth is the change in the number of people living in urban areas, which is a count and can rise while the proportion falls.
   1. A country whose cities gain two million residents in a decade while its countryside gains three million has urban growth and no urbanization at all.
3. The rate of urbanization is the speed at which the urban share changes, which is why an already urbanized country can urbanize slowly while a mostly rural one urbanizes fast.
4. An urban share and an urban headcount can move in opposite directions over the same decade, so both measures are needed to describe urban change.

The Statistical Office Draws the Urban Boundary and the Number Follows

1. Aggregating each country's own definition of urban puts 58 percent of the world's population in urban areas in 2025.
2. The Degree of Urbanization classifies every square kilometre of land by how densely it is peopled and then groups the cells, sorting settlements into cities, towns and semi-dense areas, and rural areas.
3. On that classification 81 percent of the world's population lived in cities and towns in 2025, against the 58 percent the aggregated national definitions give.
   1. The two figures describe the same people in the same year, so the 23 percentage point gap is a property of the definitions.
4. Redrawing a definition reclassifies people without moving them, which is the third component of urban population change.

Cities Hold 45 Percent of the World's People and Rural Areas Hold 19 Percent

1. 45 percent of the world's 8.2 billion people lived in cities in 2025 on the United Nations Population Division's grid-based count, with 36 percent in towns and semi-dense areas and 19 percent in rural communities.
2. The city share has more than doubled from 20 percent in 1950, when the world held 2.5 billion people.
   1. Over the same period the town share fell from 40 percent to 36 percent.
3. Two-thirds of world population growth between now and 2050 is projected to happen in cities.

The Rural Population Is Near Its Peak and Sub-Saharan Africa Is the Exception

1. The global rural population is expected to peak in the 2040s and to decline after that.
2. Sub-Saharan Africa is the only region that has seen substantial rural population growth in recent decades.
   1. That region is projected to account for nearly all future rural growth.

Natural Increase Does More of the Work than Migration in Most Urbanizing Countries

1. Natural increase is the number of births minus the number of deaths in a population over a period, before any movement in or out is counted.
2. The rate of natural increase is the crude birth rate minus the crude death rate, written per 1,000 people or as a percentage.
3. A city's natural increase depends on its age structure as much as on how many children each woman has, because crude rates are taken over everybody.
   1. Migration delivers young adults, so it raises the share of a city's population in the childbearing ages and lifts the crude birth rate with no change in fertility per woman.
4. That is why a city's population keeps climbing for years after the arrivals slow, through the children of people who moved earlier.
   1. That lag is called population momentum, which is why a city's growth can outlast the conditions that started it.
5. China recorded a crude birth rate of 5.63 per 1,000 against a crude death rate of 8.04 per 1,000 in 2025, giving a rate of natural increase of minus 2.41 per 1,000.

Differences in Natural Increase Explain Most Urbanization Across 129 Countries

1. Ashira Menashe-Oren and Philippe Bocquier estimated the joint contribution of migration and administrative reclassification to the urban transition in 129 countries over 1985 to 2015, using United Nations urban and rural population data by age and sex.
2. They report that differences in natural increase between the rural and urban sectors explain most of the urbanization over those 30 years.
   1. Their paper adds that the role of migration and reclassification declines over the urban transition and becomes negligible.
3. The claim rests on the difference between the two sectors, so urban natural increase need only exceed rural natural increase for the urban share to rise.

Nigeria's Own Survey Puts Urban Birth Rates Below Rural Ones

1. The Nigeria Demographic and Health Survey 2023-24 recorded a total fertility rate of 3.9 children per woman in urban areas against 5.6 in rural areas.
2. The same survey put the crude birth rate at 28 per 1,000 in urban areas and 38 per 1,000 in rural areas.
   1. Urban fertility in Nigeria therefore sits below rural fertility, which rules out higher urban birth rates as the source of any urban advantage in natural increase.
3. Whether urban natural increase exceeds rural natural increase in Nigeria does not follow from these birth rates alone, because that comparison also needs a crude death rate for each sector.

