Nearly half the world's population and three quarters of all westerners live in cities. Between 1960 and 1992, the number of city-dwellers worldwide rose by 1.4 billion and over the next 15 years it is expected to rise by around 1 billion more. Most of this city growth has occurred in developing countries, where the number of people living in cities is increasing by the equivalent of the population of Spain every year. China, for example, now has something like 90 cities of at least 1 million people. Cities with more than 10 million inhabitants are known as mega-cities. Figure 1 shows the size of some of these mega-cities in 1994. It is predicted that by the end of the decade there will be 17 mega-cities in developing countries, and that by 2015, 22 of the 27 mega-cities will be in less-developed countries. In 1995, London, was not classified as a mega-city, with a population of 7.3 million.
Urban growth varies considerably between countries. With regard to particular cities, rates of urban population growth currently range from under 1% per annum in most of the developed world's large cities (except for Tokyo where it is slightly over 1%) to over 4% (in many developing countries' large cities).
Rapid rates of urban growth first occurred among the nineteenth-century leaders in the industrial revolution. In less developed parts of the world urban growth rates did not reach significant levels until after the Second World War. Consequently a relatively small fraction of the Third World's population - only about one-fourth - is urban. The corresponding fraction for the developed world is close to seven-tenths. Because of their corresponding larger share of the world's population, however, developing countries have a larger urban population than the developed countries.
Almost all developing countries that have been concerned with the size and growth of their urban populations believe that internal (rural-urban) migration has been the prominent factor contributing to urban growth. Although high population growth is a serious problem in most developing countries, internal migration puts even greater strain on cities. Because of the lack of data on migration in many developing countries, the difference between the urban growth rate and the natural (total population) growth rate is used as a rough measure of migration. Table 1 shows how urban population growth rates exceed total population growth rates in a selected sample of developing countries. In addition, statistics show that rural migrants constitute anywhere from 35 to 60% of recorded urban population growth.