As the US population grows toward 300 million, the growth is producing headaches for Americans fed up with traffic congestion, sprawl and dwindling natural resources.
But the alternatives are pretty scary, too. Just look at Europe and Japan, which are on the verge of such big population losses that several countries are practically begging women to have babies.
"Europe and Japan are now facing a population problem that is unprecedented in human history -- declining population over time with an increase in the percentage of old people," said Bill Butz, president of the Population Reference Bureau, a Washington think tank.
Countries have lost people because of wars, disease and natural disasters but never -- at least in modern history -- because women stopped having enough children, Butz said.
The US is the fastest growing industrialized nation in the world, adding about 2.8 million people a year. That's a little less than 1 percent, but enough to mitigate the kinds of problems facing Japan and many European countries.
Europe, with 728 million people, saw its population shrink by 74,000 since the beginning of the decade, according to the UN. By 2050, it is projected to lose a total of 75 million people.
That ought to give motorists on Germany's Autobahn some extra room to change lanes. But experts warn it could cause labor shortages while straining retirement and health programs, ultimately threatening economic competitiveness.
The problem is that birth rates are so low there aren't enough young people entering the work force to support an aging population, said Hans-Peter Kohler, an associate professor of sociology at the University of Pennsylvania.
"Presumably, many people would not be so concerned about the numbers declining if it wasn't combined with an aging population," Kohler said. "I think it's more the age structure that gives rise to these concerns, and these concerns are well justified."
Russian President Vladimir Putin is so concerned he recently proposed paying women money to have children. Last year, France increased monthly stipends to parents who take time off work to care for a third child.
Last Friday, Japan announced that it is now the world's most elderly nation, with more than a fifth of its people 65 or older.
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