Norway enjoys the world's highest quality of life, while Niger suffers the lowest, a U.N. agency said Monday, as it released a ranking that highlights the wide disparities in well-being between rich and poor countries.

The annual Human Development Index, unveiled in Bangkok by the United Nations Development Program, takes into account life expectancy, literacy, school enrollment and per capita gross domestic product in 182 countries.

"A child born in Niger can expect to live to just over 50 years, which is 30 years less than a child born in Norway. Furthermore, the differences in per capita income are huge for every dollar earned per person in Niger, US$85 are earned in Norway," UNDP said.

Norway was followed by Australia and Iceland on the list, which drew on statistics dating from 2007, before Iceland was hit hard in global economic crisis. Afghanistan and Sierra Leone rounded out the bottom of the ranking.

The United States was listed 13th.

Trends in the index since 1980 showed an average improvement of 15 percent in countries' scores. The greatest long-term improvements have been shown by China, Iran and Nepal, but progress has been concentrated in education and health rather than income, said the U.N agency.

Afghanistan is new to the list this year — reliable statistics were not previously available — but otherwise leaders and laggards are largely the same.

However, five countries rose three or more places — China, Colombia, France, Peru and Venezuela — while seven countries dropped more than two places — Belize, Ecuador, Jamaica, Lebanon, Luxembourg, Malta, and Tonga.

The index was released as part of the UNDP's annual Human Development Report, which this year highlighted migration.

"Most migrants, internal and international, reap gains in the form of higher incomes, better access to education and health and improved prospects for their children," said the report. "These gains often directly benefit family members who stay behind as well as countries of origin indirectly."

It also suggested that as the populations age in developed countries, they could benefit from increased migration to boost their work forces.

But it cautioned that encouraging migration should not substitute for "efforts by developing countries to achieve growth and improve human well-being."