Almost everywhere in the world, people are having fewer and fewer babies. How will Europe, Japan and the United States cope with providing for the elderly? And how might demographic changes reshape developing countries in the Middle East and Africa?
In the year 2000, fertility rates in Europe and other parts of the developed world fell to levels never before recorded. Europe's population is now expected to shrink significantly in coming decades.
The United Nations says that birth rates of 2.1 children per woman are needed to replace the population. Yet only four developed countries in the world have birth rates above the replacement level.
Fertility in the United Kingdom has dropped to 1.6 children per woman. Germany's is now 1.4. And women in Italy have an average of just 1.2 children.
Bill Frey, a demographer at the Brookings Institution research group in Washington D.C., projects that European labor forces and economies will soon shrink along with the population.
"I've always used this metaphor that Europe was going to become this geriatric ghetto among countries of the world. They're going to become increasingly smaller; the continent of Europe is going to lose 100 million people over the next 50 years. It's going to become increasingly aged," he said.
Mr. Frey projects that in 2050, Europe will have a median age of about 52. In the United States, by contrast, the median age will be under 40 because of comparatively high fertility and immigration levels.
In 30 or 40 years, Mr. Frey says that there will only be a 1:1 ratio between each worker and retiree in Europe. He says that by mid-century in the United States, the ratio of working-age people supporting each retiree will still be just over 2:1.
As the Bush administration warns of a looming emergency in the U.S. retirement program known as social security, Mr. Frey argues that the aging crisis is far worse in the rest of the developed world.
"We're much better off here in the United States than in most of Europe and in Japan simply because we have a bit more breathing room, I guess you could say. So if the administration thinks we have a crisis here, I don't know what they should be thinking in Italy or in England or in Germany. Because they have a catastrophic crisis there," Mr. Frey said.
Many European countries have introduced incentives to encourage residents to have more children. This small town of Laviano in Italy is offering couples almost $12,000 for each newborn baby.
But these bonuses appear to have little effect on birth rates. Some analysts argue that more comprehensive social programs are needed to offset the expense of having a child.
Phil Longman is a demographer at the New America Foundation research group in Washington D.C.
"We live in a society in which more and more it really doesn't make economic sense to have children. That's not only unfair, it's imprudent in a society that's increasingly consuming more human capital than it's producing," he noted.
Some analysts believe that shrinking populations in Europe and other developed countries will profoundly affect global economics and even security.
Richard Jackson is head of the global aging initiative at the Center for Strategic and International Studies, CSIS, in Washington D.C.
"Graying means paying more for pensions, more for health care, more for nursing homes, more for social services for the elderly," he said.
Mr. Jackson says by the 2030s, developed countries will have to spend almost a quarter of their gross domestic product on services for the elderly, up from about 12 percent today. The fiscal strain will be especially great in Japan and Europe. He says rising retirement costs will divert resources from defense and international affairs, and developed countries may have difficulty recruiting for their militaries.
Mr. Jackson says that demographic changes may well determine which countries remain great powers in the future.
"Historians have observed that the rise and fall of civilizations is often linked to demographic trends, and that contracting populations give way not just militarily but economically and ultimately culturally to expanding populations," Mr. Jackson said.
While the population of Europe will soon begin to contract, Mr. Jackson says that populations in the Middle East and Central Asia will expand for decades to come. Fertility has declined in these areas, but demographic momentum will keep the population relatively young until mid-century.
One of the only regions in the world to maintain high fertility rates is sub-Saharan Africa, with more than five children per woman on average. But the AIDS and HIV pandemic have ravaged populations there.
The only large, developed country to maintain stable fertility levels is the United States, where the population is expected to grow significantly in coming decades.