[Pro-Life Infonet Note: Nicholas Eberstadt holds the Henry Wendt Chair in Political Economy at the American Enterprise Institute. This article is adapted from a longer one in the current issue of Foreign Policy magazine.]

It may not be the first way we think of ourselves, but all of us alive today are children of the "world population explosion." Thanks to sweeping mortality declines, human numbers leapt from about 1.6 billion or 1.7 billion in 1900 to more than 6 billion in 2000.

In certain circles within Washington (and outside the United States), that unprecedented leap in human numbers fueled an anti-natalist obsession. But continuing preoccupation with high fertility and rapid population growth leaves us poorly prepared to comprehend (much less respond to) emerging demographic trends.

Three of these are poised to refigure our global profile in surprising -- and not always beneficial -- ways. The first is the spread of "sub-replacement" fertility regimens: patterns of childbearing that will eventually result, all else being equal, in indefinite population decline.

According to the U.S. Census Bureau, 83 countries and territories are now thought to experience below-replacement fertility. Those places encompass nearly 2.7 billion people -- roughly 44 percent of the world's total population.

Today's global march toward smaller family size flies in the face of many prevailing assumptions about when rapid fertility decline can and cannot occur. Poverty and illiteracy (especially female illiteracy) are widely regarded as impediments to fertility decline, yet they have not prevented Bangladesh from reducing its fertility rate by more than half over the past quarter-century. By the same token, "traditional" religious attitudes are commonly seen as a barrier against low fertility. Yet over the past two decades, Iran, under the tight rule of a militantly Islamic clerisy, has slashed its fertility level by fully two-thirds, and now apparently it stands on the verge of sub-replacement.

What accounts for the worldwide plunge in fertility? The honest answer is that nobody really knows -- at least, with any degree of confidence. If you can find the shared determinants of fertility decline in such disparate below-replacement societies as the United States, Guadeloupe, Thailand and Tunisia, then your Nobel Prize is in the mail.

While causes might be uncertain, results are quite predictable. Global population growth will decelerate markedly over the coming generation. By current projections, in fact, slightly fewer babies will be born worldwide in the year 2025 than at any point over the previous four decades.

Thanks to extreme birth dearth, depopulation is now imminent for both Europe and Japan. In Europe, immigration must nearly quadruple -- to an average of almost 4 million net entrants a year -- to prevent a decline in the size of the 15- to-64-year-old "working age" population over the next 50 years. In Japan, where net immigration approximates zero, more than 600,000 newcomers a year will be needed to keep the working age population from shrinking.

Will these territories opt for indefinite decline -- or for ethnic transformation? Given the arithmetic, they have no other options. Low and decreasing fertility levels will accelerate the tempo of social aging -- the second great demographic trend of the coming era.

We all know about the coming pensioner problem in Western countries -- but Western countries are rich. Many of today's developing countries, by contrast, will become "gray" before they become "rich." One of the most arresting cases of population aging is now set to unfold in China. Between 2000 and 2025, China's median age will soar -- in fact, it may exceed America's within 25 years. By 2025, roughly 200 million Chinese will be 65 or older. Caring for China's elderly will inexorably become a domestic, and global, political issue -- for nothing remotely resembling a national pension system is yet in place in that country.

The third, and most ominous, demographic trend of the coming era involves unexpected and brutal mortality spikes. In our era, we have come to presume that death rates inevitably decline during times of peace and order. That happy presumption must now be discarded. By Census Bureau projections, nearly 40 countries and territories will have lower life expectancies in 2010 than they enjoyed in 1990. More than 750 million people -- one-sixth of the world's current population -- live in such spots. Many of these countries are today's sub-Saharan victims of the HIV-AIDS epidemic.

But the international health setback is not just about Africa and AIDS. In Russia -- an urbanized, industrialized, peacetime society -- lifespans are shorter today than 40 years ago. In a dozen other post-Communist countries, life expectancy is lower today than in the 1970s.

Since virtually no one predicted these foreshortenings of national lifespan, we cannot yet claim to know which countries will be afflicted by -- or spared from -- uncontrollable bouts of mortality in the years to come. Before too long, unfortunately, our current era's widespread anxiety about health-driven global population growth may look remarkably quaint and naive.

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