The role of metalinguistic knowledge in third-language development among heritage and late bilinguals

Aims and objectives: This study examined whether bilinguals' metalinguistic knowledge (MK) across both of their languages as well as heritage/late bilingual experience contributed to the initial development of third-language (L3) morphosyntax using Japlish, a semi-artificial language.Methodology: 48 heritage and 63 late English-Spanish bilinguals were exposed aurally to Japlish sentences containing word order patterns and case markers. Participants completed an acceptability judgment task (AJT) and a picture-matching task (PMT) immediately after exposure and again 2 weeks later as well as English and Spanish MK tests.Data and analysis: Logistic mixed effects analyses were run on AJT and PMT accuracy scores, grammatical/ungrammatical items, linguistic structure, and time of testing. MK scores and heritage/late bilingual status were included as predictors in the statistical model.Findings: MK significantly contributed to overall AJT scores, including grammatical and ungrammatical items and time of testing. MK also contributed to ungrammatical word order and case markers. For the PMT, MK significantly contributed to scores, especially during delayed testing. A weak correlation emerged between participants' MK across both of their languages and their rule-based knowledge of Japlish linguistic structures. Heritage and late bilingual experiences did not significantly contribute to these results.Originality: This study included two different bilingual groups to investigate how variation in bilingual experience shapes additional language learning. It also advances our knowledge on the role of MK in the learning of linguistic structures not involved in transfer. Furthermore, a more nuanced examination of MK is considered as it relates to type of assessment, linguistic structure, and time of testing.Significance: The findings highlight the critical role of MK for both bilingual populations and its facilitative role in the development of initial L3 morphosyntax. The results are interpreted under the lens of structural sensitivity theory.