Recursion in Grammar and Performance

In the last 50 years of cognitive science, linguistic theory has proposed more and more articulated structures, while computer science has shown that simpler, flatter structures are more easily processed. If we are interested in adequate models of human linguistic abilities, models that explain the very rapid and accurate human recognition and production of ordinary fluent speech, it seems we need to come to some appropriate understanding of the relationship between these apparently opposing pressures for more and less structure. Here we show how the apparent conflict disappears when it is considered more carefully. Even when we regard the linguists’ project as a psychological one, there is no pressure for linguists to abandon their rather deep structures in order to account for our easy production and recognition of fluent speech. The deeper, more recursive structures reflect insights into similarities among linguistic constituents and operations, but a processor can compute exactly these structures without the extra effort that deeper analyses might seem to require. To show how this works, we review §1 standard notions of recursive depth and some basic ideas about how computations can be implemented, §2 a consensus position about linguistic structure, and §3 some large classes of parsing models that compute exactly the consensus structures in such a way that the depth of the linguistic analysis does not correspond to processing depth. The apparent tension is resolved in this last step, with a proper understanding of how computations can be implemented. The needed perspective on what it is to represent and manipulate structures, to implement a computation, is completely standard. From this unifying perspective, we will argue that adequate performance models must be more superficial than adequate linguistic models, that this is unsurprising, and that the two perspectives are reconciled by a substantial theory of how linguistic computations are implemented. In a sense that will be made precise, the recursive depth of a structural analysis does not correspond in any simple way to depth of the calculation of that structure in linguistic performance.