Complement clause comprehension across languages: The case of bilingual children with and without Autism Spectrum Disorder

Complement clause comprehension plays a key role in language and cognitive development, particularly in supporting Theory of Mind (ToM). This study examines how bilingual exposure and Autism Spectrum Disorder (ASD) affect children's understanding of finite complement clauses introduced by speech verbs (e.g., 'say'). Children with ASD frequently experience difficulties in complement clauses, especially when these structures encode false beliefs or require complex mental-state reasoning. In contrast, bilingualism has been associated with advantages in executive functions (e.g., working memory, cognitive flexibility) and enhanced ToM abilities, both of which relate to complement clause comprehension. These findings raise the possibility that bilingual experience may mitigate some of the linguistic and cognitive difficulties typically observed in ASD. To investigate these possibilities, we tested 126 children (79 with ASD, 47 neurotypical) using a cross-linguistic picture selection task designed to minimize ToM demands, in either English, French, or German. We explored the role of cognitive (working memory, nonverbal IQ, inhibitory control) and linguistic (syntax, vocabulary) predictors, and continuous measures of bilingual language experience. Results showed that children with ASD performed significantly lower than their neurotypical peers, with more pronounced age-related gains in the neurotypical group. While bilingualism per se did not predict performance, richer and more proficient use of the language of testing positively influenced complement clause comprehension, particularly in children with ASD. Visuospatial working memory, nonverbal IQ, and receptive vocabulary emerged as significant positive predictors of complement clause comprehension, whereas broader syntactic ability and inhibitory control did not. Complement clause comprehension predicted ToM outcomes in children with ASD, but not in neurotypical children, suggesting that complement understanding may serve as a compensatory mechanism for ToM reasoning in autism. These findings highlight that bilingualism is not inherently associated with either advantages or disadvantages for syntactic development and further underscore the intricate links between complements and cognition in ASD.