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Description: Vocabulary size is one of the most important early metrics of language development, often used as a screener to identify children who may be at risk of language impairment or delay. While there are well-established norms and guidelines for monolingual infants and children, assessing vocabulary in bilinguals is complex because bilinguals learn words in two languages, which include translation equivalents (cross-language synonyms). We investigated how translation equivalents should be counted to yield a comparable metric across toddlers of monolingual and bilingual backgrounds. We collected expressive vocabulary data from English and French monolinguals (n = 220), and English–French bilinguals (n = 220) aged 18–33 months, via parent report using the American English and Québec-French adaptations of the MacArthur-Bates Communicative Development Inventories (CDIs; Fenson, Marchman, Thal, Dale, & Bates, 2007; Trudeau, Frank, & Poulin-Dubois, 1999). Our results showed that traditional approaches overestimate (word vocabulary; credit given for both words in a translation equivalent pair) or underestimate (concept vocabulary; credit given for only one word in a translation equivalent pair) bilinguals’ vocabulary knowledge. Instead, we propose a new metric: the bilingual adjusted vocabulary. Uniquely, this approach counts translation equivalents differently at different ages, giving the youngest toddlers (18–23 months) credit for both words in a pair, middle toddlers (24–31 months) partial credit for the second word, and oldest toddlers (32–33 months) credit for only one word in a pair. This developmentally-informed bilingual vocabulary measure reveals differences in word learning abilities across ages, and provides improved recommendations for clinicians to appropriately assess vocabulary in bilingual toddlers.


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