Population and Migration

Quick Recap from Unit 5

These five ideas came up in People and Cultures. Each link goes back to that section if you want a refresher.

• Density and distribution: people are spread very unevenly across Earth. Most live in a few crowded areas, often near rivers and coasts. Review density in Unit 5.
• Growth: the world passed 8 billion people in November 2022. Growth is now slowing. Review growth in Unit 5.
• Push and pull: push factors drive people away from a place, and pull factors draw them to a new one. Review migration in Unit 5.
• Urban and rural: a growing share of people live in cities rather than in the countryside. Review urban and rural in Unit 5.
• Megacities: some cities now have more than 10 million people each. Unit 5 lists the biggest ones, so this unit looks at how cities grow instead. Review megacities in Unit 5.

Two Ways to Measure Density

Arithmetic density is the number of people divided by the total land area. It is the "population density" you usually see in atlases. But it treats every square kilometer the same, whether it is farmland, desert or mountain.

Physiological density is the number of people divided by the area of arable land, which is land that can grow crops. National Geographic describes it as a way to show how much pressure a population puts on the land that feeds it. A high number means many people depend on each patch of farmland.

Worked example: Egypt

Unit 5 showed that most Egyptians live along the Nile. Physiological density turns that idea into a number. These figures are from the World Bank.

1 Arithmetic density

116,538,258 people (2024) ÷ 995,450 km² of land = about 117 people per km².

2 Physiological density

116,538,258 people ÷ 32,740 km² of arable land (2024) = about 3,560 people per km² of farmland.

Only about 3.3% of Egypt's land is arable. So its physiological density is about 30 times its arithmetic density. Most of the country is desert, which is why farmland along the Nile and in its delta matters so much.

Geographers also use agricultural density: the number of farmers, or people living in rural areas, per unit of arable land (National Geographic).

The Ecumene: Where People Can Live

The ecumene is the land that people live on all year round (Population Education, 2024). Places outside it are usually too dry, too high, too cold or too wet for people to grow food and get what they need. Central Australia and northern Canada are two examples. People do visit, work and travel there, but few live there permanently. You saw the reasons in Unit 4, Climate and Biomes: deserts, ice and tundra are hard places to farm.

Birth Rates, Death Rates and Doubling Time

Demographers are scientists who study population. They count births and deaths per 1,000 people each year.

• The crude birth rate is the number of births per 1,000 people in a year.
• The crude death rate is the number of deaths per 1,000 people in a year.
• Natural increase is the birth rate minus the death rate. Divide it by 10 to get a percentage. A birth rate of 20 and a death rate of 5 give a natural increase of 15 per 1,000, or 1.5%.
• The growth rate adds migration to natural increase. A country can have more deaths than births and still shrink only slowly if people move in.

The rule of 70

How long would it take a population to double if it kept growing at the same rate? A quick estimate is the rule of 70: divide 70 by the yearly growth rate in percent (Population Education). A population growing 2% a year doubles in about 70 ÷ 2 = 35 years. Real growth rates change over time, so doubling time is an estimate, not a forecast.

Japan has about twice as many deaths as births. Its population still fell by only about half a percent in 2024, partly because about 153,000 more people moved in than moved out (UN, 2024).

Reading Population Pyramids

A population pyramid is a pair of bar graphs back to back. Each bar is one age group, five years wide, with the youngest at the bottom. Males are on the left and females on the right. The length of each bar shows that group's share of the whole population. The shape tells you a lot.

The median age is the age that splits a population in half. Half of Niger's people are younger than about 15. Half of Japan's are older than about 49. Each shape brings different needs. A young country needs schools and, soon, many new jobs. An older country needs more health care and pensions, with fewer workers to pay for them.

The Demographic Transition Model

The demographic transition model (DTM) describes how birth and death rates have changed in many countries over time. It is a model, so it simplifies. It was built mostly from the history of European countries, and it leaves out migration (Population Education).

Why do the rates fall?

In stage 2, death rates drop first, thanks to cleaner water, better food and medicine. Birth rates stay high for a while, so the population grows fast. In stage 3, birth rates fall too. Population Education links this to better schooling and health care for girls and women, and to family planning. Egypt shows the change: its birth rate was 47 per 1,000 in 1960 (Population Education) and 20.9 per 1,000 in 2024 (UN). Niger's 2024 rates, a birth rate of 41.4 and a death rate of 8.7, fit the stage 2 pattern.

