Large brains are associated with increased cognitive skills, enabling animals to use new environments and resources more successfully. Such behavioral flexibility is theoretically expected to have macroevolutionary consequences. First, populations of big-brained individuals should more easily become established in new locations, increasing opportunities for allopatric speciation and decreasing chances that the species as a whole becomes extinct. Second, the ability to use new resources should place new selection pressures on populations, promoting adaptive diversification, a process termed "behavioral drive." In this article, we show that the average brain size of a bird family explains a significant fraction (R2 =0.12, P < .0001 , N= 120 families) of the rate at which body size diversifies within the family. The association is independent of the number of species in the family, geographic range, and correlates of speciosity, providing the first general support for the importance of behavioral drive in evolution.