Khuyen N. Le

When encountering a novel word, adults and children as young as 12 months old often reason that it refers to a novel object rather than one with an existing name – making a ‘mutual exclusivity inference.’ We explored the mechanism of this inference, aiming to differentiate between three hypotheses: whether mutual exclusivity arises due to reasoning about a specific speaker’s knowledge, projection of one’s own egocentric knowledge, or reasoning generally about the conventionality of labels. Adults and 3.5-5-year-old children in our experiment heard a label being either taught or invented on the spot. They were then asked to decide what another label, produced by a speaker who was absent during the first label’s introduction, referred to. Our results revealed that adults and older children made more mutual exclusivity inferences when the first label was taught compared to when it was invented. Additionally, they were more likely to exclude the previously-labeled object in the referential choice if they judged that the speaker also knew the label, regardless of how the label was introduced. Both adults and older children also showed sensitivity to the speaker’s explicit statements of knowledge, excluding an object that the speaker explicitly stated that he did not know the name of from their referential choice. These results suggested that adults and older children reasoned about a specific speaker’s knowledge of labels in order to make mutual exclusivity inferences, likely in the form of a Gricean inference where they reasoned about possible alternative utterances a speaker could have said.

Numerate adults know that when two sets are equal, they should be labeled by the same number word. We explored the development of this principle—sometimes called “cardinal extension”—and how it relates to children’s other numerical abilities. Experiment 1 revealed that 2-to 5-year-old children who could accurately count large sets often inferred that two equal sets should be labeled with the same number word, unlike children who could not accurately count large sets. However, not all counters made this inference, suggesting that learning to construct and label large sets may be a necessary but not sufficient step in learning how numbers represent exact quantities. Experiment 2 found that children who extended labels to equal sets were not actually sensitive to exact equality and that they often assigned two sets the same label when they were approximately equal, but differed by just one item (violating one-to-one correspondence). These results suggest a gradual, stagelike, process in which children learn to accurately count, learn to extend labels to perceptually similar sets, and then eventually restrict cardinal extension to sets that are exactly equal.