Certain problems do exist, however. For instance, increases in real income that are achieved need to be maintained, which means keeping population numbers in check. Illiteracy and social customs for large families tend to work against governmental efforts to increase the STANDARD OF LIVING of its citizens. Also, most of the foreign exchange earned by such countries is by exporting, mainly commodities (see INTERNATIONAL TRADE). See ECONOMIC DEVELOPMENT, STRUCTURE OF INDUSTRY, DEMOGRAPHIC TRANSITION, POPULATION TRAP, INTERNATIONAL COMMODITY AGREEMENTS, UNITED NATIONS CONFERENCE ON TRADE AND DEVELOPMENT, INTERNATIONAL DEBT.