We find a general, language-independent pattern in child language acquisition in which there is a clear difference between subject and object noun phrases. On one hand, indefinite objects tend to be interpreted nonreferentially, independently of word order and across experiments and languages. On the other hand, indefinite subjects tend to be interpreted referentially in most contexts, even in contexts where adults would favor a nonreferential reading. In this article we offer an explanation for this pattern within the framework of bidirectional Optimality Theory. This explanation focuses on clarifying in what sense children's interpretations deviate from the adult interpretations and clarifying the nature of the linguistic knowledge that the 4-year-old child will need to acquire to become a competent, adultlike speaker and hearer of her language.