The majority of the world's children grow up learning two or more
languages. The study of early bilingualism is central to current
psycholinguistics, offering insights into issues such as transfer and
interference in development. From an applied perspective, it poses a
universal challenge to language assessment practices throughout
childhood, as typically developing bilingual children usually
underperform relative to monolingual norms when assessed in one language
only. We measured vocabulary with Communicative Development Inventories
for 372 24-month-old toddlers learning British English and one
Additional Language out of a diverse set of 13 (Bengali, Cantonese,
Dutch, French, German, Greek, Hindi-Urdu, Italian, Mandarin, Polish,
Portuguese, Spanish and Welsh). We furthered theoretical understanding
of bilingual development by showing, for the first time, that linguistic
distance between the child's two languages predicts vocabulary outcome,
with phonological overlap related to expressive vocabulary, and word
order typology and morphological complexity related to receptive
vocabulary, in the Additional Language. Our study also has crucial
clinical implications: we have developed the first bilingual norms for
expressive and receptive vocabulary for 24-month-olds learning British
English and an Additional Language. These norms were derived from
factors identified as uniquely predicting CDI vocabulary measures: the
relative amount of English versus the Additional Language in
child-directed input and parental overheard speech, and infant gender.
The resulting UKBTAT tool was able to accurately predict the English
vocabulary of an additional group of 58 bilinguals learning an
Additional Language outside our target range. This offers a pragmatic
method for the assessment of children in the majority language when no
tool exists in the Additional Language. Our findings also suggest that
the effect of linguistic distance might extend beyond bilinguals'
acquisition of early vocabulary to encompass broader cognitive
processes, and could constitute a key factor in the study of the debated
bilingual advantage.
