Children's Grammars Grow More Abstract with Age

Children’s Grammars Grow More Abstract with Age – Evidence from an Automatic Procedure for Identifying the Productive Units of Language Gideon Borensztajn (gideon@science.uva.nl) Willem Zuidema (jzuidema@science.uva.nl) Rens Bod (rens@science.uva.nl) Institute for Logic, Language and Computation, University of Amsterdam Plantage Muidergracht 24, 1018 TV, Amsterdam, The Netherlands Abstract memory and attentional abilities (Chomsky, 1980); most of the knowledge of language is in place from birth, and only their parametrization needs to be set by the environmental triggers (Clahsen, 1996). Hence, in this tradition children are assumed to have, at least in competence, the same syn- tactic categories and rules as adults; this is referred to as the continuity assumption (e.g. Crain & Thornton, 2005). These opposing views on the units of language acquisition are, of course, best investigated empirically, on the basis of actual language usage. Several in-depth case-by-case anal- yses on the productive units in children’s corpora have been reported. For instance, Hodges, Krugler, and Law (2004) ana- lyzed the item-based nature of the acquisition of the complex construction ‘I V (NP) to VP-INF’, as in I want (you) to play. Lieven, Behrens, Speares, and Tomasello (2003) traced back the sources of creativity of target utterances in the child’s speech. The target utterances were reconstructed (manually) from a set of utterances used in the previous 6 weeks. They found that 74% of all novel target utterances produced in one day by a 2 year old child could be reduced to previously pro- duced utterances by using a single combinatorial operation. Their finding supports the hypothesis that the smallest units used in language production are often memorized multi-word constructions, rather than single words. To resolve the controversy, however, we believe it is essen- tial to move beyond the typical handful of linguistic examples that support one view over the other. In the current study, we develop computational tools for automatically identifying the most likely primitive units that were used by the child to pro- duce the utterances in a given corpus. We apply these tools to a well-known English-language corpus (the Brown corpus in CHILDES) with longitudinal data from three children. We then present a qualitative and quantitative analysis of the pro- ductive units that these children employ in progressive stages of acquisition. Note that we do not model the actual process of language acquisition or attempt to directly choose between usage-based and generative theories of language acquisition. Rather, we aim at providing a new way to evaluate predic- tions from theories of language performance in either tradi- tion about the productive units in child language. We develop an approach to automatically identify the most probable multi-word constructions used in children’s utter- ances, given syntactically annotated utterances from the Brown corpus of CHILDES. The found constructions cover many in- teresting linguistic phenomena from the language acquisition literature, and show a progression from very concrete towards abstract constructions. We show quantitatively that for all chil- dren of the Brown corpus grammatical abstraction, defined as the relative number of variable slots in the productive units of their grammar, increases globally with age. Keywords: First language acquisition; Usage Based Gram- mar; Constructions; Data-oriented Parsing Introduction Many contemporary theories of language acquisition assume that the basic units of language acquisition are constructions: associations between a semantic frame and a syntactic pat- tern, for which the meaning or form is not strictly predictable from its component parts. Learning, in this framework, con- sists of the gradual acquisition of a structured inventory of constructions, a constructicon, where the constructions are of various sizes and varying degrees of complexity and abstract- ness (Goldberg, 2006; Tomasello, 2003). Empirical studies in this tradition (e.g., Peters, 1983; Tomasello, 2003) show that, first, the primary units of speech of children in their first stage of language acquisition are not words but complete utterances, or holophrases. Second, in the earliest stages the child’s language is item-based in nature (Tomasello, 2000). Verb constructions are typically learned case-by-case (so-called verb islands), without reference to a general verb-class. The scope of the syntactic rules is limited to specific constructions, and system-wide syntactic rules or categories are mostly lacking. Third, in subsequent stages the child breaks down the item-based constructions, introducing variables, such as in Where’s the X? , I wanna X, etc. The acquisition of constructions with variable slots forms the beginning of abstraction and category formation, and it marks the beginning of grammar. Such Usage Based theo- ries of language acquisition assume a dynamically changing grammar that follows a route from simple and concrete to complex and abstract constructions. This view is in sharp contrast with the view on language acquisition taken in many versions of generative grammar. Here, grammar rules and categories are assumed to be universally and innately spec- ified by a Universal Grammar. The reason that children do not produce adult-like grammatical sentences is their limited Choosing the right representation In this section, we develop a formal definition of the produc- tive units of language and a probability model that defines the likelihood of various hypotheses on the units used. The for-