Why are nouns often over-represented in children's early words? Prior research has suggested a range of possible explanations for the observed bias in the syntactic category composition of early vocabularies, including cognitive, linguistic, and contextual factors. However, these factors are not mutually exclusive and can be difficult to disambiguate empirically. Across three analyses of the vocabularies of children acquiring two languages (N = 1997), we investigated the role of different language combinations, different levels of language exposure, and different environmental contexts, finding some support for cross-linguistic modulation of syntactic category bias, as well as variation across countries. These results suggest that both linguistic and contextual factors contribute to syntactic category biases in young children's vocabularies, and highlight the importance of understanding the broader background of the child when studying language acquisition.