What are the motivations for which direction syntactic trees are built in (top down or bottom up)?

Question

When I learned x-bar theory, there seemed to be an implicit assumption that trees were built top-down, from IP or CP to the VP and its complement, etc. However, as I am learning more about Minimalism (keep in mind I still know very, very little), I see that it posits the opposite direction for tree building. I don't really understand this, because it seems to me that we perceive speech and plan speech, well, from the beginning to the end. I have wondered that if the "bottom up" approach were true, shouldn't we expect to see more speech errors where people stumble on the beginning of the sentence and then finish executing it perfectly, because they have successfully planned the bottom part of the tree/sentence but haven't gotten to or correctly built the top? (Instead of the opposite which usually seems to happen: people start out creating a grammatical sentence and then have an error and the rest becomes incoherent.)

Bear in mind, I do not know a lot about Minimalism and don't really want a Minimalism-centric answer. I am just wondering what general theoretical motivations are there for building directionality (for lack of a better term) -- or if syntacticians disregard this matter completely, why.

Answer 1

In general, talk of syntactic derivations as top-down versus bottom-up is understood to be purely metaphorical: the theory of grammar aims to explain static knowledge, not dynamic process. A forthcoming paper considers how to understand directionality of derivation for syntactic processing (and what that may imply for an adequate theory of syntactic competence). From the introduction:

Generative grammars are typically framed as theories of speakers’ task‐independent knowledge of their language, and these are understood to be distinct from theories of how specific communicative tasks might put that knowledge to use. [...] In the most popular current version of the derivational approach, derivations proceed ‘upwards’, starting from the most deeply embedded terminal elements in the sentence, which are often towards the right of a sentence (e.g., Chomsky, 1995; Carnie, 2006). Such derivations tend to proceed in a right‐to‐left order, which is probably the opposite of the order in which sentences are assembled in everyday tasks such as speaking and understanding. ... [yet there] are a number of recent proposals that various linguistic phenomena can be better understood in terms of derivations that incrementally assemble structures in a (roughly) left‐to‐right order. [...] it is hard to avoid the question of whether it is mere coincidence that left‐to‐right derivations track the order in which sentences are spoken and understood. It is also natural to ask how left‐to‐right derivations impact the psychological commitments of grammatical theories.

In brief, whether speech error data is relevant to the question of derivational order in syntax depends on just how much explanation syntactic theory owes to a theory of linguistic performance.

Answer 2

Regarding top-down vs. bottom up, I am afraid there is no one answer here. Every theory has its own preference. One scholar writes:

Despite the widely accepted claim that listening requires an active process on the part of the learners, there are still intense debates as to how the oral input is processed. Some argue that the processing operates in purely bottom-up fashion, while others tend to the top-down one. To mediate these polarized claims, some scholars suggest that both are necessary so the cognitive processing works interactively.

It is important to distinguish between theoretical linguistics models and psycholinguistic models (not to mention computational models). They have different motivations. For example, linguists may be motivated by language change (sociological), while psycholinguists may be motivated by sentence processing strategies. These may suggest different top-down vs. bottom-up theories.

My favorite book on linguistic theory and sentence processing is "Syntactic Carpentry: An Emergentist Approach to Syntax", William O'Grady. It attempts to reduce the theory of grammar to the theory of sentence processing and how working memory stores information. O'Grady (2001):

Traditional syntactic theory focuses its attention on the architecture of sentence structure, which is claimed to comply with a complex grammatical blueprint. In Principles and Parameters theory, for instance, well-formed sentences have a Deep Structure that satisfies the X-bar Schema and the Theta Criterion, a Surface Structure that complies with the Case Filter and the Binding Principles, a Logical Form that satisfies the Empty Category Principle, and so on. The question of how sentences with these properties are actually built in the course of language use is left to a theory of ‘carpentry’ that includes a different set of mechanisms and principles (parsing strategies, for instance).

I propose a different view. Put simply, there are no architects; there are only carpenters. They design as they build, limited only by the materials available to them and by the need to complete their work as quickly and efficiently as possible. Indeed, drawing on the much more detailed proposals put forward in O’Grady (2005), I suggest that efficiency is THE driving force behind the design and operation of the computational system for language.