Is stage 5 real?

Stage 5 is for countries where deaths outnumber births. Population Education calls it a "possible" stage, and it notes that the debate is about whether a country belongs in stage 5 if deaths outnumber births but immigration keeps its population from falling. Germany is one example: in 2014, Population Education noted that its deaths had outnumbered its births every year since 1972, yet its population was still growing. Bulgaria's population is shrinking, with a birth rate of 9 and a death rate of 16 per 1,000.

Why People Move: Beyond Push and Pull

In Unit 5 you sorted reasons to move into push factors and pull factors. Real moves are more tangled than that. Geographers use a few more ideas to describe them.

Intervening obstacles

An intervening obstacle is something that makes a move harder, even when the push and pull are strong. Population Education describes it as an environmental or cultural feature that hinders migration. A sea, a desert or a mountain range can be one. So can the cost of a ticket, a language you don't speak or a visa you can't get. In 1966 the demographer Everett Lee made intervening obstacles one of the four parts of his theory of migration.

Chain migration

In chain migration, people move to a place because relatives or neighbors from home already live there. Earlier migrants share news about jobs, help with travel and offer a first place to stay (MacDonald and MacDonald, 1964). This is why people from one village often end up in the same neighborhood of a far-off city.

Step migration

In step migration, people move in stages. A family might move from a farm to a small town, and years later from the town to a capital city. In 1885 the geographer Ernst Ravenstein noticed that many migrants moved this way.

Voluntary and forced migration

In voluntary migration, people choose to move, often for work, school or family. In forced migration, people are moved against their will or must flee to survive (Britannica). Many moves fall somewhere in between. A family that leaves during a famine chooses when to go, but staying might mean starving. Keep this range in mind as you read about the five migrations below.

Five Migrations That Shaped the World

These five migrations happened at very different times and for very different reasons. Two of them were forced. Each one changed the places people left and the places they reached.

1 The Bantu expansion

Starting roughly 6,000 to 4,000 years ago, groups who spoke early Bantu languages began to spread out from western Africa, near today's border of Cameroon and Nigeria. Over thousands of years, their descendants moved first through the Congo rainforest, then east and south. Today about 350 million people speak more than 500 Bantu languages, such as Swahili and Zulu. That is about 30% of Africa's people (Nature, 2023). Scientists use language, archaeology and DNA to study the spread, and they still debate the exact paths.

2 The transatlantic slave trade

From 1501 to 1866, slave ships carried about 12.5 million African people from Africa toward the Americas (Slave Voyages database). They were captured, sold and shipped against their will. Packed below deck in chains, with little food, water or air, about 1.8 million people died during the crossing, called the Middle Passage, and about 10.7 million survived it (Slave Voyages). Captives resisted in many ways, including revolts on board. More captives were taken from West Central Africa than from any other region. Brazil received more people than any other place, about 4.9 million. Nearly three-fifths of the trade took place in the 1700s (Britannica). The enslaved people and their descendants shaped the languages, music, food and faith of the Americas.

3 European emigration in the 1800s

In the 1800s and early 1900s, tens of millions of Europeans crossed the Atlantic. Britannica calls this the Great Atlantic Migration. An early wave came from Ireland and Germany in the 1840s. In Ireland, a potato disease caused the Great Famine of 1845–49. About 1 million people died, and perhaps 2 million left. Ireland's population fell from 8.4 million in 1844 to 6.6 million in 1851. From the 1880s, more emigrants came from southern and eastern Europe. About 17 million people entered the United States from 1880 to 1910 alone.

4 The Great Migration

From the 1910s to the 1970s, about 6 million Black Americans moved from the rural South to cities in the North, Midwest and West (National Archives). They left to escape racial violence and the Jim Crow laws, and to find better jobs and schools. In the first phase (1910–1940), many went to New York, Chicago, Detroit and Pittsburgh. During and after World War II, many also went west to Oakland, Los Angeles, San Francisco, Portland and Seattle. Many still faced unfair treatment, such as housing discrimination, in their new cities. This migration happened inside one country.

5 The Partition of India, 1947

On August 15, 1947, British India was split into two countries, India and Pakistan. New borders cut through the provinces of Punjab and Bengal. About 15 million Hindus, Muslims and Sikhs fled across these borders, most of them on foot. Estimates of the number killed in the violence range from 200,000 to 2 million (Britannica). It was one of the largest forced migrations in history.

