Knowing How You Know: Toddlers Re-evaluate Words Learnt from an Unreliable Speaker, 2016-2019

There has been little investigation of the way source monitoring, the ability to track the source of one’s knowledge, may be involved in lexical acquisition. In two experiments, we tested whether toddlers (mean age 30 months) can monitor the source of their lexical knowledge and reevaluate their implicit belief about a word mapping when this source is proven to be unreliable. Experiment 1 replicated previous research (Koenig & Woodward, 2010): children displayed better performance in a word learning test when they learned words from a speaker who has previously revealed themself as reliable (correctly labeling familiar objects) as opposed to an unreliable labeler (incorrectly labeling familiar objects). Experiment 2 then provided the critical test for source monitoring: children first learned novel words from a speaker before watching that speaker labeling familiar objects correctly or incorrectly. Children who were exposed to the reliable speaker were significantly more likely to endorse the word mappings taught by the speaker than children who were exposed to a speaker who they later discovered was an unreliable labeler. Thus, young children can reevaluate recently learned word mappings upon discovering that the source of their knowledge is unreliable. This suggests that children can monitor the source of their knowledge in order to decide whether that knowledge is justified, even at an age where they are not credited with the ability to verbally report how they have come to know what they know.As anyone who has learnt a foreign language or travelled abroad will have noticed, languages differ in the sounds they employ, the names they give to things, and the rules of grammar. However, linguists have long observed that, beneath this surface diversity, all human languages share a number of fundamental structural similarities. Most obviously, all languages use sounds, all languages have words, and all languages have a grammar. More subtly and more surprisingly, similarities can also be observed in more fine-grained linguistic features: for instance, George Zipf famously observed that, across multiple languages, short words tend also to be more frequent, and in my own recent work I have shown that languages prefer to use words that sound alike (e.g., cat, mat, rat, bat, fat, ...). Why do all languages exhibit these shared features? This project aims to tackle exactly this key question by studying how languages are shaped by the human mind. In particular, I will explore how the way we learn language and use it to communicate drives the emergence of important features of lexicons, the set of all words in a language. To simulate the process of language change and evolution in the lab, I will use an experimental paradigm where an artificial language is passed between learners (language learning), and used by individuals to communicate with each other (language use). This paradigm has been successfully applied in previous research showing that key structural features of language can be explained as a consequence of repeated learning and use; my contribution will be to apply the same methods to study the evolution of the lexicon. I will then use two complementary techniques to evaluate the ecological validity of these results. First, do the artificial lexicons obtained after repeated learning and communication match the structure of lexicons found in real human languages? We will assess this by analyzing real natural language corpora using computational methods. Second, are these lexicons easily learnable by young children, the primary conduit of natural language transmission in the wild? This will be assessed using methods from developmental psychology to study word learning in toddlers. The present project requires an unprecedented integration of techniques and concepts from language evolution, computational linguistics and developmental psychology, three fields that have so far worked independently to understand the structure of language. The outcomes of the project will be of vital interest for all these communities, and will provide insights into the foundational properties found in all human languages, as well as the nature of the constraints underlying language processing and language acquisition. This project will provide a springboard for my future work at the intersection of computational and experimental approaches to language and cognitive development.