"The world has had a great experience of dealing with rapid population growth," Wilmoth said at a news conference. "World population doubled between 1960 and 2000, roughly. World food supply more than doubled over that time period."
"The problem is more one of extremes," he added. "The main story is to avoid the extreme of either rapid growth due to high fertility or rapid population aging and potential decline due to very low fertility."
Among the fastest-growing countries is Nigeria, whose population is expected to surpass the U.S. population before the middle of the century and could start to rival China as the second-most populous country in the world by the end of the century, according to the report. By 2050, Nigeria's population is expected to reach more than 440 million people, compared to about 400 million for the U.S. The oil-rich African country's population is forecast to be nearly 914 million by 2100.
The report found that most countries with very high levels of fertility — more than 5 children per women — are on the U.N. list of least-developed countries. Most are in Africa, but they also include Afghanistan and East Timor.
But the average number of children per woman has swiftly declined in several large countries, including China, India, Indonesia, Iran, Brazil and South Africa, leading to a reduction in population growth rates in much of the developing world.
In contrast, many European and eastern Asia countries have very low fertility levels.
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