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 in a few countries (or maybe just one). This logic explains how each country may specialize in producing a few brands of any given type of product, instead of specializing in different types of products.
Krugman modeled a 'preference for diversity' by assuming a CES utility function like that in a 1977 paper by Avinash Dixit and Joseph Stiglitz. Many models of international trade now follow Krugman's lead, incorporating economies of scale in production and a preference for diversity in consumption. This way of modeling trade has come to be called New Trade Theory