The 20th century began with a world populated by 1.6 billion people and will end with 6 billion inhabitants - with most of the growth occurring in poor countries, demographers estimate.

The Population Reference Bureau, a nongovernmental and nonprofit research organization, said Tuesday that most young people in or near childbearing age live in developing countries and those who do tend to have more children.Ninety percent of young people in or near the early childbearing ages of 15 to 19 live in developing countries, it said in an annual stock-taking of the world's people.

In the most fertile part of the world, middle Africa, 18 percent of women of early childbearing age give birth each year, the bureau said. In Western Europe, only 1 percent do. In the United States the rate is 5 percent, and in Asia it is 4 percent.

Economics chiefly account for the sharp differences in growth rates between Europe and the United States, Carl Haub, senior demographer at the bureau, told a news conference.

With unemployment rates reaching toward 20 percent in some European countries and housing both costly and scarce, European families are limiting the number of children they have. By contrast, a prosperous United States offers jobs and plenty of space for families to build homes, accounting for bigger American families.

Early childbearing speeds population growth by cutting the number of years between generations, Haub said.

"When we ask what the future size of world population will be, we are really asking how many children today's youth's will have," he said.

By 1950, world population had reached 2.5 billion. That was an increase of 900 million people in 50 years.

But according to the bureau's estimates, the increase in population in the second half of the century will be 3.5 billion, nearly six times as great. That will give the world a population of 6 billion as the new century begins.

The last half-century was "a period of explosive growth," and it likely "will be at least duplicated during the first half of the next century," the bureau said.