Intentional bilingualism does not hinder morphosyntactic development in L1: A study with children speaking Polish

Background: Intentional bilingualism (IB) refers to caregivers’ deliberate, planned introduction of an additional language at home—beyond what community exposure alone would provide—and is characterized by caregivers’ use of their non-native language with the child. While scholarly debate continues about whether IB affects first-language (L1) outcomes, evidence on morphosyntax specifically remains limited. Therefore, we examined L1 morphosyntax using the short version of the Polish sentence repetition (SRep) task. Methods: Seventy-one children aged 5 years 0 months–6 years 11 months (IB = 23; community-induced bilingualism, CommBil = 24; monolinguals, Mono = 24), matched on age and gender, completed the 20-item Polish SRep task. We also collected the Cross-Linguistic Lexical Task (CLT) and nonverbal reasoning (RCPM) data. Data were analyzed with generalized linear mixed-effects models including group and sentence structure as fixed effects. Results: IB children performed similarly to monolinguals on the SRep task. By contrast, the CommBil group scored lower than both IB children and monolinguals. Conclusions: Within the scope of this study, IB was not associated with disadvantages in L1 morphosyntax. The lower SRep performance observed in the CommBil group may reflect differences in input quality/quantity rather than an inherent “bilingualism cost.”