Is Structure Dependence Shaped for Efficient Communication?: A Case Study on Coordination (2024)

Abstract: Natural language exhibits various universal properties.But why do these universals exist?One explanation is that they arise from functional pressures to achieve efficient communication , a view which attributes cross-linguistic properties to domain-general cognitive abilities.This hypothesis has successfully addressed some syntactic universal properties such as compositionality and Greenbergian word order universals.However, more abstract syntactic universals have not been explored from the perspective of efficient communication.Among such universals, the most notable one is structure dependence , that is, grammar-internal operations crucially depend on hierarchical representations.This property has traditionally been taken to be central to natural language and to involve domain-specific knowledge irreducible to communicative efficiency. In this paper, we challenge the conventional view by investigating whether structure dependence realizes efficient communication, focusing on coordinate structures.We design three types of artificial languages: (i) one with a structure-dependent reduction operation, which is similar to natural language, (ii) one without any reduction operations, and (iii) one with a linear (rather than structure-dependent) reduction operation.We quantify the communicative efficiency of these languages.The results demonstrate that the language with the structure-dependent reduction operation is significantly more communicatively efficient than the counterfactual languages.This suggests that the existence of structure-dependent properties can be explained from the perspective of efficient communication.

## introduction to understand the universals of natural language, it is crucial to address why such universals exist, as well as how such universals can be theoretically described. this raises the question: what kinds of pressures shape these universals? one explanation is that the universals of natural language are shaped as a result of functional pressures to achieve efficient communication @xcite @xcite @xcite @xcite . efficient communication refers to a situation where the amount of information conveyed is maximized while the effort required for production and comprehension is minimized under human cognitive constraints. if some structural property of languages is shaped to achieve efficient communication, it can be optimized under two competing functional pressures: the need to be as simple as possible and the need to be as informative as possible. the hypothesis that two competing pressures for utilities shape the form of human language has long been assumed in linguistics @xcite @xcite . in recent years, methodologies have been established to examine this hypothesis by quantifying the simplicity and informativeness of languages by using information-theoretic criteria @xcite @xcite . to date, this hypothesis has been successfully examined at the lexical level @xcite @xcite @xcite @xcite @xcite @xcite @xcite @xcite . at the syntactic level, there are pieces of empirical evidence that grammar itself is shaped to achieve efficient communication @xcite @xcite @xcite , and it has been shown that the existence of syntactic universals such as compositionality and greenbergian word order universals @xcite can be explained by this hypothesis @xcite . but we do not yet know whether this type of competition-based account can be extended to more abstract types of linguistic knowledge that go beyond mere sensitivity to structure such as compositionally and word order. a representative type of such abstract knowledge comes from cases of what one might call structure dependence, by which we broadly refer to operations that directly manipulate structural representations at some level of linguistic representation. structure dependence has traditionally been taken to be a characteristic and central property of human language @xcite @xcite ; in fact, a key underlying theme throughout the whole history of mainstream generative grammar is extreme skepticism of the idea that such properties can be reduced to communicative principles. the dogma in this line of thought has it that abstract syntactic properties of language are thought to be governed by domain-specific efficient computation necessary for deriving the structure of language @xcite @xcite , while communication is viewed as not essential to the core linguistic competence @xcite . it is thus crucial to investigate whether even such syntactic properties can be accounted for from the perspective of domain-general efficient communication. in this paper, we directly address this issue by examining structure dependence. specifically, we investigate whether structure dependence realizes efficient communication by focusing on coordinate structures. we design three types of languages: (i) one with a structure-dependent reduction operation, which has coordinate structures similar to those in natural language, (ii) one without any reduction operations, and (iii) one with a linear (rather than structure-dependent) reduction operation. the latter two are conceptually possible but counterfactual languages. we adopted @xcite artificial probabilistic context-free grammars (pcfgs) to create the three languages. then we quantify the simplicity and informativeness of these languages and compare their communicative efficiency. the results demonstrate that the languages with a structure-dependent reduction operation are significantly more communicatively efficient than their counterfactual counterparts. this suggests that the structure-dependent properties in human language can be explained in terms of efficient communication. ## results a distribution of the values of communicative efficiency for the three types of languages, as calculated by rnngs, is shown in figure 3 . for an interpretation of the trade-off parameter λ, the predictability and parsability values of all languages are z-transformed (i.e., centered and divided by the standard deviation) before being substituted into eq 4. the lines in the figure show the transitions of the value of communicative efficiency for λ with a 95% confidence interval (ci). by finding the coordinates where the lines intersect, we can observe the behavior of communicative efficiency for each language depending on the value of λ. the lower bound of the 95% ci for structure-reduction intersects with the upper bound of the 95% ci for linear-reduction at λ = 0.18. in the same way, the upper bound of the 95% ci for structure-reduction intersects with the lower bound of the 95% ci for linear-reduction at λ = 0.93. this indicates that structure-reduction languages are the most communicatively efficient, at least within the range of λ values between 0.18 and 0.93. additionally, when we examine only predictability