The "human capital earnings function," in which earnings are expressed as a quadratic in potential experience, is probably the most widely accepted empirical specification in economics. In spite of its widespread acceptance, the human capital earnings function provides a very poor approximation of the true empirical relationship between earnings and experience. The standard formulation understates early career earnings growth by about 30%-50% and overstates midcareer growth by 20%-50%. However, simple alternative specifications that fit the data are available.

Our analysis of growth assumes endogenous fertility and a rising rate of return on human capital as the stock of human capital increases. When human capital is abundant, rates of return on human capital investments are high relative to rates of return on children, whereas when human capital is scarce, rates of return on human capital are low relative to those on children. As a result, societies with limited human capital choose large families and invest little in each member; those with abundant human capital do the opposite. This leads to two stable steady states. One has large families and little human capital; the other has small families and perhaps growing human and physical capital.

Our analysis of growth assumes endogenous fertility and a rising rate of return on human capital as the stock of human capital increases. When human capital is abundant, rates of return on human capital investments are high relative to rates of return on children, whereas when human capital is scarce, rates of return on human capital are low relative to those on children. As a result, societies with limited human capital choose large families and invest little in each member; those with abundant human capital do the opposite. This leads to two stable steady states. One has large families and little human capital; the other has small families and perhaps growing human and physical capital.