Trajectories of syntactic complexity and subordination across Spanish and English in bilingual children with and without developmental language disorder

Purpose This longitudinal study examined the development of complex syntax in Spanish–English bilingual children with and without developmental language disorder (DLD) from prekindergarten (PK) to 3rd grade. We evaluated syntactic complexity and grammaticality using Subordination Index (SI) and ungrammaticality (UG), overall subordinate clause use using the percentage of complex utterances, and subordinate clause-type development (nominal, adverbial, and relative clauses) in English and Spanish. Method Fifty-four bilingual children (39 typically developing TDL, 15 DLD) completed narrative retells in English and Spanish in PK, 1st, and 3rd grade. Language samples were analyzed using SALT-derived SI and UG measures, and utterances were coded for subordinate clause presence and type. Linear mixed models examined the effects of Grade and Group separately by language and outcome. Results Both groups showed developmental increases in syntactic complexity and subordination across grades, but patterns differed by language. In English, TDL and DLD children showed broadly similar growth, with no significant group differences in SI, overall subordinate clause use, or clause-type production. In Spanish, the heritage language, group differences were more pronounced. TDL children showed significant gains in overall subordination and nominal clause use, whereas children with DLD showed limited growth after 1st grade. Clause-type analyses revealed reduced growth and lower diversity of subordinate clauses in Spanish among children with DLD, particularly for nominal clauses. Conclusions Complex syntax developed in both groups across languages; however, Spanish showed greater vulnerability than English, particularly for children with DLD. Clause-type analyses revealed group differences not fully captured by global syntactic measures, highlighting the importance of examining complex syntax in the heritage language. This study provides longitudinal evidence of distinct syntactic trajectories across the two languages in bilingual children with and without DLD.