A growing set of studies suggests that our ability to infer, and reason about, mental states is supported by the assumption that agents maximize utilities -the rewards they attain minus the costs they incur. This assumption enables observers to work backwards from agents' observed behavior to their underlying beliefs, preferences, and competencies. Intuitively, however, agents may have incomplete, uncertain, or wrong beliefs about what they want. More formally, agents try to maximize their expected utilities. This understanding is crucial when reasoning about others' behavior: it dictates when actions reveal preferences, and it makes predictions about the stability of behavior over time. In a set of 7 experiments we show that 4 and 5 year-olds understand that agents try to maximize expected utilities, and that these responses cannot be explained by simpler accounts. In particular, these results suggest a modification to the standard belief/desire model of intuitive psychology. Children do not treat beliefs and desires as independent; rather, they recognize that agents' have beliefs about their own desires and that this has consequences for the interpretation of agents' actions.

Many words are associated with more than a single meaning. Words are sometimes "ambiguous," applying to unrelated meanings, but the majority of frequent words are "polysemous" in that they apply to multiple related meanings. In a preregistered design that included 2 tasks, we tested adults' and 4.5to 7-year-old children's ability to learn 4 novel polysemous words or 4 novel ambiguous words. Both children and adults demonstrated a polysemy over ambiguity learning advantage on each task after exposure, showing better learning of novel words with multiple related meanings than novel words with unrelated meanings. Stimuli in the polysemy condition were designed and then normed to guard against learners relying on a simple definition to distinguish the multiple target meanings for each word from foils. We retested available participants after a week-long delay without providing additional exposure and found that adults' performance remained strong in the polysemy condition in 1 task, and children's performance remained strong in the polysemy condition in both tasks. We conclude that participants are adept at learning polysemous words that vary along multiple dimensions. Current results are consistent with the idea that ambiguous meanings of a word compete, but polysemous meanings instead reinforce one another.

Four experiments show that four-and five-year-olds (total N=112) can identify the referent of under-determined utterances through their Naïve Utility Calculus-an intuitive theory of people's behavior structured around an assumption that agents maximize utilities. In Experiments 1-2, a puppet asked for help without specifying whom she was talking to ("Can you help me?"). In Experiments 3-4, a puppet asked the child to pass an object without specifying

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Because words have multiple meanings, language users must often choose appropriate meanings according to the context of use. How this potential ambiguity affects first language learning, especially word learning, is unknown. Here, we present the first large-scale study of how children are exposed to, and themselves use, ambiguous words in their actual language learning environments. We tag 180,000 words in two longitudinal child language corpora with word senses from WordNet, focusing between 9 and 51 months and limiting to words from a popular parental vocabulary report. We then compare the diversity of sense usage in adult speech around children to that observed in a sample of adult-directed language, as well as the diversity of sense usage in children's own productions. To accomplish this we use a Bayesian model-based estimate of sense entropy, a measure of diversity that takes into account uncertainty inherent in small sample sizes. This reveals that sense diversity in caregivers' speech to children is similar to that observed in a sample of adult-directed written material, and that children' use of nouns --- but not verbs --- is similarly diverse to that of adults. Finally, we show that sense entropy is a significant predictor of vocabulary development: children begin to produce words with a higher diversity of adult sense usage at later ages. We discuss the implications of our findings for theories of word learning.