In 1968, Stanford University Professor Paul Ehrlich created a scare with his book "The Population Bomb," warning of the consequences of too many people.
But ever since that year, when the U.N. Population Division predicted the world population, now 6.3 billion, would grow to at least 12 billion by 2050, the agency has regularly revised its estimates downward. Now it expects population to level off at 9 billion.
The slowdown is attributed to declining birth rates and improved public health measures that have reduced mortality rates.
"On a farm, children help with the pigs or chickens," said Joseph Chamie, director of the U.N. Population Division. Nearly half the world's people live in cities now, he said, "and when you move to a city, children are not as helpful."
Barring war, famine, epidemic or disaster, a country needs a birthrate of 2.1 children per woman to hold steady. As late as 1970, the world's median fertility level was 5.4 births per woman, but in 2000, it was 2.9, the report said.