This paper is a review of the literature in economics on the effect on earnings of human capital investment from Adam Smith to the early 1980s. It discusses the use of the net present value (NPV) technique by Walsh and Friedman and Kuznets, and Becker's use of the NPV to estimate the internal rate of return to schooling. The first regression-based approach was the Becker and Chiswick 'schooling-earnings function', which was expanded by Mincer to the 'human capital earnings function' (HCEF) by incorporating on-the-job training. Extensions of the HCEF in the 1970s and early 1980s are discussed.