The aim of this paper is not to supply a synthesis of the Chicago School human capital theory, but rather to define and analyze the earnings functions and their relations with the human capital concept. The earnings functions are an easy and flexible tool, for the analysis of the investments in education that, in the formulation introduced in this paper do not require hardly available data. But they don't seem to be a suitable model for the human capital analysis in its ample definition. Using Italian, Canadian and US data provided by Luxembourg Income Study, some different formulations of the Mincer's earning function are estimated and compared, analyzing the effects of some socio-demographic variables on the rate of return of education. Results reflect the different labor market structure across the studied country.