Probabilistic Constraints in Language Acquisition Mark S. Seidenberg, Joseph Allen, & Morten H. Christiansen Program in Neural, Informational and Behavioral Sciences, University of Southern California marks, joeallen, morten@gizmo.usc.edu Abstract An approach to language acquisition is described in which the foundational questions are about how the child acquires adult-like language processing capaci- ties. This approach assumes that the child’s primary task is not to identify the grammar of the target lan- guage, but rather to learn to comprehend and produce utterances in the service of communicating with oth- ers. In this framework the input to the child is seen as providing a rich source of probabilistic information, and notions of the target language are provided by constraint satisfaction models of adult performance. The methodology involves implementing connection- ist models that simulate detailed aspects of perfor- mance. 1. Introduction Our paper is about a new approach to language ac- quisition that has begun to emerge over the past few years. The sources of this new approach are the re- newed interest in the statistical and probabilistic as- pects of language on the part of many language re- searchers; connectionism, which provides new in- sights about knowledge representation, learning, and processing; and studies of the remarkable learning ca- pacities of infants and young children. This approach entails different assumptions about what the important questions are in language acquisi- tion research and where the answers are likely to be found. First, we view the child’s task as learning to use language, not grammar identification. Second, we ask how the child acquires adult-like processing ca- pacities, not the capacity to distinguish grammatical from ungrammatical sentences. Third, the “target” or “steady state” the child must achieve is provided by constraint satisfaction models of adult performance, not an idealized competence grammar. What does this reorientation accomplish? For one thing, it provides the framework for a unified account of acquisition and processing. For another it makes language learning a tractable problem for the child. The standard approach leaves it unclear how a child could ever acquire a language: poverty of the stimulus arguments that have been taken to prove that gram- mar must be innate (Chomsky 1965) create a paradox because they also apply to language-specific types of knowledge that must be learned from experience. The parameter setting approach attempts to avoid classic learnability problems, but introduces new ones (Gib- son & Wexler 1994). In addition, the new approach is beginning to yield fresh analyses of and insights about particular lan- guage acquisition puzzles. For example, the approach is yielding a different view of what is innate. The old approach focuses on alternatives that date from 16th century philosophy: either tabula rasa empiri- cism or innate knowledge of grammar. The newer view takes its account of what is innate from mod- ern developmental neuroscience, which suggests that learning entails both structural and functional changes to the systems responsible for performance, and that these changes are governed by an interaction between the nature of the events to which the learner is exposed and a system structured so as to develop capacities for dealing successfully with particular tasks (Elman, Bates, Johnson, Karmiloff-Smith, Parisi & Plunkett 1996). In other words, brains don’t come with knowledge of specific grammars, faces, or objects already wired in; rather, brains are structured so as to facilitate the emergence of certain kinds of knowledge given cer- tain kinds of environmental events. Our approach is concerned with the factors that govern the character of such emergent knowledge. 2. The Standard Approach To place this research program in context, consider two of the standard assumptions that are widely held among acquisition researchers: there is a grammat- ical competence underlying language behavior, and the task of the child is to discover which of a very restricted set of innately specified grammars her lan- guage is described by. In this section we critically ex- amine these assumptions in turn. The starting point for the standard approach is the competence grammar, defined as that knowledge em- bodied in the ideal speaker-hearer of a given speech community (Chomsky 1965). The competence gram- mar self-consciously abstracts away from many of the factors that actually govern whether an utterance will be used on any occasion by any individual, includ- ing memory limitations, individual differences, facts about the computational system that implements the grammar, and so on. Grammar is a characterization of the knowledge that allows the speaker-hearer to pro- duce and understand an infinite number of sentences.