IntroductionThis essay focuses on two questions of fundamental importance to cognitive science.i. Why does language have the particular properties that it does?ii. How is language acquired in response to the particular types of experience that children receive in the first years of life?In a sense, the answer to both questions is obvious. Human language is the way it is, and children learn it so successfully, because it is a good fit for the brain. Everyone believes this, and the point is frequently made in one form or another by scholars whose views otherwise differ (e.g., Chomsky 2007, Christiansen & Chater 2008, to take just two examples). But what features of the human brain create and mediate the fit? The list of possible contributors, summarized by Chomsky ( 2005), is practically self-evident: (i) inborn principles of grammar, if there are any; (ii) a capacity for extracting regularities from experience; and (iii) the influence of perception, processing, economy, and cognition (the so-called 'third factor').The first factor has been the central concern of work on classic Universal Grammar (UG), as developed by Chomsky (1981) and many others. Within much of cognitive science, though, increasing attention has been devoted to the second factor, as evidenced by the growth of research focused on statistical learning (e.g.,