Casey Lew-Williams

Children learn to understand and speak from the speech around them, yet how that everyday input shapes learning remains unresolved. Large language models show that language is learnable from data, but their scale and training conditions differ sharply from a child’s natural experience. We trained cognitively feasible, child-specific models on longitudinal home recordings from eight children across their first 1000 days of life. Each model learned incrementally from a single child’s ordered speech input without prior linguistic knowledge, developing phoneme-like and word-form representations that tracked each child’s learning pace. Learning was dependent on dense daily sampling and replay of recent input, indicating that both experience- and memory-related processes are necessary. These findings show that everyday experience contains sufficient structure for language learning and establish child-specific modeling for testing developmental theories.

The ability to learn and apply rules lies at the heart of cognition. In a seminal study, Marcus et al. (1999) reported that 7‐month‐old infants learned abstract rules over syllable sequences and were able to generalize those rules to novel syllable sequences. Dozens of studies have since replicated that finding and extended it using different rules, modalities, stimuli, participants (human adults and non‐human animals) and experimental procedures. Yet questions remain about the generalizability of the core findings in Marcus et al., and sources of variation across these findings. In the current study, we address this issue by testing 839 infants (650 after exclusions) of a wide age range (5;0–12;0 months) in a multi‐laboratory (30 laboratories, [31 samples]) conceptual replication of the Marcus et al. study. This study and the analysis plan were submitted as a registered report. A mixed‐model analysis of the looking times at consistent versus inconsistent test trials indicated no significant effect. The current study therefore finds no evidence for rule‐learning; to the contrary, a Bayesian analysis of the data indicated strong evidence for a null effect. This was similarly the case for the moderating factors that were part of the design: mono‐ versus multilingualism, age, and experimental paradigm; none of these factors showed significant effects. Robustness analyses indicated that the results were not different in the case of different choices in the (pre‐)processing of the data. A follow‐up analysis for single labs indicated that there was no evidence of rule‐learning in 30 of the 31 contributed samples. The current finding raises fundamental issues about rule‐learning that are presented in the Discussion section.

The ability to learn and apply rules lies at the heart of cognition. In a seminal study, Marcus et al.(1999) reported that 7-month-old infants learned abstract rules over syllable sequences and wereable to generalize those rules to novel syllable sequences. Dozens of studies have since replicatedthat finding and extended it using different rules, modalities, stimuli, participants (human adultsand non-human animals) and experimental procedures. Yet questions remain about thegeneralizability of Marcus et al.’s (1999) core findings, and sources of variation across thesefindings. In the current study, we address this issue by testing 839 infants of a wide age range(5;0-12;0 months) in a multi-laboratory (30 laboratories, [31 samples]) conceptual replication ofthe Marcus et al. (1999) study. This study and the analyses were submitted as a registered report.A mixed-model analysis of the looking times at consistent vs. inconsistent test trials indicated nosignificant effect. The current study therefore finds no evidence for rule-learning; to the contrary, aBayesian analysis of the data indicated strong evidence for a null effect. This was similarly thecase for the moderating factors that were part of the design: mono- vs. multilingualism, age, andexperimental paradigm; none of these factors showed significant effects. Robustness analysesindicated that the results were not different in the case of different choices in the (pre-)processingof the data. A follow-up analysis for single labs indicated that there was no evidence ofrule-learning in 30 of the 31 contributed samples. The current finding raises fundamental issuesabout rule-learning that are presented in Discussion.

A decade of ManyBabies research, testing thousands of babies across hundreds of labs, has shown that some, but not all findings in infant research replicate well. Collectively, these projects have shown us that our methods carry limitations that larger samples alone cannot resolve. Here we present three lessons that point toward a more reliable, inclusive developmental science.

A decade of ManyBabies research, testing thousands of babies across hundreds of labs, has shown that some, but not all findings in infant research replicate well. Collectively, these projects have shown us that our methods carry limitations that larger samples alone cannot resolve. Here we present three lessons that point toward a more reliable, inclusive developmental science. A decade of ManyBabies research, testing thousands of babies across hundreds of labs, has shown that some, but not all findings in infant research replicate well. Collectively, these projects have shown that current methods carry limitations that larger samples alone cannot resolve. Here three lessons that point toward a more reliable, inclusive developmental science are presented.

