Largest Urban Agglomerations, 1975, 2000, 2025

Source: United Nations, World Urbanization Prospects: The 2007 Revision.

Teachers Guide: Discussion guide

Question and Answer: What are the social implications of rapid population growth in less developed countries?

Through most of history, the human population has lived a rural lifestyle, dependent on agriculture and hunting for survival. In 1800, only 3 percent of the world's population lived in urban areas. By 1900, almost 14 percent were urbanites, although only 12 cities had 1 million or more inhabitants. In 1950, 30 percent of the world's population resided in urban centers. The number of cities with over 1 million people had grown to 83.

The world has experienced unprecedented urban growth in recent decades. In 2008, for the first time, the world's population was evenly split between urban and rural areas. There were more than 400 cities over 1 million and 19 over 10 million. More developed nations were about 74 percent urban, while 44 percent of residents of less developed countries lived in urban areas. However, urbanization is occurring rapidly in many less developed countries. It is expected that 70 percent of the world population will be urban by 2050, and that most urban growth will occur in less developed countries.

What is an urban area? An urban area may be defined by the number of residents, the population density, the percent of people not dependent upon agriculture, or the provision of such public utilities and services as electricity and education. Some countries define any place with a population of 2,500 or more as urban; others set a minimum of 20,000. There are no universal standards, and generally each country develops its own set of criteria for distinguishing urban areas. The United States uses a population density measure to define urban with a minimum population requirement of 2,500. The classification of metropolitan includes both urban areas as well as rural areas that are socially and economically integrated with a particular city.

When comparing countries it is often helpful to look beyond the proportion of populations that are rural or urban and instead consider the size of cities. Countries differ markedly in the distribution of their urban population. For example, many urban dwellers in Africa live in cities of fewer than 10,000 residents. In Argentina, 92 percent of the 2007 population was urban, and 32 percent of these people lived in just one city, Buenos Aires. In 2007, 38 percent of the world's urbanites lived in agglomerations of 1 million or more inhabitants, and 15 percent resided in agglomerations of 5 million or more. Only 8 percent of Americans live in cities of 1 million or more.

Migration or Natural Increase

A city grows through natural increase—the excess of births over deaths—and because the in-migration of people from other cities, rural areas, or countries is greater than out-migration. More developed and less developed countries of the world differ not only in the percent living in cities, but also in the way in which urbanization is occurring.

During the 19th and early 20th centuries, urbanization resulted from and contributed to industrialization. New job opportunities in the cities spurred the mass movement of surplus population away from the countryside. At the same time, migrants provided cheap, plentiful labor for the emerging factories. While the proportion increased through rural to urban migration, high death rates in the cities slowed urban growth. Cities were unhealthy places because of crowded living conditions, the prevalence of contagious diseases, and the lack of sanitation. Until the mid-1800s, the number of deaths exceeded births in many large European cities. Migration accounted for as much as 90 percent of city growth during this period.

Urbanization in most less developed countries in the past 50 years contrasts sharply with the experience of the more developed countries. Death rates have fallen faster in urban areas because of greater access to health services. Because birth rates are relatively high in most less developed countries, the rates of natural increase are also quite high in cities. Migration also fuels urban growth in less developed countries as people leave the countryside in search of better jobs.

Growth of Urban Agglomerations, 1950–2025

Source: United Nations, World Urbanization Prospects: The 2007 Revision.

The figure "Growth of Urban Agglomerations" shows population growth in selected cities. New York and London are typical of large cities in more developed countries that arose in the 1800s and early 1900s, reached their current size mid-century, and have since experienced slow growth or decline. Cities in some less developed countries, such as Mexico City, grew very rapidly between 1950 and 1980, and are growing more slowly now. Many Asian and African cities, such as Lagos and Bombay, are experiencing very rapid growth now and are projected to continue at this pace.


As the population increases, more people will live in large cities. Many people will live in the growing number of cities with over 10 million inhabitants, known as megacities. As the map "Largest Urban Agglomerations" shows, just three cities had populations of 10 million or more in 1975, one of them in a less developed country. Megacities numbered 16 in 2000. By 2025, 27 megacities will exist, 21 in less developed countries.

Top 10 Largest Urban Agglomerations in 1975, 2000, and 2025

1975 2000
1. Tokyo, Japan 26.6 1. Tokyo, Japan 34.5
1. Tokyo, Japan 36.4
2. New York- Newark, USA 15.9
2. Mexico City, Mexico 18
2. Bombay, India 26.4
3. Mexico City, Mexico 10.7
3. New York-Newark, USA 17.9
3. Delhi, India 22.5
4. Osaka-Kobe, Japan 9.8
4. São Paulo, Brazil 17.1
4. Dhaka, Bangladesh 22
5. São Paulo, Brazil 9.6
5. Bombay, India 16.1
5. São Paulo, Brazil 21.4
6. Los Angeles-Long Beach-Santa Ana, USA 8.9
6. Shanghai, China 13.2
6. Mexico City, Mexico 21
7. Buenos Aires, Argentina 8.8
7. Calcutta, India 13.1
7. New York-Newark, USA 20.6
8. Paris, France 8.6
8. Delhi, India 12.4
8. Calcutta, India 20.6
9. Calcutta, India 7.9
9. Buenos Aires, Argentina 11.9
9. Shanghai, China 19.4
10. Moscow, Russian Federation 7.6
10. Los Angeles-Long Beach-Santa Ana, USA 11.8
10. Karachi, Pakistan 19.1

Source: United Nations, World Urbanization Prospects, The 2007 Revision.


Death rate (or crude death rate): The number of deaths per 1,000 population in a given year.

Less developed countries: Less developed countries include all countries in Africa, Asia (excluding Japan), and Latin America and the Caribbean, and the regions of Melanesia, Micronesia, and Polynesia.

Megacities: A city with a population of 10 million or more residents.

Metropolitan area: A large concentration of population, usually an area with 100,000 or more people. The area typically includes an important city with 50,000 or more inhabitants and the administrative areas bordering the city that are socially and economically integrated with it.

More developed countries: More developed countries include all countries in Europe, North America, Australia, New Zealand, and Japan.

Rate of natural increase: The rate at which a population is increasing (or decreasing) in a given year due to a surplus (or deficit) of births over deaths, expressed as a percentage of the base population.

Urban: Countries differ in the way they classify population as "urban" or "rural." Typically, a community or settlement with a population of 2,000 or more is considered urban. A listing of country definitions is published annually in the United Nations Demographic Yearbook.

Urban agglomeration: Refers to the population contained within the contours of a contiguous territory inhabited at urban density levels without regard to administrative boundaries. It usually incorporates the population in a city or town plus that in the sub-urban areas lying outside of but being adjacent to the city boundaries.

Urbanization: Growth in the proportion of a population living in urban areas.