The aggregate rate of R&D in a competitive economy is compared with the optimal rate. The optimal rate of R&D is shown to be the same for all preferences in a broad family, while the competitive rate is sensitive to the form of substitutability among products and so can vary dramatically within a family. The second-best level of R&D is shown to be also common within a family and equal to the optimal rate. Numerical examples suggest that diminishing returns in the innovation technology is the most important potential source for excessive R&D in a competitive economy.

Recent estimates of the potential growth effects of tax reform vary wildly, ranging from zero to eight percentage points. Using an endogenous growth model, we assess which model features and parameter values are important for determining the quantitative impact of tax reform. We find that the critical parameters are factor shares, depreciation rates, the elasticity of intertemporal substitution, and the elasticity of labor supply. The elasticities of substitution in production, on the other hand, are relatively unimportant. The quantitative estimates in several recent papers are compared with each other and with some of the evidence from U.S. experience. We find that Robert Lucas's conclusion, that tax reform would have little or no effect on the U.S. growth rate, is theoretically robust and consistent with the evidence.

Recent estimates of the potential growth effects of tax reform vary wildly, ranging from zero to eight percentage points. Using an endogenous growth model, the authors assess which model features and parameter values are important for determining the quantitative impact of tax reform. The quantitative estimates in several recent papers are compared with each other and with some of the evidence from U.S. experience. The authors find that Robert Lucas's conclusion, that tax reform would have little or no impact on the U.S. growth rate, is theoretically robust and consistent with the evidence.