Bilingual children learn their languages through rich interactions with caregivers within dynamic family contexts. However, little is known about how families align their two languages to support bilingual acquisition and how this varies across bilingual communities. This study examines language choice alignment across two communities: French–English families in Quebec (Canada) and Spanish–English families in New Jersey (United States). Thirty-nine children aged 18–35 months and their families were video-recorded during two 20 min home play sessions—one with a primary caregiver only and one including additional household members. Utterances were coded for speaker identity and language. We found strong turn-by-turn alignment between primary caregivers and children across both communities and sessions, with observed alignment exceeding chance in ⅔ of analyzed sessions at rates 20–22% above baseline. Other family members showed weaker correspondence with children’s language choices. Children’s alignment was modulated by language exposure and age, whereas caregivers’ alignment only decreased when additional members were present. These findings demonstrate that primary caregivers and their bilingual children align language choices consistently across diverse family configurations and communities. This linguistic coupling may support bilingual development across diverse interaction contexts, highlighting primary caregivers’ central role in early bilingual experiences across societies where bilingualism is and is not the norm.

Do toddlers and adults engage in spontaneous Theory of Mind? This multi-lab collaboration examined whether 18- to 27-month-olds’ and adults’ anticipatory looks distinguish between two basic forms of epistemic states: knowledge and ignorance. In adults (n = 703, 68% female), we found clear evidence that they do: they showed simple goal-based action anticipation and differentiated between knowledge and ignorance conditions. In toddlers (n = 521, 49% female), results were less conclusive. While demonstrating goal-based action anticipation, they did not differentiate between knowledge and ignorance as predicted. Future research can explore adults’ sensitivity to more complex epistemic states like true/false beliefs and clarify whether toddlers’ results reflect competence or performance limitations.

Language switching is a central feature of bilingual toddlers’ input, yet the everyday patterns that infants experience remain poorly understood. To characterize the switching that bilingual toddlers’ encounter, we recorded naturalistic play interactions on two occasions: a dyadic session between the toddler and a primary caregiver and a multi-person session that included additional household members. We calculated the number of language switches per minute produced by speakers in toddlers’ environments and classified these switches as within-speaker (a single person alternating languages) or across-speaker (different speakers using different languages in sequence). Patterns of language switching were then compared across Spanish-English (New Jersey) and French-English (Montreal) bilingual communities. Compared to sessions with only a primary caregiver, toddlers heard approximately 50% more speech and twice as much language switching in sessions with multiple household members present. This shift in switching was primarily driven by increased across-speaker switches, a pattern that was especially pronounced in the French-English community. Although there was considerable variability in switching across families, primary caregivers showed relatively stable switching behavior across sessions, suggesting that they may provide a source of consistency within otherwise dynamic language environments. Together, these findings underscore the importance of recognizing that language learning unfolds within interpersonal contexts, and that variation in social configurations may be particularly consequential for bilingual learners’ language input.

This preference has been linked to infants’ language processing and word learning in experimental settings, and also correlates with later language outcomes. Recently, the cross-cultural consistency of infants’ IDS preference has been confirmed by large-scale, multisite replication studies, but conclusions from these studies were primarily based on participants from North America and Europe. The current study addressed this sampling bias via a large-scale, multisite study of infants (3-15 months) across communities in Africa. We investigated whether participants showed a preference for IDS over ADS, and if so, whether the magnitude of their preference differs from effects documented in other populations of infants. Across six sites (total N = 200), we observed a preference for IDS βIDS vs. ADS = 0.06), suggesting that infants look on average 6% longer on the IDS trials than the ADS trials. There was no significant difference between African infants in this study and a method-matched subsample of infants from prior studies of IDS preference. This study provides new evidence on the generalizability of IDS preference and looking-time methods more broadly, while also highlighting some of the challenges of global big team science.

Bilingual children learn their languages through rich interactions with caregivers within dynamic family contexts. However, little is known about how families align their two languages to support bilingual acquisition. This study examines language choice alignment across two communities: French-English families in Quebec (Canada) and Spanish-English families in New Jersey (United States). Thirty-nine children aged 18–35 months and their families were video-recorded during two 20-minute home play sessions —one with a primary caregiver only and one including additional household members. Utterances were coded for speaker identity and language. We found strong turn-by-turn alignment between primary caregivers and children across both communities and sessions, with observed alignment exceeding chance in ⅔ of analyzed sessions at rates 20-22% above baseline. Other family members showed weaker correspondence with children's language choices. Children's alignment was modulated by language exposure and age, whereas caregivers' alignment only decreased when additional members were present. These findings demonstrate that primary caregivers and their bilingual children align language choices consistently across diverse family configurations and communities. This linguistic coupling may support bilingual development across diverse interaction contexts, highlighting primary caregivers' central role in early bilingual experiences across societies where bilingualism is and is not the norm.