(46) Economic growth involves increases over time in the volume of a country"s per capita gross national product (GNP) of goads and services. Such continuing increases can raise average living standards substantially and provide a stronger base for other policy objectives. (47) It is only in the last two centuries that continued growth in living standards has been realized for a number of now—developed countries, and this process has broadened in the 20th century to include a number of developing countries. (48) However, the fairly steady expansion in the third quarter of the 20th century gave way to a period of slower and more erratic growth for both-high-and low-income countries, while some of the economically poorest countries were thus far unable to establish a serf-sustaining pattern of development. It also became increasingly evident that there were serious environmental problems associated with some types of growth in production. In examining the record of economic growth and development, economists offer some explanations for the changes involved, and the attempts by governments to plan these changes. Five major issues are involved. The first is why economic growth occurs more quickly in some countries and periods than in others. It is the increase in the size and quality of the factors of production that underlies growth, but certain forces deserve special attention. A variety of models of economic growth give expression to the understanding of these forces. Increasing attention has been paid in these models and in policy to the international aspects of growth. This trend is partly a reflection of the growing internationalization of economic activity. It also reflects a number of potentially destabilizing changes in the international economy that became evident during the 1970s. (49) A second issue is the challenges facing the low-income countries, namely, to move from subsistence levels of per capita income to a level that would generate self-sustaining growth and also to reduce the gap between themselves and the higher-income countries. A third issue, productivity, is central to changes in living standards and to-the analysis of international competitiveness. A fourth major issue is the attempt to maintain growth and increase development through economic planning. (50) Planning became a widespread phenomenon during and just after World War II and was given further emphasis in many newly independent countries that were industrializing. Beginning in the 1970s the emphasis shifted to more decentralized planning, with deregulation and privatization of industry as two aspects of this process. Underlying economic growth and planning is a fifth issue, the attempt to predict economic activity. Modern forecasting involves a variety of computer-based techniques at the level of the firm, the country, and the international economy. The accuracy of forecasting has been reduced by increased uncertainty in the global and national economies since the early 1970s.