model, the exponent P on the fixed factor of production L has been reduced from 0.6 to 0.33. This lower value of /3 is consistent with the data on income shares because total wage payments consist of payments to both human capital and unskilled labor. If K and H vary together across countries, this specifica- tion implies that it takes about a three-fold increase in investment (an increase by the factor 0.1-"."0 be precise) to offset a 10-fold increase in output per worker in a comparison across nations. Once one takes account of variation in investment in schooling as well as in investment in physical capital, a factor of three is roughly consistent with the variation in total investment rates observed in the Summer-Heston sample of countries.
Although Mankiw, Romer and Weil do not examine the state data, it is clear what their style of explanation would suggest. They would assume that the same technology was available in the North and the South. Suppose that Northern states had levels of both human capital and physical capital that were higher than those in the Southern states in the same ratio. A value of P equal to
1/3, together with the fact that output per capita was about one-third as large in the South in 1880, would imply that rate of return on physical capital and the wage for human capital were both about (1/3)-"."or about 1.7) times higher in the Southern states than they were in the New England states. Compared to the factor of 5 implied by the model without human capital, these parameters would imply much smaller incentives to shift all capital investment to the South. (They would imply, however, that human capital would tend to migrate from the North to the South.)
The implication from this work is that if you are committed to the neoclassical mode, the kind of data exhibited in Figures 1 and 2 cannot be used to make you recant. They do not compel you to give up the convenience of a model in which markets are perfect. They cannot force you to address the complicated issues that arise in the economic analysis of the production and diffusion of technology, knowledge, and information.
An Evaluation of the Convergence Controversy
The version of the development of endogenous growth theory outlined above skips lots of detail and smooths over many conlplications that made this seem like a real controversy at the time. In retrospect, what is striking is how little disagreement there is about the basic facts. Everyone agrees that a conventional neoclassical model with an exponent of about one-third on capital and about two-thirds on labor cannot fit the cross-country or cross-state data. Everyone agrees that the marginal product of investment cannot be orders of magnitudes smaller in rich countries than in poor countries. The differences between the different researchers concern the inferences about models that we should draw from these facts. As is usually the case in macroeconomics, many different inferences are consistent with the same regression statistics.