Over the centuries cities have grown as economic output rose, as transport improved, and as sanitation and other measures made it possible for larger numbers of people to live in close proximity to one another. But large-scale urbanization is a relatively recent phenomenon in world history. Homer Hoyt has estimated that in 1800 only slightly more than 1 percent of the world's population lived in cities of 100,000 or more population, but that by 1960 the proportion had grown to 20 percent of the total. In that year, 42 percent of the people of the Americas lived in these large cities, but in Asia the proportion similarly urbanized was only 12 percent and in Africa it was only 8 percent. At whatever date, if the urban cutoff point were lower than Hoyt's 100,000--at the 2,500 level which defines "urban" in the United States, for instance--the percentages would be much larger.
The pace of urbanization has picked up sharply in the past decade or two, especially in the economically less-developed countries. There, improvement in health conditions has lowered deathrates while birthrates have remained unchanged, thus leading to rapid increases in total population. Economic and social opportunities in the rural areas have been, or have seemed, less attractive than in the cities, and rural people have flocked to the cities. Man, as a species, is congregating himself into urban complexes at a rate and on a scale that leaves the ultimate result much in doubt today. Within many of these complexes decentralization, or movement toward the suburbs, is going on simultaneously, especially in the higher-income countries.
One aspect of this worldwide urbanization is the emergence of "world cities." Such cities are usually centers of political power, both national and international; this attracts a host of institutions whose business is dealing with government; they are typically centers of trade, often are seaports, and today possess great international airports as well. Such cities have also typically become centers for professional talents in medicine, law, science, arts, and literature. They are centers of wealth and power, a position that gives them influence beyond that of the size of their population alone. Hall identifies twenty-four metropolitan centers in the world with 3 million or more population in 1960 (or in the nearest year for which data were available), but he considers only seven of these large metropolitan centers as truly "world cities." He shows that not only have these cities grown in total . . .