This article presents a new type of comparative linguistic survey, analyzing (socio)linguistic variation in a database of 1,155 grammatical constructions drawn from 42 diverse languages. We focus in particular on variation in the expression of grammatical meanings, and the extent to which grammatical variation differentiates geographic dialects. This is the first study we know of to present a systematic, crosslinguistic survey of dialect differentiation. We identify three main structural types of grammatical variation—form, order, and omission—and find that in situations of close contact between dialects, where signaling of distinct group identities is more relevant, form variables are more likely to differentiate dialects than the other two types. Order and omission variables usually only differentiate dialects that have minimal contact. Our survey suggests that social signaling may have a substantial role in the divergence of grammars, and provides systematic support for previous proposals regarding convergence and divergence under contact.