Cost-Based Pragmatic Inference about Referential Expressions

Cost-Based Pragmatic Inference about Referential Expressions Judith Degen (jdegen@bcs.rochester.edu) Michael Franke (m.franke@uva.nl) Dept. of Brain and Cognitive Sciences University of Rochester Institute for Logic, Language and Computation Universiteit van Amsterdam Gerhard J¨ager (gerhard.jaeger@uni-tuebingen.de) Institute of Linguistics University of T¨ubingen Abstract experimental data from referential language games. Its the- oretical contribution is a novel probabilistic model of back- and-forth reasoning that synthesizes recent Bayesian models (Frank & Goodman, 2012) and game theoretic approaches (Camerer, Ho, & Chong, 2004; Rogers, Palfrey, & Camerer, 2009; Franke, 2011; J¨ager, 2013). Our experiments probed interlocutors’ perspective-taking ability in a task where an artificial language left some rel- evant meaning features inexpressible or made some expres- sions costly. When critical meaning features are inexpress- ible, the situation is reminiscent of scalar implicature calcu- lation (Grice, 1975). We manipulated how many steps of such reasoning were needed for communicative success and tested both comprehension (Exp. 1) and production (Exp. 2) to in- vestigate question 1. Our design was chosen so as to im- prove on previous related studies where non-linguistic picto- rial messages were used (Degen & Franke, 2012) and where the availability of alternative expressions was not explicitly controlled (Stiller, Goodman, & Frank, 2011; Frank & Good- man, 2012). In addition, rather than making some messages entirely unavailable, we investigated the effect of variable production costs on interpretation behavior to address ques- tion 2 (Exp. 3). Rohde, Seyfarth, Clark, J¨ager, and Kaufmann (2012) showed that listeners take into account message costs when messages are assigned an explicit dollar value. Here we investigate whether these results replicate when costs, as in real language use, are implicit. The data from our experiments is explained well by a probabilistic model of back-and-forth reasoning. The model parameterizes how deeply interlocutors reason about each other’s perspective and how close they are to being ratio- nal. Parameter values that best explain our data suggest that participants were reasonably rational and applied a small but non-negligible amount of theory of mind reasoning and that they took production costs into account in comprehension. We present data from three experiments addressing how much theory of mind reasoning is involved in production and inter- pretation of ambiguous referential expressions in an artificial language task, and how this interacts with the cost and avail- ability of alternative utterances. When an unambiguous alter- native is not available, listeners tend to draw simple Quantity inferences reminiscent of scalar implicatures (Grice, 1975). When an unambiguous alternative is available, fewer infer- ences are observed, but gradiently more as the cost of unam- biguous alternatives increase. We outline a novel game the- oretic model of pragmatic reasoning based on probabilistic back-and-forth reasoning about interlocutors’ rational choices and beliefs. The model provides a good fit to the data and raises interesting issues for future research. Keywords: Pragmatics; Game theory; Referential Expres- sions; Language production; Language comprehension. Introduction People are lazy: when they speak, they like to save effort. But if speakers are too lazy and say too little, their listeners will not understand them. A good example is the choice and inter- pretation of referential expressions. A rational speaker who wants to establish reference should choose the most economic (shortest, easiest, least effortful) description that, according to his beliefs about the listener’s dispositions to interpret ut- terances, will allow for the listener to safely infer the correct referent. A rational listener should take the speaker’s produc- tion costs 1 into account and so a rational speaker should in turn take that into account, etc. But this is an idealized pic- ture. From an empirical point of view two related questions arise: 1) How much do speakers and listeners take into ac- count each other’s perspective? 2) How much influence do economy considerations have; do listeners weigh the speaker’s production costs? When it comes to referential language use, the latter ques- tion has not been investigated thoroughly, but the former question has been addressed in a variety of ways, both the- oretically (Clark & Marshall, 1981) as well as experimen- tally (Hanna, Tanenhaus, & Trueswell, 2003; Keysar, Barr, & Brauner, 2000). This paper aims to adress both questions and to bring theoretical and empirical approaches closer to- gether. The paper’s empirical contribution is to report on Referential Language Games Referential communication can be conceived of as a signal- ing game (Lewis, 1969): a sender (speaker) S knows which referent he wants to talk about, but a receiver (listener) R does not; S chooses a referential description; if R can identify the intended referent, communication is successful, otherwise a failure. Different games ensue for different sets of potential referents and referential expressions. In the critical trials of our experiments the referential games were isomorphic to the 1 Many factors have been identified as contributing to production preferences (see e.g. Jaeger & Tily, 2011). Here we take partici- pants’ empirically estimated (Exp. 3) relative preference for shorter over longer messages as a measure of subjective production cost.