Assessing Corpus Evidence for Formal and Psycholinguistic Constraints on Nonprojectivity

Abstract

Formal constraints on crossing dependencies have played a large role in research on the formal complexity of natural language grammars and parsing. Here we ask whether the apparent evidence for constraints on crossing dependencies in treebanks might arise because of independent constraints on trees, such as low arity and dependency length minimization. We address this question using two sets of experiments. In Experiment 1, we compare the distribution of formal properties of crossing dependencies, such as gap degree, between real trees and baseline trees matched for rate of crossing dependencies and various other properties. In Experiment 2, we model whether two dependencies cross given certain psycholinguistic properties of the dependencies. We find surprisingly weak evidence for constraints originating from the mild context-sensitivity literature (gap degree and well-nestedness) beyond what can be explained by constraints on rate of crossing dependencies, topological properties of the trees, and dependency length. However, measures which have emerged from the parsing literature, e.g., edge degree, end-point crossings, and heads’ depth difference differ strongly between real and random trees. Modeling results show that cognitive metrics relating to information locality and working-memory limitations affect whether two dependencies cross or not, but they do not fully explain the distribution of crossing dependencies in natural languages. Together these results suggest that crossing constraints are better characterized by processing pressures than by mildly context-sensitive constraints.

Memory limitations are hidden in grammar

“…Whereas a recent study of similar classes of grammars suggested that crossing dependencies are constrained by either grammar or cognitive pressures rather than occurring naturally at some rate (Yadav et al, 2019), our findings strongly demonstrate that it is not grammar but rather non-linguistic cognitive constraints, that limit the occurrence of crossing dependencies in languages. Since we released the first version of this article in August 2019, https://arxiv.org/abs/1908.06629, other researchers have confirmed that dependency distance minimiza-tion contributes significantly to the emergence of formal constraints on crossing dependencies (Yadav et al, 2021(Yadav et al, , 2022. Yadav et al, 2021 have also confirmed the findings of previous research indicating that the effect of dependency distances alone leads to overestimate the actual number of crossing dependencies (Gómez-Rodríguez and Ferrer-i-Cancho, 2017); a critical point is that Gómez-Rodríguez and Ferrer-i-Cancho (2017) use a normalized score leading to the conclusion that such overestimation implies a small relative error.…”

Do Speakers Minimize Dependency Length during Naturalistic Dialogue?

“…2 Explanations for right-extraposition have traditionally been cast in terms of phrasal length or complexity (e.g., Yngve, 1960;Wasow, 2002) However, this point is not relevant for the current discussion as right-extraposition due to increased phrasal length leads to DLM (see, Yadav et al, 2022a).…”