In the year 1900 the Earth was home to about 1.6 billion people. The total had grown by 600 million in the 100 years since 1800, the year that the first billion was reached; but the change in the 19th century gave no hint of things to come. By the middle of the present century another billion had been added, in the remarkably short span of only 50 years. Moreover, and significantly, 80 percent of the growth had taken place in the world's poorer, or "developing," nations. In 1995, but 45 years later, world population had risen by an additional three billion, with most of the increase, as before, in Asia, Africa, and Latin America.

The dramatic fashion in which new numbers have been added to the world's population since 1950 is shown in this figure. While it took the several million years of human history to reach the first billion, and 130 years to reach the second, today each new billion is added in but 11 years.

It came as something of a surprise. The fact that world population would grow by billions in but a few decades was not anticipated by demographers in the initial postwar period. The earliest United Nations projections showed more concern with the possibility of devastating mortality striking the developing countries. As a result, U.N. projections made in 1951 predicted the 1980 population at anywhere from 3 to 3.6 billion, but the higher limit was considered optimistic and unlikely. The actual figure, as best we now know, proved to be about 4.4 billion: a large difference in a less than 30 year projection.