'It's a big world': understanding the factors guiding early vocabulary development in bilinguals

How many words is a bilingual 2-year-old supposed to know or say in each of her languages? Speech and language therapists or researchers lack the tools to answer this question, because several factors have an impact on bilingual language skills: gender (Kern, 2007), amount of exposure (De Houwer, 2007; Hoff et al, 2012), mode of acquisition (Place & Hoff, 2011), socio-economic status (SES, Gathercole et al., 2010) and the distance between L1 and L2 (Havy et al., 2015). Unfortunately, these factors are usually studied separately, making it difficult to evaluate their weight on a unique measure of vocabulary.

The present study measures the contribution of the following factors to the vocabulary scores of bilingual toddlers: i) gender; ii) sibling ranking; iii) relative amount of exposure to each language; iv) mode of exposure; v) SES; vi) linguistic distance; vii) language spoken between parents.

Close to the child’s second birthday, parents of 278 UK-based bilinguals completed successively: a 100-word version of the Oxford-CDI (Hamilton et al., 2000), the CDI in the child’s Additional Language, a family questionnaire (taken from the UK-CDI study, Alcock et al., in prep.), and the Language Exposure Questionnaire (Cattani et al., 2014).

Thirty-six British-English-AL pairs were considered, with languages contrasted on a second-language-learning scale (Chiswick and Miller, 2005): for example, Dutch and French are close to British-English, while Polish or Cantonese are more distant. Data from the corpus were included in two mixed-effect models, one with the English scores in comprehension as the dependent variable, and the other with production scores. The seven factors listed above were included as predictors.

The amount of English exposure was the strongest predictor of comprehension scores (?2(13) = 9.35, p < .005, ? = 0.02, t = 3.08, p <.005), followed by the language that parents speak between themselves (?2(13) = 14.94, p < .001, ? = 1.37, t = 3.76, p <.0005), linguistic distance (?2(13) = 6.92, p < .01, ? = -0.74, t = -2.66, p <.01) and age (?2(13) = 4.86, p < .05, ? = 0.55, t = 2.17, p <.05). In production, gender (?2(13) = 13.57, p < .0005, ? = -0.91, t = -03.72, p <.0005), amount of exposure to English (?2(13) = 13.57, p < .0005, ? = -0.91, t = -03.72, p <.0005), the language that parents speak between themselves (?2(13) = 11.85, p < .005, ? = 1.09, t = 3.41, p <.001), and the mother’s occupation (?2(13) = 4.51, p < .05, ? = 0.63, t = 2.13, p <.05) were the significant predictors.

The more English parents use to address one another, the more English words the child says and understands. This surprising result could be simply explained by the fact that parents who speak English together are also more likely to speak English to their child.

The main results of this study is that linguistic distance is a powerful predictor of toddlers’ vocabulary in English, with children learning two close languages growing their vocabulary faster than those learning distant languages.