Sources of variability in children’s language growth
Introduction
The present paper concerns the relation of caregiver speech to child language development. A major issue for acquisition theories concerns the sources of language growth, especially for syntax. While all investigators recognize that exposure to incoming speech is essential for acquiring a language, different theories make alternative claims about the influence of input on child language. Constructivists argue that variations in input are critical to language outcomes in children (e.g., Aslin et al., 1999, Gomez and Gerken, 1999, Saffran, 2001, Saffran and Wilson, 2003, Tomasello, 2000), whereas nativists argue that individual variations in input are less important: in their view, innate grammatical principles are the major determiners of children’s language outcomes (e.g., Lidz, 2007, Lidz and Gleitman, 2004, Lidz and Waxman, 2004).
It has been difficult to establish the role of input because studies of the relation between caregiver and child speech typically are correlational, and results can be explained in a variety of ways. For example, while correlations might be driven by variations in input, they might instead reflect variations in the language ability of different children, which, in turn, affect how caregivers speak to them. Further, correlations between caregivers and children based on observations at a single time point might reflect temporary factors, such as particular topics of conversation during a single session.
Although we use correlational data in the present study to examine the role of input in language growth, we take steps to address some of the difficulties arising in earlier studies. We use lagged correlations across different observations (time points) to explore enduring relations between caregiver and child speech. Since our central question concerns the possible influence of caregiver speech on later child language, we are especially interested in lagged correlations where caregiver speech precedes child speech. If correlations between caregiver speech at an earlier time and child speech at a later time are much greater than those between child speech at an earlier time and later caregiver speech, it would suggest that caregiver input is a source of language growth, thus supporting a constructivist account of acquisition.
Much of the current empirical support for a constructivist view involves studies using quantitative measures of input, i.e., of the amount of speech individual caregivers produce (e.g., Hart and Risley, 1995, Huttenlocher et al., 1991). However, these measures are limited because they do not differentiate repeated use of the same elements from use of different elements. The present study incorporates a more theoretically relevant measure of input, namely the diversity of the speech of individual caregivers, i.e., the variety of words, phrases, and clauses they produce.
If acquisition involves accumulation of instances of particular words and syntactic structures, and use of this information in the construction of language, more diverse input should lead to greater language growth. This is because, for a passage of speech of fixed length, greater diversity will provide a more complete sample of possible forms of expression. For example, in this account, a child receiving input from a caregiver who uses few structurally complex sentences might be expected to construct a simpler grammar than a child receiving input from a caregiver who uses a larger variety of structurally complex sentences. In a study of the diversity of caregiver speech, Huttenlocher, Vasilyeva, Waterfall, Vevea, and Hedges (2007) found wide variations in diversity across different caregivers.
In addition to using diversity measures of caregiver speech in evaluating the role of input in language growth, we also use diversity measures to assess children’s language outcomes. The variety of words and syntactic forms children produce when progressing from single words to structurally complex sentences provides an index of their growing mastery of their language. Diversity of caregiver and child speech has been examined previously in the literature (cf., Naigles & Hoff-Ginsberg, 1998), and is especially familiar in the measure of word types (e.g., Brown, 1973). However, previous studies have been limited to diversity of words (number of word types), and of specific syntactic forms (e.g., verb phrases). In the present study, we have devised an overall analytic scheme which we use to examine diversity at lexical, phrasal and clausal levels in both caregivers and children.
In the last 30 years, longitudinal studies have shown that there are individual differences in various aspects of caregiver speech to children, and corresponding differences in the speech of children. Studies have examined caregiver speech at one point in time in relation to child speech at the same or a later time (e.g., Hoff-Ginsberg, 1985, Hoff-Ginsberg, 1986, Hoff-Ginsberg, 1998, Newport et al., 1977, Pan et al., 2004). For vocabulary, individual differences have been found in the numbers of word tokens and word types by caregivers, and these are related to differences in children’s vocabulary size (e.g., Hart and Risley, 1995, Hoff, 2003a, Hoff, 2003b, Hoff-Ginsberg, 1998, Huttenlocher et al., 1991).
