In a number of South Asian and Latin American countries, and also in many developed countries, the leading city is many times larger than any other city of the same country. Such metropolises constitute too great a concentration of the country's human resources and productive potential. The tendency for one city to remain supreme is usually reinforced by various types of inertia. A migratory stream, once established, is likely to continue in the form of chain migration. New investments are usually made in an environment of already existing specialized industries; and superior educational institutions are to be found in such cities. Entrepreneurs, doctors, teachers and other specialists are unwilling to leave a leading city for some other, far less diversified, environment. In many countries, therefore, qualified services are highly concentrated in one big city, to the detriment of the rest of the country. Comparable initiatives are unlikely to arise in other cities. Once a leading city has outdistanced others, its pre-eminence tends to be further increased by all subsequent developments.
The social ills prevailing in big cities of the less developed regions are particularly striking because of their concentrated form. It has been variously estimated that in the big cities of Asia and Latin America up to 30 or 40% of the population is that of transitional settlements or shanty-towns. Despite governmental efforts at slum clearance or relocation, the shanty-town population appears to be growing faster than the remainder of the urban population. If trends are allowed to continue, an increasing proportion of the national populations may accumulate in the shanty-towns of metropolises.