This article presents evidence in the development of Russian for a theory that says that children at a terrain age know a syntactic principle (Principle B of the Binding Theory) that governs the distribution of pronouns but that they do not know a pragmatic or semantic principle (Principle P) that restricts the situations in which NPs may be contraindexed. The major result concerns Russian possessive pronouns that must be disjoint from subject antecedents. We show that this result follows from the raising of pronouns at Logical Form together with Principles B and P. The prediction thus is that children who do not know Principle P will accept this sentence. We experimentally confirm this result. We also predict that children will reject sentences with possessive pronouns that are bound by quantifiers. We also confirm this prediction. Along the way we replicate in Russian known results in English and other languages. The pattern of results provides strong support for the Principle P/modularity theory.