Migration Today

Unit 5 gave the total number of international migrants. Here is how that number has changed, and what lies behind it. An international migrant is someone living in a country other than the one where they were born.

So about 96% of people live in the country where they were born. The largest single route, or corridor, is from Mexico to the United States, with about 11 million people born in Mexico living in the United States in 2024 (IOM, 2026).

Labor migration

Most migrants move for work. The International Labour Organization estimated nearly 168 million migrant workers in 2022 (IOM, 2026). This links to Unit 7: just as goods move to where they are needed, so do workers.

Refugees and internally displaced people

Some people are forced to leave home by war, violence or persecution. A refugee has crossed an international border to find safety. An internally displaced person (IDP) has also fled home, but is still inside their own country. UNHCR, the UN Refugee Agency, counts both.

More forcibly displaced people stay inside their own country than cross a border. The total fell in 2025 for the first time in a decade, mainly because many people returned home.

Remittances

Remittances are money that migrants send home to their families. The World Bank estimated that $685 billion was sent to low- and middle-income countries in 2024. That is more than those countries received from foreign investment and foreign aid combined. India received the most, about $129 billion, followed by Mexico, about $68 billion. In Tajikistan, remittances were equal to about 45% of the whole economy (World Bank, December 2024 estimates).

How Cities Grow

Unit 5 compared cities, towns and rural areas in 2025. Look back further and the change is huge. In 1950, about 20% of the world's 2.5 billion people lived in cities. By 2025 it was 45% of 8.2 billion. The number of cities more than doubled, from 5,851 in 1975 to 12,140 in 2025 (UN World Urbanization Prospects 2025).

Why cities grow

The UN lists three ways a city's population grows:

1. Natural increase: more births than deaths among people already living there.
2. Migration: people move in from rural areas or from other countries.
3. Reclassification: a growing town crosses the line and is counted as a city, or a city's boundary grows to take in nearby land.

The mix depends on the place. In China, between the 2010 and 2020 censuses, 56% of urban growth came from people moving in from rural areas, 26% from reclassification and 18% from natural increase. But in many countries, the UN found, natural increase and reclassification add more people to cities than migration does.

Informal settlements

When a city grows faster than it can build homes, roads and pipes, people build their own housing on land that is not planned for it. These informal settlements often lack clean water, toilets and sturdy buildings. More than 1 billion people live in them worldwide (UN, 2025). In Nairobi, Kenya, more than half of the people live in informal settlements. The city's population grew from 2.6 million in 2000 to 6.1 million in 2025, while its land area grew from 378 to 638 km². Kenya and the city have run programs since 2004 to build housing and bring water and power to these areas.

Sprawl or density?

Urban sprawl is unplanned, low-density building around a city, often on land that used to be rural (National Geographic). Worldwide, built-up land has grown almost twice as fast as population. In 1975 there were about 43 m² of buildings and pavement for each person. By 2025 there were about 63 m². About 60% of the land turned into urban space since 1970 was once farmland (UN, 2025).

Denser cities use land more efficiently. In 2025, city dwellers used about 36 m² of built-up land each, far less than the world average. Medellín, Colombia, grew from 2.8 million people in 2000 to 3.4 million in 2025, and it is still dense, with about 14,000 people per km². It built a cable car system that connects hard-to-reach neighborhoods to the city center (UN, 2025).

Links to Earlier Units

• People and Cultures: the basics of density, growth, push and pull, and city life. This unit goes deeper on each one.
• Climate and Biomes: dry, cold and high places sit outside the ecumene. Drought and famine can push people to move.
• Resources and Trade: plantation crops such as sugar drove the slave trade. Today workers and remittances cross borders much like goods do.
• World Regions: migration helps make a region. The Bantu expansion shaped the languages of much of Sub-Saharan Africa, and Partition drew borders across South Asia.

Game: Which Migration?

Read each clue. Which of the five historic migrations is it about? Every clue comes from the "Five Migrations That Shaped the World" section.

Quiz: Population and Migration

Ten questions. Choose one answer for each, then check your score. You can try as many times as you like.

Credits

Written by Marco for Jugami's World Geography course. The population pyramids, the demographic transition chart and the historic migrations map are original SVG illustrations made for this page. The pyramids are drawn from UN World Population Prospects 2024 data. The transition chart is a sketch of the model, not real data. The map uses public-domain land shapes from Natural Earth, and its arrows are placed by hand to show general directions.