摘要：Econometric studies of the effects of research on productivity have typicallyimposed arbitrary restrictions on the length and shape of the R&D lag profile. Theserestrictions are likely to have biased up both the measured effects of R&D on productivityand the estimated rates of return to research. This paper argues that the useful stock ofpublic knowledge depreciates, if at all, only gradually, and we use this notion to develop anew model, which we test using data on aggregate U.S. agriculture. We reject theconventional specification in favor of a more flexible, dynamic, alternative model, in whichthe impact of R&D on productivity lasts much longer than in previous studies. Consequently, the real, marginal rate of return to public agricultural R&D in the UnitedStates is estimated to be less than 10 percent per annum, much smaller than the typicalrates of return reported in scores of previous studies, based on conceptually flawed andinappropriately restrictive dynamic specifications. We show that conventional approachesusing the same data would have resulted in a much greater (biased) estimate of the rate ofreturn to research.
