Many countries that have experienced rapid population growth for several decades are showing signs of demographic fatigue, researchers at the Worldwatch Institute, a Washington, DC-based environmental research organization, announced last week.

Countries struggling with the simultaneous challenge of educating growing numbers of children, creating jobs for swelling ranks of young job seekers, and dealing with the environmental effects of population growth, such as deforestation, soil erosion, and falling water tables, are stretched to the limit. When a major new threat arises-such as AIDS or aquifer depletion-governments often cannot cope.

Problems routinely managed in industrial societies are becoming full-scale humanitarian crises in many developing ones. As a result, some developing countries with rapidly growing populations are headed for population stability in a matter of years, not because of falling birth rates, but because of rapidly rising death rates.

"This reversal in the death rate trend marks a tragic new development in world demography," said Lester Brown, President of Worldwatch and co-author with Gary Gardner and Brian Halweil of Beyond Malthus: Sixteen Dimensions of the Population Problem. In the absence of a concerted effort by national governments and the international community to quickly shift to smaller families, events in many countries could spiral out of control, leading to spreading political instability and economic decline, concludes the study funded by the David and Lucile Packard Foundation.

Marking the bicentennial of Thomas Malthus' legendary essay on the tendency for population to grow more rapidly than the food supply, this study chronicles the stakes in another half-century of massive population growth. The United Nations projects world population to grow from 6.1 billion in 2000 to 9.4 billion in 2050, with all of the additional 3.3 billion coming in the developing countries. However, this study raises doubts as to whether these projections will materialize.

Today, two centuries after Malthus, we find ourselves in a demographically divided world, one where national projections of population growth vary more widely than at any time in history. In some countries, population has stabilized or is declining; but in others, population is projected to double or even triple before stabilizing.

In 32 countries, containing 14 percent of world population, population growth has stopped. By contrast, Ethiopia's population of 62 million is projected to more than triple to 213 million in 2050. Pakistan will go from 148 million to 357 million, surpassing the U.S. population before 2050. Nigeria, meanwhile, is projected to go from 122 million today to 339 million, giving it more people in 2050 than there were in all of Africa in 1950. The largest absolute increase is anticipated for India, which is projected to add another 600 million by 2050, thus overtaking China as the most populous country.

To understand these widely varying population growth rates among countries, demographers use a three-stage model of how these rates change over time as modernization proceeds. In the first stage, there are high birth and high death rates, resulting in little or no population growth. In the second stage, as modernization begins, death rates fall while birth rates remain high, leading to rapid growth. In the third stage, birth rates fall to a low level, balancing low death rates and again leading to population stability, offering greater possibilities for comfort and dignity than in stage one. It is assumed that countries will move gradually from stage one to stage three.

Today there are no countries in stage one; all are either in stage two or stage three. However, this analysis concludes that instead of progressing to stage three as expected, some countries are in fact falling back into stage one as the historic fall in death rates is reversed, leading the world into a new demographic era.

After several decades of rapid population growth, many societies are showing signs of demographic fatigue, a result of the struggle to deal with the multiple stresses caused by high fertility. As recent experience with AIDS in Africa shows, some countries in stage two are simply overwhelmed when a new threat appears. While industrial countries have held HIV infection rates among their adult populations under 1percent or less, a 1998 World Health Organization survey reports that in Zimbabwe, for example, 26 percent of the adult population is HIV positive. In Botswana it is 25 percent, Zambia 20 percent, Namibia 19 percent, and Swaziland 18 percent.

Barring a miracle, these societies will lose one fifth or more of their adult population within the next decade from AIDS alone. These adult deaths, the deaths of infants infected with the virus, and high mortality among the millions of AIDS orphans, along with the usual deaths, will bring population growth to a halt or even into decline. With these high mortality trends, more reminiscent of the Dark Ages than the bright new millennium so many had hoped for, these countries are falling back to stage one.

New diseases are not the only threat to demographically fatigued stage two countries. Because population growth affects so many dimensions of a society, any of several different stresses can force a country back into stage one.

For example, in many developing countries food supplies are threatened by aquifer depletion. A forthcoming study by the International Water Management Institute (IWMI) reports that in India, a country heavily dependent on irrigation, recent growth in food production and population has been based partly on the unsustainable use of water. Nationwide, withdrawals of underground water are at least double the rate of recharge and water tables are falling by 1 to 3 meters per year. IWMI authors estimate that as India's aquifers are depleted, its grain harvest could fall by as much as one fifth. In a country where food and population are precariously balanced and which is adding 18 million people per year, such a huge drop in food output could create economic chaos.

"The question is not whether population growth will slow in the developing countries," said Brown, "but whether it will slow because societies quickly shift to smaller families or because ecological collapse and social disintegration cause death rates to rise. The challenge for national governments is to assess their land and water resources, determine how many people they will support at the desired level of consumption, and then formulate a population policy to reach that goal."

At the international level, the challenge is to quickly expand international family planning assistance, so that the millions of couples who want to limit family size but lack access to family planning services will be able to do so. Beyond this aid, investing in the education of young people in the third world, especially females, is a key to shifting to smaller families. In every society where data are available, the more education women have the fewer children they bear.

As the world enters the new millennium, it faces many challenges, but perhaps none so important-or as urgent-as the need to quickly slow population growth.