By the end of the Second World War Kuznets moved into a new research area, related to the tie between changes in income and growth. He proposed a research program that involved extensive empirical studies on the four key elements of economic growth. The elements were demographic growth, growth of knowledge, in-country adaptation to growth factors, and external economic relations between the countries. The general theory of economic growth should explain the development of advanced industrial countries, and the reasons that prevent the development of backward countries, include both market and planned economies, large and small, developed and developing countries, consider the impact on growth of foreign economic relations.