Migrants Weigh the Urban Wage Against the Chance of Getting It

1. Rural-urban migration is the component of urban population change that an industrializing city feels most sharply, because the jobs it creates are concentrated in a small number of places.
2. The standard account sorts the causes into push factors at the origin and pull factors at the destination.
   1. Push factors include small landholdings, low farm incomes, crop failure and the absence of secondary schooling within reach.
   2. Pull factors include wages paid in cash, a wider range of employers and services the origin lacks.
3. Push and pull describe the conditions at each end, while the decision itself needs a rule for weighing one against the other.

Expected Income Explains Migration Into Cities That Already Have Unemployment

1. John Harris and Michael Todaro argued in 1970 that a migrant compares the rural wage with the urban wage discounted by the probability of finding urban work.
2. Their paper states that rural-urban migration will continue so long as the expected urban real income at the margin exceeds real agricultural product.
   1. On that rule a city with heavy unemployment still draws migrants, provided the urban wage is high enough that a chance of getting it beats the certainty of the rural one.
3. China's National Bureau of Statistics put per capita disposable income at 56,502 yuan for urban residents in 2025 and 24,456 yuan for rural residents, a ratio of about 2.3 to 1.
   1. Rural income grew faster that year, at 5.8 percent against 4.3 percent, so the ratio between the two is narrowing even as the absolute gap widens.

Migration Streams Are Selective by Age and by Sex

1. Migration streams are age-selective, drawing disproportionately on people in their late teens, twenties and thirties.
2. Streams are also sex-selective, with the direction set by which industry is doing the hiring.
3. Bangladesh's readymade garment sector has supplied more than 80 percent of the country's export earnings in every year since 2020-21, on Export Promotion Bureau figures compiled by the manufacturers' association.
   1. An International Labour Organization study of 260 garment enterprises found women at 60.5 percent of their workers in 2018, down from 63.4 percent in 2010.

A Household Registration System Turns Migration Into a Legal Status

1. China's hukou system registers every person to a place, so somebody living elsewhere keeps the entitlements attached to the place of registration.
2. The 2020 census counted 492,762,506 people living somewhere other than the place of their household registration.
   1. Of those, 116,945,747 were living in another district of the same city, leaving a floating population of 375,816,759.
3. The same census put China 63.89 percent urban, a share that had risen 14.21 percentage points in ten years.
4. A State Council opinion issued in July 2014 abolished the distinction between agricultural and non-agricultural registration and set settlement rules that tighten as city size rises.
   1. The largest cities were told to run points-based systems, so the migrant who reaches the biggest labour market is the one least likely to be registered there.

Shenzhen Added Seven Million Residents Between Two Censuses

1. Shenzhen's own statistics bureau recorded 17,560,061 permanent residents at the 2020 census.
2. That is 7,136,088 more than the 10,423,973 counted in 2010, a rise of 68.46 percent across the decade.
   1. The average annual growth rate over those ten years was 5.35 percent.
3. With the national rate of natural increase now below zero, the arithmetic that built Shenzhen over that decade no longer has a growing national population behind it.

Inner City Gentrification Is the Centripetal Movement of Post-Industrial Cities

1. Re-urbanization names the return of population to an urban area that had been losing it.
2. Gentrification names something narrower, a change in the social composition of a neighbourhood alongside reinvestment in its housing stock.
   1. A city can re-urbanize with no single neighbourhood gentrifying.
   2. A neighbourhood can gentrify inside a city whose total population is still falling.
3. Ruth Glass named the process in London: Aspects of Change in 1964, writing about Notting Hill and Islington.

Once this process of gentrification starts in a district it goes on rapidly until all or most of the original working class occupiers are displaced and the whole social character of the district is changed.

