Automatic Classification of Transitivity Alternations in Child-Directed Speech

Automatic Classification of Transitivity Alternations in Child-Directed Speech Rose M. Scott (rmscott2@uiuc.edu) Department of Psychology, 603 E. Daniel Street Champaign, IL 61820 USA Cynthia Fisher (clfishe@uiuc.edu) Department of Psychology, 603 E. Daniel Street Champaign, IL 61820 USA Abstract Children can use the characteristic entailment patterns of verb classes to learn new verbs (Naigles, 1996), but how do they ac- quire verb classes? One possibility is that surface features that reflect argument structure can separate verb classes (Merlo & Stevenson, 2001). We tested this hypothesis by running cluster- ing analyses on surface features of 29 verbs extracted from a large corpus of child-directed speech. Results suggest that ani- macy plays an important role in verb classification. Introduction The syntactic bootstrapping hypothesis (Gleitman, 1990; Landau & Gleitman, 1985) holds that children use syntax to guide verb learning. For a given utterance of a verb, the refer- ential scene offers a plethora of potential meanings. Given principled relationships between clause syntax and verb meaning (e.g., Grimshaw, 1990; Jackendoff, 1990; Levin & Rappaport-Hovav, 2005), the syntactic structure, or frame, in which a verb occurs can act as a kind of linguistic “zoom lens” that constrains those interpretations. This hypothesis has been supported by many studies showing that children as young as 21 months old assign different meanings to verbs presented in different sentence structures. For example, if children hear a new verb in a transitive sen- tence, they interpret the verb as referring to the action of one participant on another; if the verb is intransitive, they interpret it as referring to an event requiring only one participant (e.g., Fisher, 2002; Naigles, 1990; Yuan & Fisher, in press). Fur- thermore, each advance in learning the syntactic and morpho- logical features of the native language yields new constraints on verb learning. For example, English-learning infants as young as 21 months old use word order in a transitive sen- tence to guide interpretation of a new verb (Gertner, Fisher, & Eisengart, in press). A single sentence frame, however, is a limited guide to verb meaning. First, it provides only highly abstract semantic information. The transitive frame occurs with such disparate meanings as He became a doctor and He shot a doctor. The verbs that appear in this frame share, not the specifics of the events they describe, but similar formal structure: becoming and shooting both require two core arguments (though only one person in the case of become). A sentence frame yields information mainly about the number and type of arguments associated with the verb–what Grimshaw (1994) has termed its semantic structure–rather than its semantic content. Observation of events must provide the semantic content (see Fisher, 2000, for a review). Experimental results suggest that 2-year-olds are appropriately open-minded about the possible meanings of transitive verbs: when children are presented with a novel verb in a transitive sentence, they can interpret it as referring to a caused-motion event (e.g., pushing) or to a simple contact event (e.g., patting; Naigles & Kako, 1993). Second, a single frame yields information only about the semantic structure of the verb when used in that frame. Most verbs occur in more than one frame, each of which results in a different semantic structure, as in (1). (1) a. She explained that he left. b. She explained the problem to me. Gleitman and colleagues have proposed that children can gain further constraint on verb learning by appealing to the set of subcategorization frames in which a verb appears (Gleit- man et al., 2005). For example, explain occurs with sentence complements (1a); this frame implies that one of the verb's arguments has propositional content. Explain also occurs with both direct and indirect objects (1b), a frame consistent with transfer, or motion toward a goal. The combination of these frames considerably narrows the possible meanings of ex- plain: it is a verb describing transfer of propositional content. But even a set of subcategorization frames can be ambigu- ous. Take for instance the verbs break (2) and dust (3): (2) a. Anna broke the lamp. b. The lamp broke. (3) a. Anna dusted the lamp. b. Anna dusted. Both can be transitive and intransitive. Given only their oc- currence in these two frames, a learner would have to treat both verbs as members of a single syntactically defined class and use world observation to infer the semantic content of each verb in the class. The learner who does this, however, would be missing a very useful distinction between verbs like break and verbs like dust, which can be seen if the entailment relations across the transitive and intransitive sentences in each pair are taken into account. The transitive sentence (2a) describes an event with two parts: the application of force to the lamp and the ensuing breaking sub-event. This sentence necessarily entails the sub- event (that the lamp broke), expressed in the intransitive sen- tence (2b). In contrast, sentence (3a) does not describe a