The importance of the standard concept of human capital, in contrast to social and cultural capital, is fully accepted by mainstream economists. Whilst the concept did not catch on until 50 years ago or so the very idea of human capital is far older. “Economists who considered human beings or their skills as capital include such well-known names in the history of economic thought as Petty, Smith, Say, Senior, List, von Thunen, Roscher, Bagehot, Ernst Engel, Sidgwick, Walras, and Fisher” (Kiker 1966: 481). For instance, the founder of modern economics Adam Smith introduced the notion of humans as capital in his classic Wealth of Nations (1776/1937). Others, such as Alfred Marshall (1890/1930) and Irvin Fisher (1906), kept the idea alive (Walsh, 1935). Notwithstanding its long history, the theory of humans as capital remained relatively undeveloped well into the 20th century.