(March 1999) In the history of the world, no century can match the population growth of the one now coming to a close. We entered the 20th century with less than 2 billion people, and we leave it with more than 6 billion.

What is the world population outlook beyond 6 billion? The momentum created by the unprecedented growth of the last half century will carry us toward the seventh billion— probably within the next 14 years. Nearly all of this increase will occur in less developed regions. Beyond that, our vision blurs.

Will world population stop growing over the next century? Will the 21st century witness long-term population decline? Or will the new century see even more population growth than the last? Any of these scenarios is possible.

World population in the next century, as in the last, will reflect starkly different demographic trends around the world: high fertility and mortality and rapid population growth in sub- Saharan Africa, for example, and low fertility and mortality and population decline in parts of Europe.

What accounts for these differences? Are they likely to change? To answer these questions, we must examine what causes population change. We have learned a great deal about the factors linked with population change. These include economic growth or decline; public health interventions; investments in education and environmental protection; the status of women; epidemics and other health threats; and access to family planning information and services.

Some of these factors are harder to understand and predict than others. Many are intricately interconnected— so that a change in one can cause a change in another.

We know that the future world population will be influenced heavily by the 2 billion young people under age 20 in less developed countries today. As these youths enter their childbearing years, their decisions about how many children to have and when to have them will determine the size and characteristics of the world’s population in 2050 and at the end of the 21st century.

This Population Bulletin chronicles the demographic history of the world and the changes in population in less developed and more developed countries. It examines the social and economic factors that affect population change. It also discusses the heightened international concern in the second half of the century about the rapid rate of growth and large increases in population size. And, it looks at the ways that governments and private groups around the world have responded to these concerns. It describes a new world vision of what to do about population issues. This vision draws attention to particular population groups and the importance of their well-being for the quality of life for all people in the 21st century.

Alene Gelbard is director of international programs at the Population Reference Bureau. Carl Haub holds the Conrad Taeuber Chair for Population Information at the Population Reference Bureau. Mary M. Kent is editor of the Population Bulletin series at the Population Reference Bureau.