Exploring diachronic syntactic shifts with dependency length: the case of scientific E nglish (2020)

Abstract: We report on an application of universal dependencies for the study of diachronic shifts in syntactic usage patterns. Our focus is on the evolution of Scientific English in the Late Modern English period (ca. 1700-1900). Our data set is the Royal Society Corpus (RSC), comprising the full set of publications of the Royal Society of London between 1665 and 1996. Our starting assumption is that over time, Scientific English develops specific syntactic choice preferences that increase efficiency in (expert-to-expert) communication. The specific hypothesis we pursue in this paper is that changing syntactic choice preferences lead to greater dependency locality/dependency length minimization, which is associated with positive effects for the efficiency of human as well as computational linguistic processing. As a basis for our measurements, we parsed the RSC using Stanford CoreNLP. Overall, we observe a decrease in dependency length, with long dependency structures becoming less frequent and short dependency structures becoming more frequent over time, notably pertaining to the nominal phrase, thus marking an overall push towards greater communicative efficiency.

## introduction it is obvious that language use changes dynamically due to external pressures, e.g. new vocabulary emerging continuously, but what is less reflected upon is how despite these pressures language remains communicatively intact. there is accumulating evidence of the regulatory function of grammar in the process of changing language use, helping retain efficiency in communication. specifically, grammatical variation in word order, taxis and syntactic embedding is an important means to control linguistic complexity and is thus instrumental in levelling out processing effort (see e.g. @xcite ; @xcite ; @xcite ). one approach to capture syntactic complexity with a view to cognitive processing is dependency locality theory @xcite . this theory says that languages try to minimize syntactic dependencies in order to manage processing effort by reducing the time in which linguistic elements are held in working memory, e.g. by modulating word order (cf. @xcite ). for instance, @xcite shows for english that syntactic choices exhibit a preference for structures with shorter dependencies on the basis of the wsj and brown corpora @xcite provide cross-linguistic evidence of dependency length minimization. recently, there is an increasing interest in dependency length minimization as a diachronic optimization process affecting the language system in general (e.g. @xcite ; @xcite ) or specific sublanguages/registers in particular (e.g. @xcite ). in our research, we are taking a diachronic perspective looking for traces of optimization of syntactic complexity by dependency length minimization in the development of scientific english from the mid 17th century onward. our overarching hypothesis is that the changes in grammatical choice preferences can, at least partially, be explained by dependency length minimization. such changes include for instance the shift from paratactic clause complexes to single clauses with complex nps; see examples (1) and (2) below, both taken from the royal society corpus @xcite . (2) in contrast to the complete and temporary visual motion blindness which occurs during stimulation of v5, a less-prominent interference with the perception of visual motion occurs at 70-80 ms after the onset of the visual stimulus when tms is applied to v1. (1992) the two examples illustrate two kinds of syntactic organization associated with orality and informal prose (example 1) and writtenness and informativeness (example 2). example (1) is a historical example illustrating a paratactic clause complex, marked by the coordinating conjunctions but and and; example (2) is a contemporary example that is syntactically characterized by a simple structure at clause level (x occurs (at y)). its complexity is in the nominal phrases, which exhibit complex premodification with multiple adjectives as well as compounds (temporary visual motion blindness), postmodification with relative clauses (... motion blindness which occurs during stimulation of v5) and prepositional phrases (less-prominent interference with the perception of visual motion). in this light, assuming that shorter dependencies result in more efficient code, our overarching research question is whether this is borne out diachronically for scientific english: if the structures that become typical (more frequent) of scientific english over time are associated with greater dependency locality, then we may conclude that the sublanguage of scientific english develops a more efficient code. specifically, we expect dependency length minimization to arise through a shift from cross-clausal, tactic relations to a preference of single-clause structures with (complex) nominal phrases. the remainder of the paper is organized as follows. section 2 discusses more related work. section 3 provides details on the data set (royal society corpus), universal dependency parsing and the measure of dependency length. in section 4 we present our results, including a discussion of the kinds of syntactic structures involved in dependency length minimization. section 5 concludes the paper with a summary and sketch of follow-up studies. ## related work there is a good deal of research on scientific english from a