(September 2000) Managing urban population change will be one of the world’s most important challenges in the next few decades. In less developed countries, where 80 percent of the world’s population resides, central issues will be how to cope with an unprecedented increase in the number of people living in urban areas and the growing concentration of these urbanites in large cities with millions of residents. In more developed countries such as the United States, the urban future will involve dealing with complex changes in the composition of urban populations while also containing urban sprawl beyond suburbs into what remains of the countryside.

In Asia, Africa, and Latin America, the unprecedented population growth that characterized much of the 20th century has evolved into unparalleled urban growth. The United Nations (UN) projects that world population will expand from 6.1 billion to 7.8 billion between 2000 and 2025—90 percent of this growth will occur in urban areas of less developed countries. By 2020, a majority of the population of less developed countries will live in urban areas.

The population of less developed countries will become increasingly concentrated in large cities of 1 million or more residents. There were an estimated 292 such “million-plus” cities in less developed countries in 2000. Megacities, with 10 million or more residents, are also becoming more numerous and will play an important role in the world’s urban future. Many of the largest cities are likely to absorb enormous population increments. Lagos, Nigeria, for example, is expected to add nearly 10 million people between 2000 and 2015, while Dhaka, Bangladesh, will add 9 million.

The tremendous population growth in the urban areas of less developed countries can be viewed as a welcome or as an alarming trend. Historically, cities have been the engines of economic development and the centers of industry and commerce. They have spurred innovations in science and technology and in systems of law and government. Cities have facilitated the diffusion of information through interaction among diverse cultures. The density of urban populations has offered significant cost advantages for governments in the delivery of essential goods and services, and for the private sector, in the production and consumption of such items.

Cities have also played a crucial role in reducing fertility, thereby slowing world population growth. In the 19th century, urban residents of Europe and North America were among the first people to widely practice family planning, and they helped spread the idea of fertility regulation to the countryside. Today, fertility levels are invariably lower in urban than in rural areas of less developed countries. The growing concentration of residents in urban areas, where the costs of childrearing are higher, family planning services are more available, and social norms are more conducive to small families than in rural areas, may hasten global fertility decline.

The unprecedented magnitude of urban growth has engendered debate about whether less developed countries and their large cities can accommodate the current volume of urban growth. This dispute echoes disagreements voiced a generation ago regarding limits to the number of people the world can support. Some observers claim that good urban management and governance can overcome population constraints. They note that some big cities in less developed countries are competing successfully on economic terms with their counterparts in more developed countries by offering vast supplies of relatively inexpensive labor. Moreover, there is no evidence of a threshold population size beyond which cities generate more negative than positive effects for their countries. And the information revolution enables struggling cities to improve by adopting “best practices” of successful cities.

Yet experts in other circles are highly concerned about the urban future. Experts in the health sciences, for instance, warn that uncontrolled in-migration and increased density is pushing morbidity and mortality higher in cities than in surrounding rural areas, as was the case in some U.S. cities in 1900. Some environmentalists point out that the unplanned development of big cities is depleting nonrenewable natural resources and contributing to global climate change. Further, many cities in less developed countries are built on ecologically fragile foundations, or are vulnerable to such natural disasters as earthquakes, floods, and destructive storms. Unbridled population growth in these cities increases the risk of catastrophic loss of life.

Some political scientists maintain that rampant urban growth is increasing urban poverty and inequality, which in turn could spark a weakening of the state, civil unrest, urbanbased revolutions, and radical religious fundamentalism. Economists see a shortage of decent income-earning opportunities in cities, while urban planners see a lack of livable spatial forms.

In more developed countries such as the United States, there looms a different urban future: Challenges are arising less from population growth than from changes in the composition and distribution of urban populations.

Urban planners in more developed countries confront problems that date back several decades. Residential segregation remains prominent among these issues. The departure of many affluent residents from central cities to suburbs, a trend experienced in several countries since the 1970s, has been countered by strategies of urban revitalization, the gentrification of inner-city neighborhoods by young professionals, and the return of middleaged “empty nesters” to cities. These revitalization processes generally help cities generate sales and tax revenues, but they also tend to widen the disparity in housing costs between neighborhoods and further concentrate low-income minority groups in slums and ghettos. The notion that there is a permanent urban underclass, trapped in the inner-city by inadequate educational and income-earning opportunities, is as relevant today as when it was first raised in the 1960s.

New urban challenges are emerging in more developed countries. As these countries experience population aging, their cities will house an increasing proportion of elderly persons with special needs. Immigration is diversifying the ethnic profile of urban populations in the United States and many other more developed countries, creating exciting opportunities for cultural interaction. But immigration also spurs ethnic clustering, intolerance toward minorities, and increased demands for basic services.

Also in the United States, the continued movement of “baby boomers” away from central cities, beyond suburbs and into “exurbs” that offer access to nature as well as close proximity to city amenities, portends the virtual disappearance of traditionally rural areas in many large regions. Meanwhile, a critical shortage of affordable housing in many cities is forcing some middle-income residents to move into dilapidated housing and others to homelessness.

This Population Bulletin examines trends of urban population change in less developed and more developed regions. Among more developed countries, particular attention is given to the United States. The report describes the demographic sources of urban growth in less developed countries, traits that distinguish urban from rural populations in these countries, and some of the critical challenges posed by the urban demographic revolution. Policies to manage cities and urban growth are discussed, incorporating concepts of the city that have recently emerged in urban population studies.

Martin P. Brockerhoff is an associate of the Policy Research Division of the Population Council.