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.
Japan announced that it is now the world's most elderly nation, with more than a fifth of its people 65 or older.
On average, women must have 2.1 children in their lifetimes for a society to replenish itself, accounting for infant mortality and other factors. Only one country in Europe -- Albania -- has a fertility rate above 2, according to statistics gathered by the Central Intelligence Agency. Russia's fertility rate is 1.28. In Japan, it's 1.25.
The United States has a fertility rate of 2.05, about enough to maintain a stable population. The U.S. also adds people through immigration, something many European countries have shunned.
About 40 percent of U.S. population growth comes from immigration, both legal and illegal, according to the Census Bureau. However, if the fertility rate remains unchanged, all of America's population increase will eventually come from immigration, Butz said.
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