An explanation of the decisive role of function words in driving syntactic development

Abstract

The early mastery of function words (FWs) better predicts children’s concurrent and subsequent syntactic development than their acquisition of content words (CWs). Wishing to understand why the advantage of the early mastering of a FW vocabulary, we tested the hypothesis that the learning of FWs involves learning their syntax to a higher degree than is the case for CWs. English-language parental (N=506) and young children’s speech samples (N=350) were taken from the CHILDES archive. We mapped the use of words of different form-classes in parental speech, comparing the words’ occurrence as single-word utterances and as the heads of two-word long syntactically structured sentences. The distributions showed a dramatic effect of form-class: the four FW categories subordinators, determiners, prepositions and auxiliary verbs are used by parents almost exclusively in multiword utterances. By contrast, words in the four CW categories verbs, nouns, adjectives and adverbs appear both as single-word utterances and as roots of twoword sentences. Analysis of children’s talk had similar results, the proportions correlating very highly with parents’. Acquisition of FWs predicts syntactic development because they must be learned as combining words, whereas CWs can be learned as stand-alone lexemes, without mastering their syntax. 1. The research question 1.1 FWs predict syntactic development better than CWs Grammatical words such as determiners, auxiliary verbs and prepositions had long been considered marginal for the early stages of syntactic development. Many authorities such as Radford (1990) believed that such ‘function words’ (FWs) are acquired late by typically-developing children, and that, at the early stages of acquisition, syntactic development relies on ‘content words’ (CWs) or ‘lexical words’ (nouns, verbs, adjectives and adverbs), that carry semantic relations which can be expressed as patterned speech (Brown, 1973). In the last few years the trend has turned, as recently developmental studies have been offering some new evidence for the importance of the early mastery of FWs for syntactic development. In several studies it was found that in children acquiring various languages, the early mastery of FWs such as subordinators, auxiliary verbs, prepositions and determiners strongly predicts children’s concurrent and especially subsequent syntactic development. Kedar et al. (2006) found that 18and 24-month-old infants acquiring English oriented faster and more accurately to a visual target following sentences in which the referential expression included determiners. They concluded that by 18 months of age, infants use their knowledge of determiners when they process sentences and establish reference. Le Normand et al. (2013) examined the speech of French-speaking children aged 2-4 years and correlated the diversity of word types in various form-classes with the children’s mean length of utterance in words (MLU). They found that the diversity of word types in FWs was the best predictor of MLU, much surpassing CW categories. Szagun and Schramm (2016) studied young deaf children acquiring German who received cochlear implants at an early age. They found that the early type and token frequencies of determiners predict MLU two years later more strongly than the early frequency of lexical words. In addition, Ninio (2019) established that FWs are indeed participants in the sentence’s syntactic structuring. Taking the determiner-noun relation as an exemplar of the FW-CW relationship, Ninio demonstrated that the syntactic relation of FWs to the CWs they are associated with is probably identical to the syntactic relation of Head-Dependent complementation (or Merge) that constitutes the major building process of syntax. The determiner-noun combination appears to be a type of complementation, the same syntactic operation that underlies, for example, the combination of a verb with its direct object. Possibly, some generalized knowledge of how words relate to one another is learned when producing early multiword utterances headed by FWs, and, through transfer and facilitation, it drives the construction of grammar. It seems that children learn syntactic principles through specific constructions; in the case of determiner-noun combinations, they apparently learn the principle of headcomplement relation, which then can be transferred to other syntactic constructions employing the same basic combinatory operation. Although the correlational results are a good foundation for theorizing, it is actually still unclear what is the explanation for the higher predictive power of learning FWs for syntactic development. At the early stages of syntax, children learn both a vocabulary of FWs and of CWs; and children do learn to produce verb-object combinations early, not only determiner-noun ones. Nevertheless, it is the acquisition of determiners as vocabulary items, and not of verbs, that best predicts a child’s syntactic development. In the present study, our goal is to provide an explanation for FWs’ advantage over CWs in providing a learning environment for principles of syntax while being learned as vocabulary items. Our starting point is the most significant difference between CWs and FWs which is that CWs have rich semantic content whereas FWs are grammatical words that have little or none. This implies that CWs require a learning format where words are matched to their meaning in some non-linguistic context (Macnamara, 1972), but may not necessarily need a syntactic context to acquire their meaning (Ninio, 2016). By contrast, FWs cannot be learned from the non-linguistic context; they are words whose whole purpose is to participate in some combinatory process with another word. It follows that learning them requires multiword input that will make possible to observe their syntactic behaviour. Thus, our hypothesis is that the acquisition of FWs is better connected to syntactic development than the learning of CWs because their learning necessarily involves multiword input, hence it necessitates the mastery of syntactic principles. CWs, as words possessing semantic content, can be learned from singleword utterances, as their meaning is learned from the nonlinguistic context. We therefore predict that CWs are more likely to be learned from single-word input than FWs; the latter are more likely to be learned from multiword input sentences. To make the comparison between single-word and multiword as equitable as possible, we took as multiword input sentences the shortest possible multi-word combinations, which are two-words long sentences possessing syntactic connectivity. The hypotheses were tested by two different comparisons: first, we compared the parental input of content words (CWs) and of function words (FWs) as either single-word utterances or as heads of twoword syntactic combinations. Second, we looked at words occurring in children’s single-word sentences and compared them with the heads of their two-word syntactic combinations. The hypothesis was that FWs are presented, and learned, as strongly syntactic elements, while CWs, less so. We expect a significant difference between FWs and CWs in proportion of tokens in single-word and multiword uses, both in the parental input and in children’s own spontaneous productions. 2. Method 2.1 Participants For English-language parental and child samples we systematically sampled the English transcripts in the CHILDES (Child Language Data Exchange System) archive (MacWhinney, 2000). The CHILDES is a public domain database for corpora on first and second language acquisition. The publicly available, shared archive contains documentation of the speech of more than 500 English-speaking parents addressed to their young children. The CHILDES archive stores the transcribed observations collected in various different research projects. In building our corpora, we followed closely the principles established in linguistics for constructing systematically assembled large corpora (Francis and Kučera, 1979). We selected projects among the ones available using the criteria that the observations were of normally developing young children with no diagnosed hearing or speech problems, and of their parents, native speakers of English, their speech produced in the context of naturalistic, dyadic parent-child interaction. We restricted the child s age during the observed period to three years and six months. This process resulted in the selection of parents and children from 33 research projects in the CHILDES archive: the British projects Belfast, Howe, Korman, Manchester, and Wells, and the American projects Bates, BernsteinRatner, Bliss, Bloom 1970 and 1973, Brent, Brown, Clark, Cornell, Demetras, Feldman, Gleason, Harvard Home-School, Higginson, Kuczaj, MacWhinney, McMillan, Morisset, New England, Peters-Wilson, Post, Rollins, Sachs, Suppes, Tardif, Valian, Van Houten, and Warren-Leubecker (MacWhinney, 2000). From these projects, we selected 471 observational studies involving a target child of the desired young age range, namely, below three years and six months.