Structuralism is a development theory which focuses on structural aspects which impede the economic growth of developing countries. The unit of analysis is the transformation of a country's economy from, mainly, a subsistence agriculture to a modern, urbanized manufacturing and service economy. Policy prescriptions resulting from structuralist thinking include major government intervention in the economy to fuel the industrial sector, known as import substitution industrialization (ISI). This structural transformation of the developing country is pursued in order to create an economy which in the end enjoys self-sustaining growth. This can only be reached by ending the reliance of the underdeveloped country on exports of primary goods (agricultural and mining products), and pursuing inward-oriented development by shielding the domestic economy from that of the developed economies. Trade with advanced economies is minimized through the erection of all kinds of trade barriers and an overvaluation of the domestic exchange rate; in this way the production of domestic substitutes of formerly imported industrial products is encouraged. The logic of the strategy rests on the infant industry argument, which states that young industries initially do not have the economies of scale and experience to be able to compete with foreign competitors and thus need to be protected until they are able to compete in the free market. The Prebisch–Singer hypothesis states that over time the terms of trade for commodities deteriorate compared to those for manufactured goods, because the income elasticity of demand of manufactured goods is greater than that of primary products. If true, this would also support the ISI strategy.