We propose a new generalization governing the crosslinguistic distribution of radical pro drop (the type of pro drop found in Chinese). It occurs only in languages whose pronouns are agglutinating for case, number, or some other nominal feature. Other types of languages cannot omit pronouns freely, although they may have agreement-based pro drop. This generalization can for the most part be derived from three assumptions. (a) Spell-out rules for pronouns may target nonterminal categories. (b) Pro drop is zero spell-out (i.e., deletion) of regular pronouns. (c) Competition between spell-out rules is governed by the Elsewhere Principle. A full derivation relies on an acquisitional strategy motivated by the absence of negative evidence. We test our proposal using data from a sample of twenty languages and The World Atlas of Language Structures (Haspelmath et al. 2005).