The March-April issue of the scholarly journal Foreign Policy unusually has a photograph of a baby playing in the grass on its cover. “Wanted More Babies” cries the cover line. The opening page of the excellent lead article “The Population Implosion,” by Nicholas Eberstadt, opens with an introductory text: “Be careful what you wish for. After decades of struggling to contain the population explosion that emerged from the health care revolution of the 20th Century, the world confronts an unfamiliar crisis: rapidly decreasing birthrates and declining life spans that might set back the progress of human development.”

Just about everyone alive today is a child-of the “world population explosion” — the historic demographic surge that swept over the planet during the course of the 20th century. Despite wars and extermination policies, human numbers have nearly quadrupled in just 100 years, leaping from about 1.6 or 1.7 billion in 1900 to about 6 billion in 2000. Simply put, the “population explosion” within living memory is coming to a close. Despite the lack of exactitude of up-to-the-minute estimates, both the pace and absolute magnitude of increases in human numbers are markedly lower now than they were just a few years ago. And an even more marked slowing down of global population almost certainly can be expected in the decades lying immediately ahead of us.

Instead of any population explosion, a new collection of demographic trends — each lacking any historic precedent — stands ready to recast the world’s population profile during the next quarter century. The first of these is patterns of childbearing that would inevitably bring about a population decline. The second is the rapid and drastic aging of the world population for many societies in the coming quarter century. The third is the eruption of extended and extreme mortality crises, excessive reversals in health conditions for those nations already enjoying high levels in life expectancy.

The huge 20th-century population explosion resulted from improvements in health and the expansion of life expectancy. In the last hundred years, human life expectancy at birth more than doubled — from approximately 30 years to almost 65. But during this past century, there has been an unexpected change — a decline in what demographers term “secular fertility” — a decline produced by steady reductions in the size of families because of intentional birth control practices utilized by likely parents.

Secular fertility decline is extremely new in the whole sweep of human history. Actually, it appears to have been noted for the first time two centuries back in France. Today, virtually every population of European origin is reporting fertility rates below the replacement level.

According to projections by the U.S. Census Bureau and the U.N. Population Division, the last half- century has seen a drop equivalent to over two births per woman per lifetime. Suddenly, we are seeing these patterns repeated throughout some 83 countries, representing some 44 percent of the globe’s population.

But an alarming thought to many Westerners is that by far the largest sub replacement population is in East Asia. The first non-European society to report sub-replacement fertility in time of peace and order was Japan, whose fertility rate fell below replacement in the late 1950s and has remained there for the last four decades. In addition to Japan, all four of the so-called East Asian “tigers” — Hong Kong, the Korean Republic, Singapore, and Taiwan — have reported sub-fertility levels since at least the early 1980s. The largest sub-replacement population is, of course, that of China, where the government’s anti-natal population-control plan is now entering its third decade.

But China should not divert our attention from the scale of fertility declines in other low-income locations. A large portion of humanity today lives in regions where fertility decline is proceeding at a rapid pace. Fifteen of the most populous developing countries account for three-fifths of the world’s population and, in the last quarter century, their fertility has dropped by over half.

Perhaps most surprising the fertility level for North Africa — the territory stretching from the Western Sahara to Egypt — was lower than the U.S. level of the early 1960s. Perhaps even more surprising, the fertility rate seems to be declining in many countries in sub- Saharan Africa. In Kenya, for example, the total fertility rate over the last 20 years is believed to have dropped by almost four births per woman.

Very low income levels and high incidences of female illiteracy are widely regarded as impediments, but they have not prevented Bangladesh from more than halving its total fertility rate during the last quarter century. Iran, under the tight control of a militantly Islamic clerisy, has slashed its fertility level by two-thirds and now stands on the verge of sub-replacement.

What accounts for the worldwide plunge in fertility now under way? No one really knows — at least with any degree of confidence. Two points, however, can be made with certainty. First, the worldwide drop in childbearing reflects dramatic changes in desired family size. Second, we need to do away with the notion, long promoted by demographers, that no country can be modernized without first making the move to low levels of mortality and fertility

Excepting some gigantic catastrophe, the world’s total population can be expected to grow substantially over the coming quarter century. U.S. Census Bureau projections for 2025 would place global population at over 7.8 billion — an astonishing 30 percent higher than today. The projected annual rate of world population growth for 2025 is just under 0.8 percent, far below the estimated 2.0 percent annual growth rate of the late 1960s. But the great global birth wave will have crested and begun ebbing by 2025. According to these projections, slightly fewer babies will be born worldwide in 2025 than in any year over the previous four decades.

A fact that should give modern students pause is that the natural growth of population in the more developed countries has essentially come to a standstill. The overall increase in population for the year 2000 in these countries is estimated at 3.3 million people, or less than 0.3 percent. Two-thirds of that increase is due to immigration — the total “natural increase” amounts to just over 1 million.

Despite reforms in Japanese immigration laws, a community of ethnic Koreans in Japan — many of them fourth-generation residents — still does not enjoy Japanese citizenship. Indeed Japan naturalizes fewer foreigners each year than tiny Switzerland. And for all societies with long-term fertility rates significantly below the replacement level, the only alternative to an eventual decline of total population is steady and massively enhanced immigration.

A point looking toward 2025 that we can’t overlook is 20th-century population forecasts and demographic assessments have been proved famously wrong. Depression-era demographers, for example, incorrectly predicted depopulation for Europe by the 1960s and completely missed the “baby boom.” The 1960s and 1970s saw dire warnings that the “population explosion” would result in worldwide famine, whereas today we live in the most prosperous era humanity has ever known. Any assessment of world population trends today clearly calls for a certain degree of humility.

Given today’s historically low death rates and birth rates, the arithmetic fact is that the vast majority of people who will inhabit the world in 2025 are already alive. Only a disaster on an apocalyptic scale can change that. Consequently this provides considerable insight into the shape of things to come. And we must prepare ourselves to face new demographic challenges.

A host of contradictory demographic trends and pressures will likely reshape the next quarter century. Social aging sets in motion an array of profound changes and challenges and demands far-reaching adjustments if those challenges are to be successfully met. And the longevity revolution constitutes an unambiguous improvement in the human condition. Pronounced and prolonged mortality setbacks portend the opposite: Nothing more or less but a diminution of human well-being.

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