  The ability to produce and understand an unlimited number of different
sentences is a hallmark of human language. Linguists have sought to define the
essence of this generative capacity using formal grammars that describe the
syntactic dependencies between constituents, independent of the computational
limitations of the human brain. Here, we evaluate this independence assumption
by sampling sentences uniformly from the space of possible syntactic
structures. We find that the average dependency distance between syntactically
related words, a proxy for memory limitations, is less than expected by chance
in a collection of state-of-the-art classes of dependency grammars. Our
findings indicate that memory limitations have permeated grammatical
descriptions, suggesting that it may be impossible to build a parsimonious
theory of human linguistic productivity independent of non-linguistic cognitive
constraints.
