Two empirical facts concerning the increase in innovation concentration levels of U.S. industries over the last two decades have been established: innovating firms with larger market shares invest more in R&D and enter into more industries. This paper presents a model of an imperfectly competitive patent market with heterogeneous firms to show that R&D itself increases industry concentration. In the context of an imperfectly competitive patent market structure, firms generate endogenous variable markups. In particular, the price of firms' newly invented knowledge, the profit and survival rate of the innovating firms depend on the market share of the knowledge stock. Firms pay a random fixed cost in each period which together with market share determine the decision on whether to innovate in different sectors. Firms enter different sectors sequentially: they start developing new varieties in a sector with minimal competition, building up private knowledge, and then venturing into other sectors using R&D investment and accumulated knowledge. Finally, we prove that the stationary firm innovation size distribution exhibits a Pareto tail in the steady state.
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