Before leaving Oxford, Kalecki suggested to Steindl that he examine the puzzle as to why capitalism ceased to function in the inter-war period. Steindl took up the suggestion and after completing Small and Big Business (1945b) turned his attention to the puzzle. In particular, he sought to contribute to the mature economy controversy initiated by Alvin Hansen through a concrete examination of the rise, decline, and eventual stagnation of capital accumulation in the American economy from 1869 to 1939. Believing that the explanation of economic stagnation required the articulation of a long-term theory of growth, Steindl developed Kalecki's theory of investment into a theory of growth centered on the propensity of entrepreneurs to invest, where this propensity was a function of past savings, relative indebtedness, and the degree of capacity utilization. Furthermore, he argued that the degree of capacity utilization was dependent on the magnitude of net profit margins vis-à-vis the given level of investment. Steindl therefore began his investigation by examining the forces which determined the net profit margin. The outcome of this work was a microanalysis of the net profit margin, which was completed by 1947. He then turned his attention to developing his theory of growth based on this microanalysis, which was completed in 1949. The completed manuscript was sent to the printers in January 1950, but the printing of Maturity and Stagnation in American Capitalism was delayed until 1952.