With the world population now at 5.7 billion, and increasing by about 90 million per year, we have clearly entered a zone where we can see, and may well encounter, limits on the human carrying capacity of the Earth. In this penetrating analysis of one of the most crucial questions of our time, a leading scholar in the field reviews the history of world population growth and appraises what can be known about its future.

Joel E. Cohen is head of the Laboratory of Populations at the Rockefeller University. He lives in New York City.