US economist Paul Krugman has won the 2008 Nobel Prize in Economics -- for work that integrates research in international trade and economic geography.

The Swedish Academy praised Krugman's work in explaining how a theory of trade dating back to the 1800s had become less applicable. The theory suggested that trade could be defined by initial differences among countries.

Krugman's theory also addresses issues such as what drives worldwide urbanization.

He is a professor of economics and international affairs at Princeton University in the US.

Krugman's win, which includes a $1.4 million prize, marks the ninth consecutive year that an American has solely won or shared the economics prize.