Growth and the diffusion of ideas

In a recent model of growth developed by Lucas (Lucas, R., 2009. Ideas and growth. Economica 76, 1-19), a continuum of people interact in a random manner and copy each other's productive ideas when it is economically beneficial to do so. This paper extends the Lucas model by assuming that each person's productivity also experiences random shocks due to individual discovery. A nonlinear partial differential equation is derived for the distribution of income, which admits a traveling wave solution representing a growing economy. The growth rate is an increasing function of the rate of imitation. The growth rate is also an increasing but concave function of population size and reaches a plateau in the continuum limit. Hence the scale effect is bounded. The model is extended to account for a nonzero cost of imitation, with similar results. The mathematical tools presented in this paper should prove useful in developing idea-based models of growth. (C) 2011 Elsevier B.V. All rights reserved.