The Rent Gap Explains Why Reinvestment Arrives When It Does

1. Neil Smith set out the rent gap in the Journal of the American Planning Association in 1979.
2. The gap is the difference between the capitalized ground rent, what a site earns under its present use, and the potential ground rent, what it could earn under its highest and best use.
   1. Years of disinvestment in an inner city district hold the capitalized ground rent down while the potential rises with the wider land market.
3. Reinvestment arrives once the gap is wide enough that a developer can buy cheaply, pay for the rehabilitation and still sell at a profit.
   1. On that account the timing is set inside the land market, so the arrival of new residents follows the arrival of capital.
4. The households who buy into that rehabilitation are concentrated in professional and graduate occupations, which is why a rising share of degree-holding residents is used as an indicator of gentrification.

Philadelphia's Credit Records Let Researchers Test Who Actually Moves

1. Lei Ding, Jackelyn Hwang and Eileen Divringi of the Federal Reserve Bank of Philadelphia tracked residents through an anonymized 5 percent sample of consumer credit records from 2002 to 2014.
2. They classified a census tract as gentrifying where its income sat below the city median in 2000 and it then saw above-median increases both in rents or house values and in the share of residents holding a degree.
3. They found that more vulnerable residents, including those with low credit scores or none and long-standing residents, were no more likely to leave a gentrifying neighbourhood than comparable residents elsewhere in the city.
   1. The difference in mobility for low-score residents came to under one percentage point.
4. Among those who did leave, low-score movers went disproportionately to neighbourhoods with higher unemployment and higher crime than the ones they had left.
   1. Residents who stayed in those tracts saw their credit scores rise by about eleven points over three years relative to residents of other neighbourhoods, with the gain concentrated among those who already scored well.

Berlin Regulates the Upgrading Itself Through Social Preservation Areas

1. Section 172 of Germany's federal Building Code lets a municipality designate a social preservation area, inside which work on a dwelling needs permission.
2. Berlin's Senate Department for Urban Development records 78 such areas, each one set by a district ordinance.
   1. Inside them a district can refuse permission for luxury modernization, for merging flats, for demolition and for turning homes over to other uses.
3. The permission requirement works on the building, so it slows the upgrading that raises rents by controlling what may be done to the dwelling itself.
   1. A tenant is protected here by what a landlord may not do to the flat, which is a different instrument from a rent ceiling or a housing subsidy.

The Same Word Covers Two Opposite Demographic Situations

1. What Pulls People In
   1. An industrializing city pulls with wage employment that does not exist at the origin, so the move crosses from one settlement to another.
   2. A post-industrial district pulls into jobs and housing that already stand, so the move is often from elsewhere inside the same city region.
2. Who Moves
   1. Rural-urban migration selects young adults with little capital, who arrive needing housing at the bottom of the market.
   2. Gentrification selects households with higher incomes and more schooling, who arrive able to bid at the top of a district's market.

The Housing Stock and the Birth Rate Move in Opposite Directions Too

1. What Happens to the Housing Stock
   1. An industrializing city adds dwellings, often outside the formal system, because the stock is too small for the arrivals.
   2. A post-industrial district upgrades dwellings that already stand, because the stock is adequate in number and cheap against the wider city.
2. What Natural Increase Does
   1. Arrivals in an industrializing city lift the crude birth rate and set off a second round of growth through births.
   2. Arrivals in a gentrifying district need not lift the birth rate at all, so a district can gain value while losing people.

Two Counts of the Same Place Separate Arrivals from Births

1. Two censuses of one city, taken on one definition, give the total change in its population.
2. Subtracting the births recorded over the period and adding back the deaths leaves net migration plus reclassification as a residual.
3. Where a boundary shifted between the two counts, part of the recorded growth arrived with the boundary.
4. Applied to a single neighbourhood, the same arithmetic is what identifies gentrification, with the change in who lives there standing where the migration residual stands.