Washington Japan, Germany and many other large industrialized countries face long-term population slowdowns or declines as more young adults have fewer children or delay child-rearing, demographers say.
While the world's population is expected to increase by almost 50 percent by 2050, Japan could lose 20 percent of its population in the next half-century, according to data released Tuesday by the private Population Reference Bureau.
Russia's population is expected to decline by 17 percent, and Germany's by 9 percent.
The United States is the biggest exception among industrialized countries, with its population expected to rise by 43 percent from 293 million now to 420 million at mid-century.
While the United States, like other developed nations, has an increasing number of older residents, the U.S. population is expected to keep growing in large part because of immigration.
Some European countries have considered loosening immigration curbs as a way to help fill shortages for highly skilled workers and to build a tax base to replace dwindling funds for programs for the aged.
But the underlying reasons for the population dilemma faced by industrialized nations are mainly socio-economic, says demographer Martha Farnsworth Riche, former head of the U.S. Census Bureau.
"Modernization" -- the way today's economies are built on a more educated work force -- is causing more young adults to think twice about having large families, Riche said.
They must consider direct costs, like sending a child to college, and indirect costs, such as a parent having to take time off from a career to raise a child, before starting a family.
The cost of raising a child may especially hinder young adults from having large families in countries facing economic hardships, such as in eastern Europe, said Carl Haub, author of the Population Reference Bureau's 2004 World Population Data Sheet, which was released Tuesday.
The annual study from Haub found that the world's population will increase nearly 50 percent by mid-century to almost 9.2 billion. The projection was on par with previous forecasts from the United Nations and the U.S. Census Bureau.
Nearly all the growth would come from developing nations, even though less developed countries generally have much higher rates of HIV and AIDS infections and infant mortality.
While the population of developed countries would rise 4 percent to over 1.2 billion, the population in developing nations would surge by 55 percent to over 8 billion. Countries in Africa and south Asia would see the largest increases.
China, currently the world's most populous nation at 1.3 billion, would see an overall 10 percent increase between now and 2050 to over 1.4 billion in 2050, but its peak population is anticipated to be reached by 2025, with a decline thereafter.
By 2050, India is expected to overtake China, rising almost 50 percent from under 1.1 billion now to 1.6 billion at mid-century. Nigeria's population is expected to nearly triple to 307 million, while Bangladesh would double to 280 million.