World population was transformed in the 20th century as technological and social changes brought steep declines in birth rates and death rates around the world. The century began with 1.6 billion people and ended with 6.1 billion, mainly because of unprecedented growth after 1960. The momentum created by this population growth will carry us past 7 billion by 2015. Beyond that, the future of world population is less certain.

Public discourse on population today tends to flow in one of two directions. One emphasizes the continued growth in the less developed regions, and the economic, social, environmental, and political strains associated with adding a few billion more people in the next 50 years.

The other focus centers around the unprecedented low fertility in many countries. About 40 percent of the world’s population lives in countries in which couples have so few children that the countries’ populations are likely to decline over the long term. These countries, which include China and most of Europe, must grapple with social, economic, environmental, and political challenges associated with aging and eventually dwindling populations. And, if fertility rates continue to fall around the world, more countries will face this low-fertility predicament.

Are we experiencing a population explosion or birth dearth? The answer may be both. And these two opposing trends—population growth and population decline—call for very different policies and contingencies. Those addressing population growth sometimes compete in funds, attention, and credibility with those concerned with population decline.

But the demographic reality is more complex and less certain than this simple dichotomy suggests. The United Nations warns that fertility decline in poor countries may halt unless couples have access to family planning, for example. Population decline in low-fertility countries could be slowed by massive immigration—or even by a baby boom. And researchers are looking beyond population numbers—to age, education, and other characteristics—to study links between population change and economic, environmental, and political trends.

It is almost certain that nearly all future population growth will occur in the developing regions of the world. Urban areas in these regions will absorb most of the additional people. Population is growing fastest among the poorest population groups within developing countries. In these countries, a “youth bulge” ensures that the absolute number of births will rise even as couples are having fewer children.

At the other extreme, most countries in Europe now have a “youth dearth” after decades of low fertility. Stagnant growth or even population decline is challenging more countries as fewer workers must support expanding pension and social security systems for their aging citizens.

Governments have crafted a range of population policies to address these and other issues over the last half-century. In developing countries, policies include support for family planning and reproductive health programs and efforts to improve women’s status, to enable women to have the number of children they want. In developed countries, particularly Japan and parts of Europe, governments have implemented policies to promote gender equality in the workplace and ease the burden of childrearing—all to encourage women to have more children.

The factors that drive childbearing trends—such as the economy, education, gender relations, and access to family planning—are numerous and complex, and public policies and programs to influence population trends must address many issues at once. Demographic changes often take years to be evident, making it difficult to predict how today’s actions will affect the future size and distribution of populations. Small changes in childbearing trends today have huge implications for future population size.

This Population Bulletin chronicles changes in world population in the last century, with a particular focus on the last 50 years. It examines the social and economic factors that affect population change, including wide disparities in income, education, and women’s status within countries. It also discusses the heightened international concern since the 1950s about rapid population growth, widespread fertility declines, and the new world consensus reached in the 1990s about how best to respond to population trends. It reviews the factors that have led to low fertility in Europe, Japan, and other areas and how governments there have begun to respond. Whether or not these responses bring the desired population change, their common goal is to improve the quality of life for individuals in the 21st century.

Lori S. Ashford is a technical director for policy information at PRB.

Carl Haub is a senior demographer at PRB.

Mary M. Kent is editor of the Population Bulletin series, and has edited and written numerous reports on population-related topics.

Nancy V. Yinger is director of PRB’s International Programs.