We use historical change to explore whether children filter their input for language learning. Although others (e.g., Rohde & Plaut, 1999) have proposed filtering based on string length, we explore two types of filters that assume richer linguistic structure. One presupposes that linguistic utterances are structurally highly ambiguous and focuses learning on unambiguous data (Dresher, 1999;Fodor, 1998b;Lightfoot, 1999). The second claims that children learn only from matrix clauses (Lightfoot, 1991), defining simplicity in a structural manner. We assume that certain language changes occur via mismatches during acquisition. This allows us to use patterns of change to demonstrate that filtering restrictions are necessary to model language learning. Viewing language change as a result of mismatches during learning thus constrains the learning algorithm itself.Scientists have attributed asymmetries between typical adult and child knowledge of language to filtering of the input data. For instance, Morgan (1986) claimed that children, perhaps because of general cognitive restrictions on the complexity of data that they can handle, might restrict attention to simple subparts of utterances. Elman (1993) and others have suggested that this constraint might be motivated by architectural considerations of the underlying language analyzer, modeled as a simple recurrent network-but see Rohde and Plaut (1999) for simulations that cast this assertion into doubt.A ubiquitous property of natural language is its rampant structural ambiguity. Particular strings can be analyzed in multiple ways given all of the grammatical LANGUAGE LEARNING AND DEVELOPMENT, 3(1), 43-72 Copyright © 2007, Lawrence Erlbaum Associates, Inc. Correspondence should be addressed to Lisa Pearl, Linguistics Department, 1401 Marie Mount Hall, University of Maryland, College Park, MD 20742. E-mail: llsp@umd.edu, weinberg@ umiacs.umd.edu options available cross-linguistically. The wrong analysis of such a string can lead the child to include a rule in the underlying grammar that is wrong for the language as a whole. Subsequent revision of the underlying grammar to exclude this rule can then be costly, so this is a serious concern.Dresher (1999), Lightfoot (1991), and Fodor (1998b have proposed learning strategies that bias children away from these potentially misleading cases, using a structurally based definition of simplicity to filter the data. Unfortunately, it is nearly impossible to test any filtering proposal in a natural setting. For logistical and ethical reasons, one cannot simply expose a child to an unnaturally restricted dataset during the critical period of language learning to observe the effect of that restriction on the normal course of language acquisition.Modeling language change, however, can offer a graceful solution to this predicament if we assume that certain types of change result from a misalignment of the child's hypothesis and an adult's analysis of the same data (Lightfoot, 1991(Lightfoot, , 1999. Language change models incorpor...