Of all the mysteries confronting linguistic theory, none has proven more difficult or contentious than the question of how language is acquired. In a way, this is puzzling since so much is ‘out in the open’—children acquire language based on experience that we can directly observe, and we have easy access to their immature utterances and errors. Yet, very fundamental questions remain unanswered, the most controversial of which has to do with how much is learned and how much is inborn.