or parsability, their distributions are shown in figure 4 and figure 5 , respectively. the mean values for predictability were ordered as no-@xmath0, while for parsability, the order was linear-@xmath1, in which all of the pairs have a statistically significant difference (@xmath2.05/3 by paired t-test with bonferroni correction). this indicates that when only predictability or parsability is individually considered, the structure-reduction language may not always be the best option. however, to satisfy both criteria simultaneously, i.e., to consider the weighted sum of predictability and parsability under the parameter λ ∈ [0.18, 0.93], the structure-reduction language achieves the highest score of communicative efficiency as shown in figure 3 . to further interpret the results of predictability and parsability, we plotted least squares regression lines between word position in the sentence and each word-by-word value (figure 6 ). for predictability, only the linear-reduction languages significantly decline towards the latter part of the sentence. as for parsability, although the linear-reduction languages experience a faster decrease, the other two languages, which have longer expression lengths, achieve a lower overall score. ## discussion we demonstrated that the structure-reduction languages, which have the same structuredependent reduction operation as natural language, had significantly higher communicative efficiency compared to the conceptually possible but counterfactual no-reduction and linear-reduction languages when we calculated the scores by rnngs with a trade-off parameter λ ∈ [0.18, 0.93]. this suggests that the structure-dependent reduction operation prevalent in the natural language syntax may exist due to functional pressures to support efficient communication along lines discussed in section 2.1. it should be noted that, as figure 3 shows, when λ is extremely small or large, the no-reduction or linear-reduction languages achieve the highest efficiency scores. however, λ represents the relative contribution of the two terms to the overall efficiency score. it is difficult to assume a reasonable scenario where only one term is emphasized. while we do not aim to estimate a sperelationship between predictability/parsability and word position for the three types of languages. predictability and parsability here refer to the negative surprisal and the negative loglikelihood of the best parse for each word, respectively. the lines represent the fit of a least squares regression model for these data. it should be noted that even in natural language, there are instances of linear reduction operations such as stripping @xcite , for instance shown below, or sentences that retain sentence-level coordination without reduction for pragmatic purposes such as emphasis. mary took a walk in the park, and bill too. however, these phenomena occur as alternative choices in a language that has a structure-dependent reduction operation. our claim is that a language lacking a structure-dependent reduction operation entirely is not preferable from the perspective of efficient communication. the results of this study have interesting implications for theoretical linguistics research. structure dependence, a syntactic universal property we addressed here, has traditionally been argued to be one of the characteristic features of human language @xcite @xcite . a prominent view in the mainstream generative grammar argues that natural language involves domain-specific predispositions and that syntactic properties of language-including structure dependence-are best explained from the perspective of 'efficient computation' reflecting such predispositions genetically hard-wired in the human brain @xcite @xcite . under this view, communication is taken to be a kind of epiphenomenon, not essential to the core linguistic competence @xcite . however, our results suggest that at least some structure-dependent properties present in natural language (such as coordination) can be explained from the perspective of efficient communication. this does not immediately refute the dominant research program attempting to explain linguistic properties from a 'computational' perspective, but it does indicate that abstract properties in syntax may not necessarily need to be explained solely from that perspective. this aligns with the existing body of research that attempts to explain various aspects of natural language from the perspective of efficient communication @xcite . ## conclusion in this paper, we investigated whether structure dependence, one of the syntactic universals, reflects the optimization for efficient communication. to address this issue, we focused on coordinate structures and designed three types of artificial languages: (i) one with a structuredependent reduction operation, (ii) one without any reduction operations, and (iii) one with a linear (rather than structure-dependent) reduction operation. we quantified the communicative efficiency of these languages and compared them. the results demonstrated that the languages with a structuredependent reduction operation were significantly more communicatively efficient than their counterfactual counterparts. this suggests that the structure-dependent properties of natural language can be explained from the functional perspective of efficient communication. 7 limitations there is room for improvement in the objective function for communicative efficiency. although we used the mean word-by-word surprisal, conditioned on all preceding words, as a measure of predictability, human language processing is subject to short-term memory constraints @xcite @xcite . thus, it is preferable to model predictability in a way that incorporates lossy memory representation @xcite @xcite . moreover, the psychological plausibility of the parsability metric should be critically evaluated, both conceptually and empirically. since parsability relies on an intermediate representation-syntactic structures-it does not fully capture the direct relationship between linguistic expressions and their meanings, suggesting that there is room for further conceptual refinement. to the best of our knowledge, this study is the first to investigate communicative efficiency with respect to structure dependence. we focused on coordinate structures with the artificial language paradigm as a starting point. of course, a deeper understanding of structure dependence in language will require using natural language data and extending the analysis to phenomena such as agreement and movement that are argued to be relevant to structure dependence. in future work, we plan to test the relationship between structure dependence and communicative efficiency by applying the methodology proposed here to a broader range of syntactic constructions, using treebanks like universal dependencies @xcite .