Infants who are rarely spoken to nevertheless understand many words.

Theories of language development-informed largely by studies of Western, middleclass infants-have highlighted the language that caregivers direct to children as a key driver of language learning. However, some have argued that language development unfolds similarly across environmental contexts, including those in which childdirected language is scarce. This raises the possibility that children are able to learn from other sources of language in their environments, particularly the language directed to others in their environment. We explore this hypothesis with infants in an indigenous Tseltal-speaking community in Southern Mexico who are rarely spoken to, yet have the opportunity to overhear a great deal of other-directed language by virtue of being carried on their mothers' backs. Adapting a previously established gaze-tracking method for detecting early word knowledge to our field setting, we find that Tseltal infants exhibit implicit knowledge of common nouns (Exp. 1), analogous to their US peers who are frequently spoken to. Moreover, they exhibit comprehension of Tseltal honorific terms that are exclusively used to greet adults in the community (Exp. 2), representing language that could only have been learned through overhearing. In so doing, Tseltal infants demonstrate an ability to discriminate words with similar meanings and perceptually similar referents at an earlier age than has been shown among Western children. Together, these results suggest that for some infants, learning from overhearing may be an important path toward developing language.

Empirical audit and review and an assessment of evidentiary value in research on the psychological consequences of scarcity.

Empirical audit and review is an approach to assessing the evidentiary value of a research area. It involves identifying a topic and selecting a cross-section of studies for replication. We apply the method to research on the psychological consequences of scarcity. Starting with the papers citing a seminal publication in the field, we conducted replications of 20 studies that evaluate the role of scarcity priming in pain sensitivity, resource allocation, materialism, and many other domains. There was considerable variability in the replicability, with some strong successes and other undeniable failures. Empirical audit and review does not attempt to assign an overall replication rate for a heterogeneous field, but rather facilitates researchers seeking to incorporate strength of evidence as they refine theories and plan new investigations in the research area. This method allows for an integration of qualitative and quantitative approaches to review and enables the growth of a cumulative science.

Combining observational and experimental approaches to the development of language and communication in rural samples: Opportunities and challenges.

Multiple approaches - including observational and experimental - are necessary to articulate powerful theories of learning. Our field's key questions, which rely on these varied methods, are still open. How do children perceive and produce language? What do they encounter in their linguistic input? What does the learner bring to the task of acquisition? Considerable progress has been made for the development of spoken English (especially by North American learners). Yet there is still a great deal to discover about how children in other populations proceed, especially populations in rural settings. To examine language learning in these populations, we need a multi-method approach. However, adapting and integrating methods, particularly experimental ones, to new settings can present immense challenges. In this paper, we discuss the opportunities and challenges facing researchers who aim to use a multimethodological approach in rural samples, and what the field of language acquisition can do to promote such work.

What causes the word gap? Financial concerns may systematically suppress child-directed speech.

Parents with fewer educational and economic resources (low socioeconomic-status, SES) tend to speak less to their children, with consequences for children's later life outcomes. Despite this well-established and highly popularized link, less research addresses why the SES "word gap" exists. Moreover, while research has assessed individual-level contributors to the word gap-like differences in parenting knowledge-we know little about how structural constraints that vary according to SES might affect caregivers' speech. In two pre-registered studies, we test whether experiencing financial scarcity can suppress caregivers' speech to their children. Study 1 suggests that higher-SES caregivers who are prompted to reflect on scarcity-particularly those who reflect on financial scarcity-speak less to their 3-year-olds in a subsequent play session, relative to a control group. Study 2 suggests that mid- to higher-SES caregivers engage in fewer back-and-forth exchanges with their children at the end of the month-when they are more likely to be experiencing financial hardship-than the rest of the month. These studies provide preliminary evidence that-above and beyond caregivers' individual characteristics-structural constraints may affect how much parents speak to their children.

How adults understand what young children say.

