Economic modernization theories such as Rostow's stages model have been heavily inspired by the Harrod-Domar model which explains in a mathematical way the growth rate of a country in terms of the savings rate and the productivity of capital. Heavy state involvement has often been considered necessary for successful development in economic modernization theory; Paul Rosenstein-Rodan, Ragnar Nurkse and Kurt Mandelbaum argued that a big push model in infrastructure investment and planning was necessary for the stimulation of industrialization, and that the private sector would not be able to provide the resources for this on its own.