Natural variation and diversity present in the language input of bilingual children may impact their vocabulary acquisition. In bilingual children, the quantity of input they receive is naturally divided between two languages, e.g., mother's vs. father's language, or L1 (i.e., home, heritage, or minority language) vs. L2 (i.e., community, or majority language). Thus, the nature of bilingual upbringing results in less input for each of the languages in comparison to the input received by monolingual peers (, but cf.). Reduced language input may be one of the reasons why bilingual children are repeatedly shown to score lower than monolinguals on vocabulary tasks in the majority language (e.g.,). Importantly, those vocabulary setbacks are found in different language pairs (e.g.,), across pre-school and school years, and - in the case of the majority language - are largely related to home-context vocabulary, rather than the school-context. A direct link between bilingual vocabulary development and language exposure was investigated by in a group of 5-year-old simultaneous French-English bilinguals in Canada3. Bilinguals' performance on receptive and expressive vocabulary was compared to that of their monolingual peers matched on age, socioeconomic status, and non-verbal intelligence, but differing in the amount of exposure they received in each language. A robust relationship was found between the amount of exposure to a language and children's performance in that language, although the relationship was observed to be different for the receptive and expressive vocabulary. Bilinguals exposed to both languages to the same extent scored comparably to monolingual children in the receptive vocabulary test, but they needed more input in a given language (and relatively less in the other one) to keep up with their monolingual peers in expressive vocabulary.
Access to many speakers of a given language seems to be another important factor contributing to language abilities in bilinguals. In a recent study by, the number of heritage language speakers that participants spoke to, correlated positively with their scores on a picture naming task (measured as the number of correct responses) in that language, and did not correlate negatively with their correctness in picture naming in English (community language). Importantly, the effect was independent of how frequently the participant used each language. Presumably, the greater the number of unique native speakers that a child interacts with on daily basis, the greater the variety of words used with a child, which may contribute to the child's vocabulary.