Second, trade theory: in Latin American countries early development strategies had favored an outward-oriented economic model in which countries provided primary goods, such as coffee, wheat, or copper, to the European and North American markets in accord with the Ricardian theory of comparative advantage. The depression of the 1930s revealed weaknesses inherent in this position, as demand and prices fell to the point that coffee was burnt as fuel for the Brazilian railroads. After World War II a coherent Latin American perspective on the development process was formulated in the United Nations Economic Commission for Latin America (ECLA). The commission found that conventional (classical and neoclassical) theories were inadequate for understanding the underdeveloped world. Instead, an appreciation of the different historical contexts and natural situations of these countries, their different social structures, types of behavior, and economies, required a new “structuralist” perspective. The main tenets of this theory were outlined by Raul Prebisch (1901–1986), formerly head of the Central Bank of Argentina, and Hans Singer (1910–2006), a U.S. economist, in what became known as the Singer–Prebisch thesis.