Children's early speech often bears little resemblance to that of adults, and yet parents and other caregivers are able to interpret that speech and react accordingly. Here we investigate how adult listeners' inferences reflect sophisticated beliefs about what children are trying to communicate, as well as how children are likely to pronounce words. Using a Bayesian framework for modelling spoken word recognition, we find that computational models can replicate adult interpretations of children's speech only when they include strong, context-specific prior expectations about the messages that children will want to communicate. This points to a critical role of adult cognitive processes in supporting early communication and reveals how children can actively prompt adults to take actions on their behalf even when they have only a nascent understanding of the adult language. We discuss the wide-ranging implications of the powerful listening capabilities of adults for theories of first language acquisition.

Self-directed learning by preschoolers in a naturalistic overhearing context.

Three studies investigated preschoolers' self-directed learning ability in a naturalistic context: learning from overheard speech. In Experiment 1, 4.5- to 6-year-olds were exposed to 4 novel words and 6 arbitrary facts corresponding to a set of co-present toys; in Experiment 2, 3- to 4.5-year-olds heard 5 nouns and 3 facts. In the Pedagogical conditions, children were taught the information with the aid of multiple pedagogical cues, but in the Overhearing conditions, children had to 'listen in' to one side of a phone call to learn the information. Older preschoolers (Experiment 1) learned all items above chance in both conditions. Younger preschoolers (Experiment 2) learned words and facts above chance in the Pedagogical condition, but were at chance at learning words in the Overhearing condition, despite reliably learning facts from overhearing. Experiment 3 demonstrated that younger children's difficulty at learning new words from overhearing could not be explained by only being able to hear one side of the phone conversation, as they similarly struggled when the phone call took place over speakerphone. Measures of children's touch behavior suggest that older children were better able to coordinate their attention between the overheard speech and objects, though even younger children showed evidence of attention to the overheard speech. Together, our results demonstrate that by age 5, children can learn multiple new words and facts via overhearing. This self-directed learning ability depends on being able to coordinate attention between speech and the surrounding environment, a capacity that develops throughout preschool.

Linguistic conventionality and the role of epistemic reasoning in children's mutual exclusivity inferences.

To interpret an interlocutor's use of a novel word (e.g., "give me the papaya"), children typically exclude referents that they already have labels for (like an "apple"), and expect the word to refer to something they do not have a label for (like the papaya). The goal of the present studies was to test whether such mutual exclusivity inferences require children to reason about the words their interlocutors know and could have chosen to say: e.g., If she had wanted the "apple" she would have asked for it (since she knows the word "apple"), so she must want the papaya. Across four studies, we document that both children and adults will make mutual exclusivity inferences even when they believe that their interlocutor does not share their knowledge of relevant, alternative words, suggesting that such inferences do not require reasoning about an interlocutor's epistemic states. Instead, our findings suggest that children's own knowledge of an object's label, together with their belief that this is the conventional label for the object in their language, and that this convention applies to their interlocutor, is sufficient to support their mutual exclusivity inferences. Additionally, and contrary to the claims of previous studies that have used mutual exclusivity as a proxy for children's beliefs that others share their knowledge, we found that children - especially those with stronger theory of mind ability - are quite conservative about attributing their knowledge of object labels to others. Together, our findings hold implications for theories of word learning, and for how children learn about the scope of shared conventional knowledge.

Learning language from within: Children use semantic generalizations to infer word meanings.

One reason that word learning presents a challenge for children is because pairings between word forms and meanings are arbitrary conventions that children must learn via observation - e.g., the fact that "shovel" labels shovels. The present studies explore cases in which children might bypass observational learning and spontaneously infer new word meanings: By exploiting the fact that many words are flexible and systematically encode multiple, related meanings. For example, words like shovel and hammer are nouns for instruments, and verbs for activities involving those instruments. The present studies explored whether 3- to 5-year-old children possess semantic generalizations about lexical flexibility, and can use these generalizations to infer new word meanings: Upon learning that dax labels an activity involving an instrument, do children spontaneously infer that dax can also label the instrument itself? Across four studies, we show that at least by age four, children spontaneously generalize instrument-activity flexibility to new words. Together, our findings point to a powerful way in which children may build their vocabulary, by leveraging the fact that words are linked to multiple meanings in systematic ways.