Individual differences in the early lexicon: The child is a source of variability

By the end of their first year of life, children are typically able to produce a handful of words. Over the second and third year of life, this vocabulary increases dramatically, reaching an average of 553 words by the age of 30 months (Frank, Braginsky, Yurovsky, & Marchman, 2016). Although this general pattern appears stable across children and languages, there are considerable individual differences in the words known to different children: German vocabulary data (Szagun, Stumper, & Schramm, 2009) suggests that, at 20 months, 52% of children produce the word Bagger 'digger' and 48% of children produce the word Bär 'bear'. Importantly, of the children who produce at least one of these words, 52% of them produce the one but not the other. It is not until 30 months that both words are produced by virtually all of the children. This suggests that there is a 10-month period during which some children know a word and others don't. What determines whether a child is a Bär baby or a Bagger baby? While there is little doubt that the quality and quantity of input play a role in early word learning, another source of variability in the early lexicon is the child herself. Curiosity-driven approaches to learning place the child in a more active role, such that learning is guided by the child's desire to reduce uncertainty in her knowledge about the world and an intrinsic motivation to learn (e.g., Oudeyer & Smith, 2016). This study investigated the influence of category curiosity and category density, i.e., number of category members known to the child, on the acquisition of new word-object-associations. 30-months-olds (n=22) were, first, presented with 16 familiar objects from two broad (M = 31 members) and two narrow (M = 11 members) categories and heard their corresponding labels while their pupil dilation response was measured as an index of their interest in the different categories. Next, they were exposed to novel members from each of the four categories and tested on their learning of the new word-object-associations. Analyses indicate that children are able to learn novel members from both broad and narrow categories, but learning is more robust in the broad categories. This suggests that children are able to leverage their existing semantic knowledge to learn new words, which is in line with previous research (e.g., Borovsky et al., 2015, 2016). Furthermore, pupil dilation analysis suggests that learning is impacted by children's inherent curiosity in objects from particular categories: Learning outcomes for novel members of a category are positively correlated with children's interest in familiar members of the same category, as indexed by pupillary responses to objects from these categories. Our findings suggest that the child herself is a major source of variability in early word learning. What a child knows and what a child is interested in will shape what she learns next, leading to individual differences that are observable early on.