Canada and Ireland follow the top three on the UN Human Development Index, which is dominated by rich, free-market countries.
The United States has slipped to 12th place from eighth last year.
But the index, blending 2005 figures for life expectancy, educational levels and real per capita income, finds that all 22 countries falling into its "low human development" category are in sub-Saharan Africa, with Sierra Leone last.
In 10 of these countries, two children in five will not reach the age of 40, said the compilers at the UN Development Program.
Last year's report said HIV/AIDS had had a "catastrophic effect" on life expectancy in the region.
The index ranks 175 UN member countries plus Hong Kong and the Palestinian territories. It does not include 17 countries, including Iraq, Afghanistan and Somalia, because of inadequate data.
Norway had held top spot for six years but was edged into second place by Iceland this year because of new life expectancy estimates and updated figures for gross domestic product, or GDP.
UN officials played down the significance of minor short-term shifts in the rankings, including the slide in the US position.
They said if subsequent data for the year in question had been available for last year's report, the United States would have been in 10th, not eighth place.
The United States scores high on real per capita GDP, which is second only to that of Luxembourg, but less well on life expectancy - joint last in the top 26 countries, along with Denmark and South Korea, at 77.9 years.
Japanese have the longest life expectancy (82.3 years) and Zambians the lowest, at 40.5.
The report said most countries had seen their human development index rise over the last 30 years, but in 16 it was lower than in 1990, and in three - the Democratic Republic of Congo, Zambia and Zimbabwe - lower than in 1975.
Per capita GDP is 45 times higher in Iceland than in Sierra Leone.
The United Nations has published its human development index every year since 1990. © 2007 Australian Broadcasting Corporation