Effect of bilingualism on receptive and expressive vocabulary acquisition: Evidence from the MacArthur-Bates CDI

Purpose: There is a little evidence about the effect of second language exposure on infant’s language development. Therefore, the purpose of the study was to investigate the relationship between bilingualism and amount of second language exposure with the size of receptive and expressive vocabularies. Methodology: The MacArthur-Bates Communicative Development Inventory was used to assess infants’ language development in a sample of 37 bilinguals and 116 monolinguals between 8 to 16 months. The parents were asked to fill the forms, considering total words that their infants receive and express. Data and analysis: These findings showed that the bilingual group differed significantly from the monolingual group in the field of receptive lexicon (β = 7.7; 95% confidence interval (CI): [4.1,11.4]; p < .001). However, similar to the results of the effect of second language exposure on expressive lexicon, the comparison of bilingualism and expressive vocabulary size was not significant (β = –.01; 95% CI: [–3.5, 3.9]; p > .05). Conclusion: Overall, our findings show that 8- to 16-month infants have similar expressive and larger receptive vocabularies in comparison with monolinguals. So, it is supposed that in assessing bilingual infant’s language abilities, especially in receptive vocabulary, we cannot use the monolingual norms. Originality: To our knowledge, this study is uniquely investigating the effect of bilingualism on vocabulary acquisition in bilingual Persian 8- to 16-month infants. Implications: In a practical view, our findings emphasize on using bilingual specific language tools for assessing bilingual infants and suggest considering this difference in assessment and intervention plans. Limitations: As researchers suggested, parent reports, regardless of their practical usage, may not be enough for showing infants abilities and the underestimation may have occurred, so it is suggested to expand these results by using other tools and direct assessments.