Right now, many people are nervous about the challenges presented by a global population that has reached 7 billion and is still rising. But for a lot of countries, a lack of babies is the bigger worry.
The so-called birth dearth is starting to cause problems across much of Europe and a substantial portion of Asia. With fewer children born, populations in many countries are aging rapidly. Soon, they may also be shrinking.
That presents a number of increasingly thorny problems, such as how to sustain economic growth with a declining working-age population; how to pay for rising health care costs for seniors; and questions in some nations about being able to maintain military capacity.
"Every country in the world will eventually age," says Vegard Skirbekk, who runs a research program on aging at the International Institute for Applied Systems Analysis in Austria.
In Europe, much of Asia and parts of the Arab world, people are marrying later — or not at all.
"One of the avoidable consequences of this, to a greater degree than in the past, is that older people can expect to be childless and alone," says Nicholas Eberstadt, a demographer at the American Enterprise Institute. "It's going to make things more complicated, not less."
The Decline In Numbers
By 2025, the United States will be the only major developed nation with more people under 20 than over 65 — and the only one with a working-age population that will continue to grow. Even in China, growth in the working-age population is expected to peak five years from now and start declining by 2030.
The reasons are clear. Women are marrying later and having fewer children — if they have any. Half of the world's women are now having two children or fewer, even in some developing countries such as Iran, Burma and Vietnam.
Nations including Hungary and South Korea are seeing unprecedented numbers of women staying single into their 30s — up from a handful a generation ago to 30 and 40 percent, respectively.
In places like Germany and Japan, Eberstadt says, the number of women who end up having no children at all is already approaching 30 percent.
No Possibility Of Replacement
In many wealthy nations, the fertility rate — the average number of children born to women of childbearing years — is well below 2.
By 2050, the number of children under age 5 is expected to fall by 49 million worldwide, while the number of people over 60 will go up by 1.2 billion.
In Germany, the lower fertility rate could mean that by 2100, the country will be home to fewer native-born citizens than are in Berlin today, says Fred Pearce, a British environmental consultant and author of the 2010 book The Coming Population Crash.
Italy may find itself even more emptied out by century's end. And Japan has been aging so rapidly that the government has actually predicted the date when the last Japanese baby is expected to be born.
That's not until 2959. But people are likely to see a decline in population in many countries long before then, Pearce says.
More Funerals Than Births
Population declines are already evident in Eastern Europe. Russia's population has gone down nearly every year since the breakup of the Soviet Union 20 years ago. Since 1992, the number of deaths has outpaced births by nearly 3 to 2, according to official Russian statistics.
"Globally, in the years since World War II, there has been only one more horrific surfeit of deaths over births: in China in 1959-61, as a result of Mao Zedong's catastrophic Great Leap Forward," AEI's Eberstadt writes in the current issue of Foreign Affairs.
Once funerals outnumber live births, a society can enter a downward spiral, as fewer young people will be around to produce families in the future.
Russian women in their 20s today account for two-thirds of the births in that country. Their numbers are going to fall 45 percent by 2025, according to Eberstadt.
'The Most Important Crisis'
If anything, the situation may be worse among Russia's neighbors. Places like Ukraine and Bulgaria are not only experiencing low birth rates and high death rates, but also losing population due to migration.
Bulgaria, which was home to 9 million people in 1990, is now down to 7.4 million, and is likely to drop to 6 million by 2030.
"Eastern European countries say that population trends are the single most important crisis they face," says Wolfgang Lutz, founding director of the Wittgenstein Center for Demography and Global Human Capital in Vienna.
Previously, such stark population decline has always been associated with disaster.
"Now we are for the first time in human history having population decline in a time of peace, and in the absence of any health decline," Lutz says.
Very Long-Term Strategy
Because population loss has traditionally come about as the result of catastrophe, most people assume it inevitably represents bad news, even though no disaster is to blame for current trends.
"Some believe the Japanese economy's lack of growth in the last couple of decades may be traceable to its aging population," says Pearce, the British author.
But aging societies are also the outgrowth of good health and prosperity. And Lutz argues that such nations will be able to maintain high levels of productivity, despite the drop in the traditional working-age population between 15 and 64, if they invest more in education.
Seniors in the U.S. and Western Europe are healthier and more productive than their counterparts in countries such as Mexico, China and India, largely due to better education, says Skirbekk.
Investing in today's young people so that they can remain productive in later years, however, requires not only a great deal of money, but patience.
"Of course, schooling is concentrated at a younger age, so to increase schooling among seniors, you have to wait for a generation to be replaced," Skirbekk says.