For syntax, individual differences have been found in the frequency of various structures in caregiver speech, and these differences are related to the development of those structures in children’s speech (e.g., Huttenlocher, Vasilyeva, Cymerman, & Levine, 2002). Greater frequency of auxiliary-fronted questions in the input is associated with more rapid growth of auxiliaries in child speech (e.g. Furrow et al., 1979, Newport et al., 1977). Variations in the frequency and variety of verb frames in caregiver speech predict child verb use (Naigles & Hoff-Ginsberg, 1998). Further, the average number of noun phrases per utterance in caregiver speech varies, and is a predictor of the number of noun phrases in children’s utterances (Hoff-Ginsberg, 1986). Such patterns of findings show that amount of exposure to particular syntactic forms is related to the acquisition of corresponding forms in the child, consistent with a constructivist model in which children acquire language from the input.
In summary, existing findings show that variation in the speech of different caregivers is related to the growth of those forms in children. In many studies, the directionality of the relation is ambiguous, because observed correlations could be due in part to common conversational topics in a single session, or possible child influences on the caregiver. The present study provides a comprehensive examination of the relation between caregiver and child speech, using a methodological approach aimed at minimizing the possibility of alternative interpretations.
Demographic factors are related to individual differences in the speech of both caregivers and children. Higher SES parents tend to talk more than lower SES parents (e.g., Hoff-Ginsberg, 1990). Further, SES differences are found both for vocabulary (e.g. Hart & Risley, 1992) and for syntax (e.g. Huttenlocher et al., 2002). Lower SES parents use fewer word types and tokens than higher SES parents, and these differences are predictive of child vocabulary (Hoff, 2003a, Hoff, 2003b, Hoff-Ginsberg, 1990). Also, lower SES caregivers use fewer multi-clause sentences and smaller numbers of noun phrases per sentence, and corresponding differences are found in the speech of their children (Huttenlocher et al., 2002). Huttenlocher et al. (2007) examined SES differences in caregiver speech in the same sample of families as in the present study. Substantial SES differences were found in the diversity of sentence types caregivers produced (Children’s speech was not examined in that study.)
In addition to SES, child birth order has been shown to be related to differences in caregiver and child speech. First-born children typically receive more speech than later-borns (e.g., Hoff-Ginsberg, 1998, Oshima-Takane and Robbins, 2003, Snow, 1972). Hoff-Ginsberg (1998) also found greater complexity of caregiver speech to first-borns than to later-borns, and showed that this difference was associated with more advanced language in first-borns.
While SES and child birth order are associated with individual differences in children’s syntactic skills, the mechanisms underlying this relation are not yet clear. Previous research on children’s vocabulary growth suggests that SES differences might be mediated by variations in caregiver speech (Hoff, 2003b, Rowe, 2008). Here, we address the question of whether the relation of demographic factors to syntactic growth also might be mediated by caregiver speech.
Section snippets
The present study
The present study concerns children’s language growth, especially syntactic growth, and explores how variations in caregivers’ input affect language outcomes in children. We treat children’s speech as an index of their language knowledge, and examine the relation to caregiver speech over an extended time period from 14 to 46 months. Diversity (variety) of speech is used as the language measure for both caregivers and children. To assess diversity of the lexicon, we count the number of word types
Discussion
The present study examined the longitudinal relation between language input and child language acquisition. The study incorporated several key methodological features which allowed us to obtain data supporting a constructivist interpretation of the association between caregivers’ speech and children’s language growth. These include (1) use of diversity measures of speech, (2) examination of directionality in lagged regression analyses, and (3) analysis of caregiver speech as a mediator of the
Conclusions
In conclusion, the present study provides compelling evidence that variations in language input, notably differences in the syntactic structures caregivers use, affect children’s language growth. Our study adds to the existing literature in several ways. First, we created an analytic scheme which characterizes the diversity of syntactic devices used by individuals at two syntactic levels, and specifies a set of structures at each level. Applying this scheme, we found large individual
Acknowledgments
The authors thank Susan Levine, Nora Newcombe, Mary C. Potter, Meredith Rowe, and Sandra Waxman for their helpful comments on the manuscript. This research was supported in part by a program project grant from the National Institute of Child Health and Development (Grant P01 HD040605).
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