The Contribution of Genetic Modification to Changes in Corn Yield in the United States
利用大学推广试验数据，估计基因改造性状对美国玉米产量的影响，发现基因改造性状对产量有正向作用，但多种性状组合的增益并非简单相加。
We use a large, rich dataset compiled from results of university extension trials to estimate the contribution of genetic modification (GM) to changes in corn yield in the United States from timeA to timeB. Through repeated experimental trials, we obtain consistent estimates of the effect of these traits by using both the Hausman‐Taylor estimator and a comparison of fixed effects estimates analogous to the agronomic practice of comparing near‐isolines. Our results suggest that GM traits had a positive impact on yield, but that gains associated with combining several GM traits in one hybrid are not necessarily additive.