Research on the language of heritage speakers has shown that in situations of societal bilingualism the functionally restricted language evidences the simplification of some grammatical domains. A frequent question is whether this stage of grammatical simplification is due to incomplete or interrupted acquisition in the early years of a bilingual’s life, or a result of processes of attrition of acquired knowledge of the underused language. This article considers the issue of incompleteness through an examination of the relationship between bilingual children’s developing grammars and the more or less changed bilingual systems of adult second and third generation immigrants ("heritage speakers") in the USA.

The issue of incompleteness is examined in two corpora: (1) Recordings of 50 Spanish-English adult Mexican-American bilinguals; and (2) Longitudinal data obtained during the first six years of life of two Spanish-English bilingual siblings.

Qualitative and quantitative analyses of the grammar of subjects, verbal clitics, and verb tenses of the Spanish of the bilinguals under study.

The outcome of reduced exposure and production of a minority language in simultaneous bilingual acquisition reflects the incomplete acquisition by age 6;0 of some aspects of the input language. The bilingual siblings’ unequal control of the minority language is shown to parallel the range of proficiencies identified across the adult heritage speakers.

Some linguists argue that heritage speakers’ grammars are less restrictive or "different" in some respects but not incomplete. In contrast, this article demonstrates that at least some of the reduced grammars of heritage speakers result from a halted process of acquisition in the early years of life. Furthermore, while difference is not an explanatory construct, incomplete acquisition due to interrupted development caused by restricted exposure and production offers an explanation for the range of proficiencies attested among adult heritage speakers.