key factors in the Solow growth model are s, n, and productivity growth (exogenous). What else should be considered when modeling long-run economic growth that is not captured by Solow's growth model? Endogenous growth theory by Paul Romer (JPE, 1986) explains productivity with emphasis on systematic factors that may influence output growth in the long-run. What two other factors could help move the economy up according to this view? (