Some economists claimed that the Soviet Union missed the lessons of the Solow growth model, because starting in the 1930s, the Stalin government attempted to force capital accumulation through state direction of the economy. However, Solow's calculations have been proven invalid, so this is a poor explanation. Modern research shows the main factor for economic growth is the growth of labor and capital inputs, not increases in productivity. Therefore, other factors besides capital accumulation must have been big contributors to the Soviet economic crisis.