Nature or nurture? Linguistics is a science that systematically addresses this puzzle, and its results in recent decades offer a uniquely interesting support for the answer: Both. Language is a social phenomenon, but all human languages share elaborate and specific structural properties. The conventions of speech communities arise, exhibit variation, and change within the strict confines of universal grammar, part of our biological endowment. Universal grammar is discovered through the careful study of the structures of individual languages, by cross-linguistic comparisons, and the investigation of the brain. In this way, linguistics mediates between cognitive science and social science.