synchronic as well as diachronic perspective. from a diachronic perspective, important descriptive work is found in @xcite and @xcite and corpus-based analyses are pursued e.g. by @xcite ; @xcite . existing works converge on the observation of a basic linguistic shift to heavy noun phrases and simpler clause structure resulting in higher lexical density and simpler sentence structures, such as single relational or passive clauses (cf. also @xcite , @xcite ). more recently, @xcite forward the hypothesis that scientific english, as it evolves, develops towards an optimal code for communication. using information-theoretic measures (relative entropy, average surprisal), they find that over time scientific english becomes more distinct from "general language", due to a drift towards nominal style and distinctive syntactic usage at clause level. these insights from previous work build our starting point. the operationalization of syntactic complexity we draw upon is dependency length, assuming along with others that shorter dependency length makes grammars more efficient, both computationally as well as in human language processing @xcite . dependency locality theory @xcite claims that dependencies between words should be as short as possible rendering sentence processing more efficient through a lower cost in working memory in incremental comprehension and production @xcite . vice versa, experimental studies have shown that long dependencies lead to comprehension difficulty @xcite . large-scale corpus studies show this to be true across languages @xcite with only small cross-linguistic differences (e.g. @xcite comparing english and german). several other corpus studies find dependency locality driving clause level word order @xcite @xcite . @xcite show that dependency locality is involved in noun phrase structuring and @xcite illustrates its relevance for word order in the postverbal region. for a recent, comprehensive review of research on dependency length see also @xcite . particularly relevant in our context are studies of english that inspect a range of syntactic phenomena, such as @xcite , @xcite or @xcite . also, studies that highlight dependency length minimization from a diachronic perspective, such as @xcite or lei and wen (2020) are directly relevant. here, we pursue similar goals as @xcite , taking note of the specific syntactic phenomena marked by relatively short dependency lengths that occur with high frequencies and thus contribute a lot to average dependency length minimization and overall efficiency as well as those with relatively large dependency lengths that are highly frequent and therefore have a negative impact on efficiency. correspondingly, low frequency phenomena with short/large dependency lengths will not have a big (positive or negative) impact overall. ## analyses and results we here present the results of the overall diachronic tendency regarding dependency lengths (section 4.1), followed by a micro-analysis of the types of syntactic structure that contribute to dependency length minimization (section 4.2). ## summary and conclusions using the framework of universal dependencies and dependency locality theory, we studied the diachronic development of dependency lengths in a diachronic corpus of scientific english in the late modern period. our starting assumption was that scientific english develops a more efficient code over time reflected in a statistical preference of syntactic structures with shorter dependency lengths over time. we found that overall, the average sum of dependencies per sentence goes down over time, if only slightly, and the relative frequency of longer dependencies decreases and particular shorter dependencies become more frequent, especially nominal premodification. this confirms the observed trend towards a nominal, informational style (cf. section 1). comparing the trends in sentences of 26 tokens length with sentences of the double length we found that the above mentioned trends seem to generalize across different sentence lengths. in future work we plan to extend this comparison to further (more extreme) sentence lengths to see whether the trends are stable. regarding the utility of the ud framework for the present purpose of analysis, the crucial aspect was the labeling with functions, which we found to be descriptively adequate overall with the exception of parataxis and coordination which are notorious problems in dependency parsing ( cf. ahrenberg (2019) ). this impacted negatively on our analysis, since we could not interpret the values for the parataxis function. in our ongoing research, we apply other measures of syntactic complexity, including dependency depth, to capture shifts in complexity in terms of syntactic embedding. recall that we observe nominal phrases becoming more complex over time by multiple pre-and postmodification. this cannot be captured with dependency length alone. also, we are looking to complement dependency-based analyses of complexity with information-theoretic measures, such as surprisal. for instance, we have estimated the average surprisal of nouns and verbs on a subsample of the rsc and found that while the information content of nouns vs. verbs stays fairly stable over time, nouns have higher surprisal (>9 bits) than verbs (<8 bits) on average. if nouns are generally higher in informativity than other lexical words, then we have additional evidence that "nominal style" can be explained on the grounds of efficient communication.