The global population has experienced an unprecedented reduction in birth rates over the past few decades. People in rich and poor countries alike are having fewer babies, which demographers warn will lead to a worsening problem of global aging.
For hundreds of years, the world's population has grown steadily. But demographers now believe that within several decades, the number of people on earth will actually begin to decline.
"What we're seeing now in many countries is the drop in fertility is so fast, it's literally without precedent in human history,” says Mr. Longman.
Phil Longman is a demographer at the New America Foundation research group in Washington D.C. Mr. Longman says that everywhere in the world, people are having fewer and fewer children.
"And what it's going to mean is that countries like China for example, are going to age in a single generation as much as countries like France aged in 150 years," says Mr. Longman.
This unprecedented drop in fertility is driven mostly by developing countries. The United Nations Population Division says that in the 1970s, each woman in the developing world had an average of almost six children. By the late 1990s, birth rates fell to about 3.9 children per woman. Fertility levels fell in all but four developing countries.
Demographers say there are many reasons for the drop. Over the past few decades, people have used more effective contraception and delayed marriage. Women have had increased access to education and expanded opportunities for work.
But demographer Longman says that above all, the rapid movement of people from farms to cities is causing fertility rates to fall.
"We're now on the threshold of having half the world's population live in urban areas and the economics of parenthood are just dramatically different. If you're trying to raise a child in a high-rise apartment as opposed to on a farm, children go from being an economic asset to being an economic liability," says Mr. Longman.
Demographers point out that poor countries can benefit from falling fertility levels. Families have fewer children to support, leaving adults with more money to spend and invest. But the flip side to this initial prosperity is that the share of the population that is over 65 years old begins to increase. The younger, working-age share of the population shrinks and has to bear a greater burden for taking care of the elderly.
Many analysts say that demography is destiny. Nowhere is this more true than in the world's most populous country, China.
At the beginning of the 1970s, each woman in China had an average of five-point-seven children. In 1979, the Chinese government imposed a coercive policy to control its runaway population growth. This policy succeeded in driving births down to an average of one-point-four children per woman.
Not so long ago, the fear was a population explosion that would outpace food supply, causing famine and other disasters.
Now, Richard Jackson, head of the global aging initiative at the Center for Strategic and International Studies in Washington D.C., says that falling fertility and longer life spans have created a new demographic time bomb.
"Sometimes, though, we get more than we wish for, and fertility, in fact, in China and throughout East Asia has fallen far beneath the replacement level. The problem is when you slow population growth, you not only reduce initially the growth of the population, and ultimately begin to shrink the size of the population, but you also dramatically change the age structure," says Richard Jackson.
Mr. Jackson says there will be 400 million elderly Chinese by the year 2040. In China, the vast majority of the population has no retirement protection or health insurance, so the elderly must count on their children, who are often called little emperors, to support them.
"Today's pampered little emperors may find as they grow older that the burden of filial piety is a very heavy one. The typical family in the future, particularly in the cities, where the one-child policy has been more strictly enforced, will have four grandparents, two parents and one child," says Mr. Jackson.
In other words, China is growing old before it grows rich. And it is not alone. Falling fertility poses similar challenges to other large, developing countries, such as Indonesia and Brazil. How they meet those challenges could prove pivotal to the stability of these countries in the 21st century.