Mayor, J. 1 & Plunkett, K. 2
1 FPSE, University of Geneva, Switzerland
2 Dept. of Experimental Psychology, University of Oxford, UK
Knowing words that other people know is crucial to achieving successful communication. Utterances achieve their impact when there is an alignment between the speaker's expressive vocabulary and the listener's receptive vocabulary. Yet interactions with a restricted number of people early in life may lead infants and young children to develop idiosyncratic vocabulary knowledge. We examined data collected from 14607 infants and toddlers in five countries (USA, Denmark, Norway, Germany and England). We applied a method developed by Mayor & Plunkett (2011), which was used to provide an estimate of vocabulary size from CDI scores, but can also be used to measure the amount of overlap between vocabularies. The method uses as input the proportion of infants, at a given age, who understand (or produce) a specific word and yields, amongst other measures, an index of vocabulary variability, which we monitor during development for both comprehension and production. We found that early lexicons are highly overlapping. However, beyond 100 words, toddlers share more words with other toddlers in comprehension than in production, even when matched for lexicon sizes. Lexical variability is independent from vocabulary score in comprehension whereas lexical variability correlates with vocabulary score in production. Direct measures provide further evidence that the asymmetry between comprehension and production vocabularies is not an artifact of the method used for assessing overlap between individual vocabularies. This finding demonstrates that comprehension and production differ structurally; toddlers are generalists in comprehension while developing a unique, expressive voice. Furthermore, variability in production decreases after two years of age, suggesting the end of a transitional period in which toddlers? expressive vocabulary is aligned only to an adult listener's receptive vocabulary, not to other toddlers. This paper is the first demonstration that communicative development shapes early receptive and expressive vocabularies in different ways.