Vocabulary limitations undermine bilingual children’s reading comprehension despite bilingual cognitive strengths

Previous research reported bilingual cognitive strengths in working memory, executive function and novel-word learning skills (Bialystok in Psychol Bull 143:233–262, 2017; Kaushanskaya and Marian in Psychon Bull Rev 16:705–710, 2009). These skills should also support bilingual children’s vocabulary and reading development, yet bilingual children show weaknesses in their second language vocabulary and reading comprehension skills. Our primary aim was to clarify these seemingly paradoxical reports by investigating the cognitive strengths and weaknesses associated with both bilingual experience and reading comprehension in a single study. The participants were 102 English-speaking monolingual children and 104 Hindi/Urdu-English speaking bilingual children (mean age = 118.26 months, SD = 11.23 months) in the UK. We tested children’s vocabulary, working memory, executive function (cognitive inhibition, updating memory), novel-word learning, and reading skills. All testing was conducted in English. The findings supported the previous reports of bilingual cognitive strengths in working memory, novel-word learning and cognitive inhibition skills. However, despite their cognitive strengths and adequate word reading skills, the bilingual group displayed weaker reading comprehension than their monolingual peers. As anticipated, there was a direct association between bilingual children’s smaller English vocabulary size and underperformance on reading comprehension. Along with word reading, vocabulary was the most powerful unique predictor of reading comprehension. The effects of cognitive control skills on reading comprehension were mixed and mostly indirect through word reading skills. These relations were comparable across the monolingual and bilingual groups. Together, our findings highlighted the importance of clear educational policies on oral language assessment and support in our increasingly multilingual classrooms.