A Cross-Linguistic Study of the Relationship between Grammar & Lexical Development

The relationship between grammatical and lexical development was compared in 233 English and 233 Italian children between 18 and 30 months of age, matched for age, gender, and vocabulary size on the MacArthur Communicative Development Inventories (CDI). Four different measures of Mean Length of Utterance were applied to the three longest utterances reported by parents, and to corrected/expanded versions representing the ‘target’ for each utterance. Italians had longer MLUs on most measures, but the ratio of actual to target MLUs did not differ between languages. Age and vocabulary both contributed significant variance to MLU, but the contribution of vocabulary was much larger, suggesting that vocabulary size may provide a better basis for cross-linguistic comparisons of grammatical development. The relationship between MLU and vocabulary size was non-linear in English but linear in Italian, suggesting that grammar ‘gets off the ground’ earlier in a richly inflected language. A possible mechanism to account for this difference is discussed. Cross-linguistic studies have played a central role in child language research for decades (Braine, 1976; Slobin (1985, 1992, 1997); MacWhinney & Bates, 1989; Choi & Bowerman, 1991; Berman & Slobin, 1994; Bates, Devescovi, & Wulfeck, 2001; Bowerman & Choi, 2001). As Slobin has pointed out repeatedly in his own pioneering work on the topic, this is the research strategy that stands the best chance of helping us to disentangle universal versus language-specific phenomena in language development. Through such studies, we have learned that children commit surprisingly few errors in the course of language learning (although the errors they do produce are quite informative — Slobin, 1985), that the content or concepts that 1–2-year-olds try to express are remarkably similar from one language to another (negation, possession, location, disappearance, etc. — Braine, 1976), but that variations in the forms used by very young children to express these concepts are strikingly different from one language to another (Demuth, 1990; Fortescue & Lennert Olsen, 1992). All these phenomena suggest that children are conservative, ‘sticking to their input’ as they figure out how to express a common stock of ideas (and some language-specific ideas as well — Choi & Bowerman, 1991; Bowerman & Choi, 2001). Despite these admirable advances in our understanding, serious methodological problems remain that are acknowledged by virtually all researchers who engage in cross-linguistic research. One of the most vexing problems is the establishment of equivalence between samples of children: When children from different language groups are compared to unveil similarities and differences, how shall they be matched? The simplest strategy is to match children by age, e.g. comparing the speech produced by 24-month-old Italians with the speech produced by 24-month-old children acquiring English. However, the variation that can be observed within any given language in this age range is so vast (e.g. 24-month-olds can have virtually no speech at all, or they can display complex syntax with vocabularies of more than 600 words — Dromi, 1987; Ogura, Yamashita, Murase & Dale, 1993; Fenson et al., 1994; Caselli & Casadio, 1995; Caselli, Casadio & Bates, 1999; Maitel, Dromi, Sagi & Bornstein, 2000) that this strategy is necessarily risky. This is especially true when the samples in question are small (as is often the case in detailed longitudinal studies of free speech). In many years of comparative research on early language, attempts have been made to match children based on length of utterance in content words or total words. However (as we shall also see below), this strategy does not guarantee a match in language level, since languages can vary in their ‘wordiness’ (e.g. the difference between languages that do and do not permit omission of subjects and sometimes also objects in freestanding declarative sentences; variations over languages in the obligatory status of articles and other functors). Attempts have also been made to match based on mean length of utterance in morphemes, but this strategy raises a host of definitional issues around which no consensus has emerged, as evidenced by a lively exchange on the Info-Childes mailing list (infochildes @mail.talkbank.org, as archived at ht tp : / / linguistlist.org). Indeed, the most successful and thorough efforts have typically been tailored to individual languages, with no attempt to generalize across languages (e.g. Dromi & Berman, 1982). In the present study, we will illustrate an alternative approach to the issue of cross-language matching. Recent studies have shown that, within a single language, vocabulary size is a more powerful predictor of grammatical development than age or gender, contributing significant variance to measures of grammar after age and gender are controlled (Marchman & Bates, 1994; Bates & Goodman, 1997; Dale, Dionne, Eley, & Plomin, 2000). Studies in Italian, Japanese, Spanish and Hebrew have illustrated the same point (Caselli et al., 1999; Ogura et al., 1993; Jackson-Maldonado, Thal, Marchman, Bates & Gutierrez-Clellen, 1993; Maitel et al., 2000). Taking advantage of this finding, we used the large norming data bases for the MacArthur CDI in two languages, English and Italian, to explore grammatical development and the relationship between vocabulary and grammar (controlling for