From early on, infants show a preference for infant-directed speech (IDS) over adult-directed speech (ADS), and exposure to IDS has been correlated with language outcome measures such as vocabulary. The present multi-laboratory study explores this issue by investigating whether there is a link between early preference for IDS and later vocabulary size. Infants’ preference for IDS was tested as part of the ManyBabies 1 project, and follow-up CDI data were collected from a subsample of this dataset at 18 and 24 months. A total of 341 (18 months) and 327 (24 months) infants were tested across 21 laboratories. In neither preregistered analyses with North American and UK English, nor exploratory analyses with a larger sample did we find evidence for a relation between IDS preference and later vocabulary. We discuss implications of this finding in light of recent work suggesting that IDS preference measured in the laboratory has low test-retest reliability.

We examined how noun frequency and the typicality of surrounding linguistic context contribute to children’s real-time comprehension. Monolingual English-learning toddlers viewed pairs of pictures while hearing sentences with typical or atypical sentence frames (Look at the… vs. Examine the…), followed by nouns that were higher- or lower-frequency labels for a referent (horse vs. pony). Toddlers showed no significant differences in comprehension of nouns in typical and atypical sentence frames. However, they were less accurate in recognizing lower-frequency nouns, particularly among toddlers with smaller vocabularies. We conclude that toddlers can recognize nouns in diverse sentence contexts, but their representations develop gradually.

Yarkoni's analysis clearly articulates a number of concerns limiting the generalizability and explanatory power of psychological findings, many of which are compounded in infancy research. ManyBabies addresses these concerns via a radically collaborative, large-scale and open approach to research that is grounded in theory-building, committed to diversification, and focused on understanding sources of variation.

Language mixing is common in bilingual children's learning environments. Here, we investigated effects of language mixing on children's learning of new words. We tested two groups of 3-year-old bilinguals: French–English (Experiment 1) and Spanish–English (Experiment 2). Children were taught two novel words, one in single-language sentences (“Look! Do you see the dog on the teelo?”) and one in mixed-language sentences with a mid-sentence language switch (“Look! Do you see the chien/perro on the walem?”). During the learning phase, children correctly identified novel targets when hearing both single-language and mixed-language sentences. However, at test, French–English bilinguals did not successfully recognize the word encountered in mixed-language sentences. Spanish–English bilinguals failed to recognize either word, which underscores the importance of examining multiple bilingual populations. This research suggests that language mixing may sometimes hinder children's encoding of novel words that occur downstream, but leaves open several possible underlying mechanisms.

Children tend to regularize their productions when exposed to artificial languages, an advantageous response to unpredictable variation. But generalizations in natural languages are typically conditioned by factors that children ultimately learn. In two experiments, adult and six-year-old learners witnessed two novel classifiers, probabilistically conditioned by semantics. Whereas adults displayed high accuracy in their productions – applying the semantic criteria to familiar and novel items – children were oblivious to the semantic conditioning. Instead, children regularized their productions, over-relying on only one classifier. However, in a two-alternative forced-choice task, children's performance revealed greater respect for the system's complexity: they selected both classifiers equally, without bias toward one or the other, and displayed better accuracy on familiar items. Given that natural languages are conditioned by multiple factors that children successfully learn, we suggest that their tendency to simplify in production stems from retrieval difficulty when a complex system has not yet been fully learned.

Efficiency in real-time language processing generally poses a greater challenge to adults learning a second language (L2) than to children learning a first language (L1). A notoriously difficult aspect of language for L2 learners to master is grammatical gender, and previous research has shown that L2 learners do not exploit cues to grammatical gender in ways that resemble L1 speakers. But it is not clear whether this problem is restricted to grammatical gender or whether it reflects a broader difficulty with processing local relations between words. Moreover, we do not know if immersive L2 environments, relative to typical L2 classrooms, confer advantages in learning regularities between words. In three eye-tracking experiments, 6- and 10-year-old children who were enrolled in Spanish immersion elementary schools listened to sentences with articles that conveyed information about the grammatical gender (Experiment 1), biological gender (Experiment 2), and number of referents in the visual field (Experiment 3). L1 children used articles to guide their attention to target referents in all three experiments. L2 children did not take advantage of articles as cues to grammatical gender, but succeeded in doing so for biological gender and number. Interpretations of these findings focus on how learning experiences interact with the nature of specific referential contexts to shape learners’ efficiency in language processing.