Overspecification and the Cost of Pragmatic Reasoning about Referring Expressions
Overspecification and the Cost of Pragmatic Reasoning about Referring Expressions Peter Baumann (baumann@u.northwestern.edu) Department of Linguistics Northwestern University, Evanston, IL, USA Brady Clark (bzack@northwestern.edu) Department of Linguistics Northwestern University, Evanston, IL, USA Stefan Kaufmann (stefan.kaufmann@uconn.edu) Department of Linguistics University of Connecticut, Storrs, CT, USA Abstract In current approaches to pragmatic reasoning the comprehen sion and production of referring expressions is modeled as a result of the interlocutors' mutual perspective taking under the additional assumption that speakers try to minimize their ar ticulatory effort or production cost. The latter assumption is usually not tested and instead built into the experimental tasks of referential language games by artificially restricting the set of possible referring expressions available to identify a refer ent. We present two language game experiments: a produc tion experiment, in which the speakers were allowed to freely choose a referring expression, and a comprehension experi ment to replicate earlier findings with our stimuli. Our results show that while listeners easily perform pragmatic reasoning, speakers resort to overspecification when the effort of prag matic reasoning becomes too high. Keywords: Pragmatics; Language games; Referring Expres sions; Language Production; Language Comprehension Introduction In a complex situation, like a room full of people, one may be able to identify a single person upon hearing a rather short de scription, like "the man with a hat" even in cases in which the man in question is not the only one with a hat and despite the fact that he certainly has many other characteristics besides wearing a hat. This amazing efficiency of human communi cation has made reference and the use of referring expressions a central topic in linguistic pragmatics. The most influen tial descriptive account for the efficiency of human commu nication was formulated by Grice (1975) in the form of his Maxims of Conversation, and since then, a number of propos als have been made to provide more quantitative models of pragmatic reasoning based on the Gricean maxims or more general principles of human cognition and interaction. The most prominent examples are game theoretic models based on strategic reasoning (e.g. Benz Van Rooij, 2007; Franke, 2011; J"ager, 2011) and Bayesian models grounded in so cial cognition (e.g. Frank Goodman, 2012; Goodman Stuhlm"uller, 2013) In both classes of models it is assumed that the speaker and hearer reason about each other's perspec tives: the hearer is assumed to interpret a speaker's expression as referring to the referent for which the expression is 'opti mal' under the perspective of the speaker, who in turn chooses the referring expression to be 'optimal' under the hearer's per spective, etc. A trivial solution to this recursive reasoning process is for the speaker to choose a referring expression that explicitly mentions all features of the intended referent and is thus absolutely unambiguous in the given context. Since such an expression can hardly qualify as efficient, however, the above models make the crucial additional assumption that the speaker incurs a cost for producing an utterance, thus all things being equal, speakers have a preference for the most economic (i.e. shortest and least effortful) expression. While theoretically appealing, production costs are noto riously hard to quantify, as the articulatory effort of speech production 1 is negligible (Moon Lindblom, 2003; Locke, 2008) And anecdotally one may even be tempted to reject the notion of production costs altogether: people talk a lot. However, we do not argue against a possible role for pro duction costs in pragmatic reasoning. Instead, we show that speakers' behavior cannot be explained in terms of this fac tor alone: Under certain conditions, speakers do use costlier forms than would be required to identify the intended refer ent, which suggests that the process of pragmatic reasoning (often termed implicature) is itself effortful (like any other reasoning process) and thus incurs a cost for the speaker. In language comprehension, it is well established that prag matic reasoning can be effortful: in reading, sentences involv ing implicatures take longer to process than sentences with out implicatures (Hamblin Gibbs, 2003) and in referential language games, target identification is less accurate when the message involves an implicature than when it does not (e.g. Degen Franke, 2012) In language production, on the other hand, the speaker has the option of reducing the need for pragmatic inference by overspecification, i.e. saying more than is strictly necessary. And indeed, there is plenty of empirical evidence that speak ers do make use of overspecification. In particular, research on the production of referring expressions (e.g. Koolen, Typological analyses of language efficiency (e.g. Piantadosi, Tily, Gibson, 2012) are typically based on arguments involving processing restrictions in comprehension rather than production.
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