Scholars of language development have long been challenged to understand the development of functional categories. Traditionally, it was assumed that children's language development initially relies on lexical elements, while functional elements become accessible only at later periods; and that it is lexical growth which bootstraps grammatical…

We tested 12- and 18-month-old English-learning infants on a preferential-looking task which contrasted grammatically correct sentences using the determiner "the" vs. three ungrammatical conditions in which "the" was substituted by another English function word, a nonsense word, or omitted. Our design involved strict controls…