Unsatisfied with the assumption of exogenous technological progress in the Solow–Swan model, economists worked to "endogenize" (i.e., explain it "from within" the models) productivity growth in the 1980s; the resulting endogenous growth theory, most notably advanced by Robert Lucas, Jr. and his student Paul Romer, includes a mathematical explanation of technological advancement.[1][84] This model also incorporated a new concept of human capital, the skills and knowledge that make workers productive.