Behavioral changes have long been hypothesized to be an important driver of evolutionary diversification in animals, as they expose individuals to new environmental pressures and thus favor evolutionary divergence. There have been few empirical tests of this hypothesis, however, and the mechanisms linking behavioral changes and diversification processes remain controversial. We show here that Holarctic passerines with large brain size relative to body size, a character correlated with a high propensity for behavioral changes, generally have experienced more extensive subspecific diversification. This effect appears to be largely independent of other well-known mechanisms thought to promote diversification. As suggested by path analysis, relative brain size seems to affect diversification directly rather than indirectly through its presumed effect on range expansion, which is consistent with the original formulation of the behavioral drive hypothesis. Thus, the results support the long-held, intuitive hypothesis that behavioral changes facilitate evolutionary diversification.