Since Solow's initial work in the 1950s, many more sophisticated Models of economic growth have been proposed, leading to varying conclusions about the causes of economic growth. For Example, rather than assume people save at a given constant rate that Solow did, subsequent work applied a consumer-optimization framework to derive savings behavior endogenously, allowing saving rates to vary at different points in time, depending on income flows, for Example. In the 1980s efforts have focused on the role of technological progress in the economy, leading to the development of endogenous growth theory (or new growth theory). Today, economists use Solow's sources-of-growth accounting to estimate the separate effects on economic growth of technological change, capital, and labor.