This study asks whether the diffusion of modern varieties has affected the yield stability of wheat and maize production in developing countries between 1960 and 2004. The data suggest strongly that, over the past forty-plus years, there has been a striking decline in the relative variability of grain yields in wheat and (to a lesser extent) maize. The declines are strongest when measured as the standard deviation of yield divided by the mean, but there have also been declines in other measures of variability, such as the percentage deviation from trend. Declines in yield variability do not appear to be driven by increases in intensification (e.g., irrigation), nor do they appear to extend to crops, such as oilseeds, in which there has been little concerted public sector breeding effort. Instead, they appear to reflect changes in the portfolio of varieties being grown, possibly in response to breeding efforts aimed at disease and pest problems. By reducing the fluctuations in grain yields, researchers may have played a significant role in making modern crop technology attractive and accessible to farmers around the globe.