This paper develops a search-theoretic model of technological change to explain why both patenting and the growth of productivity have remained roughly constant while research employment in the United States has increased by a factor of six over the past four decades. In the model, researchers sample from probability distributions determining the efficiency of potential new production techniques. Technological breakthroughs, resulting in patents, become increasingly hard to find as the level of technology advances. Given certain restrictions on the search distributions, the equilibrium of the model replicates the U.S. time-series pattern of research, patenting, and productivity.

This paper provides a fairly systematic study of general economic conditions under which rational asset pricing bubbles may arise in an intertemporal competitive equilibrium framework.Our main results are concerned with non-existence of asset pricing bubbles in those economies.These results imply that the conditions under which bubbles are possible inc1uding sorne well-known examples of monetary equilibria-are relatively fragile.