The structure of the lexicon reflects principles of communication

The structure of the lexicon reflects principles of communication Molly Lewis Elise Sugarman Michael C. Frank mll@stanford.edu Department of Psychology Stanford University elisesug@stanford.edu Department of Psychology Stanford University mcfrank@stanford.edu Department of Psychology Stanford University Abstract that a longer utterance should have a less predictable mean- ing. Consistent with this prediction, speakers tend to increase the duration of a word prosodically in cases where the word is unpredictable (highly informative) given the linguistic con- text (Aylett & Turk, 2004). Recent experimental data provide additional evidence for a complexity bias operating at the moment of language inter- pretation. For example, in an artificial language learning task, Fedzechkina, Jaeger, and Newport (2012) found that learners tended to use case marking (i.e. longer referential forms) in cases where the sentence meaning was less predictable in the experimental context. Bergen, Goodman, and Levy (2012) provide a more direct test of a complexity bias. In their task, partners were told that they were in an alien world with three objects of different base rate frequencies and three possible utterances of different monetary costs. Their task was to communicate about one of the objects using one of the avail- able utterances. The results suggest that both the speaker and hearer expected costlier forms to refer to less frequent mean- ings, consistent with Horn’s principle. A number of pieces of evidence thus corroborate propos- als that communicative pressures lead to a complexity bias at the moment of language interpretation. In our work here, we asked: Do these communicative pressures have conse- quences beyond in-the-moment language choices? That is, do these communicative pressures lead to the instantiation of a complexity bias in the lexicon? While this question has not been previously investigated, there is some evidence to sug- gest that they might: words that are more predictable in their linguistic context are found to be shorter than words that are less predictable in their linguistic context (Piantadosi, Tily, & Gibson, 2011b). A substantial challenge in studying a complexity bias is the difficulty of defining complexity. We give an intuitive defini- tion guiding our investigation, but we note that our opera- tionalization below is also consistent with other definitions. Imagine a space of possible meanings as compositional se- mantic primitives. In this space, a more complex meaning would be one with more primitives in it. (In a probabilistic framework, having more units would also be correlated with having a lower overall probability). We explore a complexity bias through norming and experi- mental studies. Analyses of norming data suggest that longer English words tend to have more complex meanings. Evi- dence from three experiments also suggests that a complexity bias guides inference in a novel word learning task, in both adults (Exp. 1 and 2) and preschoolers (Exp. 3). We conclude by discussing hypotheses about the source of these results and We explore a previously undescribed regularity in language: a bias for longer words to map to relatively more complex meanings. While theories of communication make the more general prediction that longer utterances should be associated with more complex meanings, this prediction has not yet been explored at the level of words. Through norming and experimental studies, we find evidence in support of a complexity bias in word meanings. We conclude by discussing hypotheses about the nature of this bias. Keywords: communication; pragmatics Introduction Is the structure of language shaped by its use in commu- nication? Hockett (1960) argued that several major fea- tures of language—from its productivity to its arbitrary map- pings from signifier to signified—emerge from communica- tive pressures. This kind of functional explanation has since been proposed for a wide range of phenomena, from the pres- ence of ambiguity in language (Piantadosi, Tily, & Gibson, 2011a) to the way languages organize the semantic space of kinship meaning (Kemp & Regier, 2012). In our current study, we identify a regularity in the lexicon—that more com- plex ideas tend to be named by longer words—and investigate a potential communicative explanation. Several theories of communication predict that longer ut- terances should be associated with more complex meanings (a complexity bias). More specifically, they predict that com- municative pressures should lead to a complexity bias that op- erates at the moment of language interpretation. Horn (1984) proposed one such theory. He argued for a relationship be- tween the cost of an utterance and the probability of mean- ing. He suggested that costlier phrases are associated with less probable meanings. For example, the phrases “I started the car” and “I got the car to start” are denotationally equiv- alent (they both denote the successful starting of a car), but they differ in cost in terms of their utterance length. Horn’s principle accounts for this difference as an asymmetry in the probabilities of meanings: The shorter form refers to the typi- cal (more frequent) starting of a car, and thus the longer form refers to an atypical case, perhaps where the driver encoun- tered difficulty. Information theory also predicts a complexity bias that op- erates during online language interpretation. Under this the- ory, speakers optimize information transfer (in terms of bits) by keeping the amount of information conveyed in a unit of language constant across the speech stream (Aylett & Turk, 2004; Frank & Jaeger, 2008). This “uniform information den- sity” hypothesis predicts as a straightforward consequence