National Bureau of Economic Research since 1979. Krugman was President of the Eastern Economic Association in 2010. In February 2014, he announced that he would be retiring from Princeton in June 2015 and that he would be joining the faculty at the Graduate Center of the City University of New York. Paul Krugman has written extensively on international economics, including international trade, economic geography, and international finance. The Research Papers in Economics project ranks him among the world's most influential economists. Krugman's International Economics: Theory and Policy, co-authored with Maurice Obstfeld, is a standard undergraduate textbook on international economics. He is also co-author, with Robin Wells, of an undergraduate economics text which he says was strongly inspired by the first edition of Paul Samuelson's classic textbook. Krugman also writes on economic topics for the general public, sometimes on international economic topics but also on income distribution and public policy. The Nobel Prize Committee stated that Krugman's main contribution is his analysis of the effects of economies of scale, combined with the assumption that consumers appreciate diversity, on international trade and on the location of economic activity. The importance of spatial issues in economics has been enhanced by Krugman's ability to popularize this complicated theory with the help of easy-to-read books and state-of-the-art syntheses. "Krugman was beyond doubt the key player in 'placing geographical analysis squarely in the economic mainstream' ... and in conferring it the central role it now assumes." New trade theory Prior to Krugman's work, trade theory (see David Ricardo and Heckscher–Ohlin model) emphasized trade based on the comparative advantage of countries with very different characteristics, such as a country with a high agricultural productivity trading agricultural products for industrial products from a country with a high industrial productivity. However, in the 20th century, an ever-larger share of trade occurred between countries with similar characteristics, which is difficult to explain by comparative advantage. Krugman's explanation of trade between similar countries was proposed in a 1979 paper in the Journal of International Economics, and involves two key assumptions: that consumers prefer a diverse choice of brands, and that production favors economies of scale. Consumers' preference for diversity explains the survival of different versions of cars like Volvo and BMW. However, because of economies of scale, it is not profitable to spread the production of Volvos all over the world; instead, it is concentrated in a few factories and therefore