How does abstract structure emerge during language learning?
On some accounts, children’s early syntax emerges from direct
generalizations from particular lexical items, while on others,
syntactic structure is acquired independently and follows its
own timetable. Progress on differentiating these views requires
detailed developmental data. Using parental reports of vocabulary
and grammar abilities, previous analyses have shown that
early syntactic abstraction strongly depends on the growth of
the lexicon, providing support for lexicalist and emergentist
theories. Leveraging a large cross-linguistic dataset, we replicate
and extend these findings, demonstrating similar patterns
in each of four languages. Moreover, the power of our dataset
reveals that there are measurable effects of age over and above
those attributable to vocabulary size, and that these effects are
greater for aspects of language ability more closely tied to syntax
than morphology. These findings suggest non-lexical contributions
to the growth of syntactic abstraction that all theories
must address.