About

Casey Lew-Williams is a Professor and the Department Chair in the Department of Psychology at Princeton University. His research focuses on how young children learn from the dynamics of their communicative environments. Using a combination of experimental, descriptive, computational, and social neuroscience approaches, he investigates how everyday experiences shape learning in infants. His work particularly emphasizes language learning and communication, studying typical learners, children facing adversity, and children growing up bilingual. His research involves measuring natural complexities such as eye movements, infant-directed speech, multimodal interactions, and infant-adult neural synchrony. Lew-Williams leads the Princeton Baby Lab, where he explores how basic cognitive and social capacities like attention, pattern detection, prediction, and memory contribute to early learning. His research aims to understand how early experiences influence participation in the community and how ideas about early learning can translate into practical ways to help children thrive. He has been recognized for his contributions to mentoring graduate students and for excellence in teaching, receiving awards such as the President’s Award for Distinguished Teaching and honors from Phi Beta Kappa.

Selected publications

• What 5000 Babies Can Tell Us About Developing Minds and How to Study Them: Lessons from the ManyBabies Consortium

  2026-04-23

  articleOpen access

  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.

  PublisherDOI

• Rhythmic scaffolding of child-directed speech and neural synchrony in child-caregiver interactions

  Open MIND · 2026-01-01

  otherOpen accessSenior author

  Previous evidence suggests there is an overlap between the rhythms of speech units (stress, syllables) and the timescales at which neurophysiological mechanisms operate in the brain, for example, neural oscillations. Previous research has also suggested that while these mechanisms become more language-specific during the first years of life, these mechanisms are highly influenced by the rhythmical properties of the environment. It is still unclear, however, how the rhythmical properties of the environment influence caregiver-child interactions. Specifically, whether the temporal regularities found in child-directed speech (CDS) can influence caregiver-child interactions, as in whether their inter-personal neural synchrony is driven by those regularities or are separate mechanisms. This project aims to understand whether and how the temporal regularities of CDS are related to neural synchrony in child-caregiver interactions, by decomposing the acoustic signal into spectral-amplitude modulations, stress rate, and syllable rate and correlating those with the time-frequency information of neural synchrony.

  PublisherDOI

• Exploring variation in infants’ preference for infant-directed speech: Evidence from a multi-site study in Africa

  2026-04-13

  articleOpen access

  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.

  PublisherDOI

• Children explore conservatively when learning novel word extensions

  2025-05-11

  preprintOpen accessSenior author

  Children are active, curious learners. How might children’s curiosity shape their curriculum during word learning? Past research suggests that children’s tendency to explore can lead them to discover novel information during learning. This exploratory tendency could be especially useful when learning word meanings: exploring potential meanings for words broadly could help children efficiently probe a word’s possible extension. To investigate this question, we tested how children (5-8 years of age) and adults sample information when presented with a novel word and tasked with uncovering the word’s extension. Overall, we found that children explored novel word extensions conservatively. Children (as well as adults) favored sampling choices that confirmed a novel word meaning, as opposed to exploring broader possible meanings. Younger children’s sampling choices were especially conservative, with children often sampling the narrowest possible generalization option. Older children were more exploratory, probing broader possible word extensions more frequently. Counter to proposals that children are generally more exploratory at younger ages, our results suggest that when children test the extent of novel word meanings, they are often more likely to confirm their hypotheses than to explore.

  PublisherOA PDFDOI

• Exploring the relationship between turn-taking and children’s learning of individual words

  Open MIND · 2025-10-23

  otherOpen access

  Word frequency is consistently one of the most robust predictors of age of acquisition (AoA; Braginsky et al., 2019), but words are heard in contexts that provide differential opportunities for early learning. For instance, hearing words in child-directed speech, compared to overhearable speech, better predicts children’s vocabulary growth (Shneidman & Goldin-Meadow, 2012; Weisleder & Fernald, 2013). The number of conversational turns within child-caregiver conversations is a related, well-established predictor of vocabulary growth (Donnelly & Kidd, 2021; Romeo et al., 2018), but to date, has not been directly linked with children’s uptake of individual words. Here, we ask how word frequency within salient turn-taking episodes is related to children’s word learning. Rather than tracking overall vocabulary growth, we model word-level trajectories, testing whether differences in turn-taking proportions relate to differences in AoA.

  PublisherDOI

• A Systems Framework of Bilingual Language Acquisition: How Development, Experience, and Contexts Interact to Shape Outcomes

  2025-02-21 · 1 citations

  preprintOpen accessSenior author

  Millions of children grow up learning multiple languages, yet their outcomes vary dramatically: some have high proficiency across languages while others have more limited abilities in some languages. This review presents a systems framework for understanding diverse trajectories in bilingual language acquisition. Drawing on systems theories, we examine how multiple levels of influence interact, from individual factors such as maturational processes that lay the foundation, to immediate language experiences with family and educational contexts that provide learning opportunities. These experiences unfold both dynamically over time, and within broader societal contexts that determine language status and community support. The framework reveals how successful bilingual development depends on alignment across system levels: children, equipped with powerful learning abilities, must meet rich and sustained language experiences, as well as supportive social conditions. This approach illuminates systematic patterns in bilingual development, and emphasizes coordinated, multi-level approaches for supporting bilingual development.

  PublisherDOI

• Dynamic Interaction of Affect and Language in Children's Home Environments

  Developmental Science · 2025-11-07 · 1 citations

  articleSenior author

  Emotion and language are very common in young children's everyday lives. Hour by hour, they play, listen, vocalize, react, and emote. Despite the centrality of emotion and language to toddlers' local environments, the dynamic interplay of these communicative signals is practically unexplored. Here, we investigated how fluctuations in caregiver and child affect are linked to caregiver-child communication and children's emerging knowledge of words. Multiday household audio recordings and densely-sampled ratings of affect revealed that, in a US-based sample, children (24-30 months) were more likely to know words that they heard frequently in moments with more positive valence or higher arousal. These moments were also associated with denser communication, suggesting that moments of higher valence or arousal facilitate word knowledge in part by supporting mutually engaging communication. This investigation underscores the importance of natural affective states for understanding how children learn language. SUMMARY: Multiday audio and affect experience sampling revealed that caregivers and children talked more in moments when they experienced higher arousal or more positive valence. Toddlers were more likely to know words heard more often in moments with higher arousal or more positive valence. Associations between affect, language input, and word knowledge highlight the importance of affect as a key factor of the early learning environment.

  PublisherDOI

• The high dimensionality of caregiver-child communication: A commentary on Karadöller, Sümer, and Özyürek

  First Language · 2025-03-17 · 1 citations

  articleSenior author

  Karadöller, Sümer, and Özyürek are doing an important service to the field by emphasizing multimodality in young children’s language learning. They integrate research on speech, gesture, and sign to highlight the independent and combined influence of these modalities on how children learn to communicate. In this commentary, we call for scientists to further broaden the study of natural caregiver-child communication by encompassing a dynamic set of interacting signals that facilitate complex information exchange.

  PublisherDOI

• Infants Do Not Reliably Track When Bilingual Speakers Switch Languages

  Behavioral Sciences · 2025-10-21 · 1 citations

  articleOpen accessSenior author

  It is a widely held belief that bilingual infants benefit from hearing each of their languages spoken by different people, as speakers could serve as a cue for separating the two languages. However, it is not yet known whether infants reliably attend to speaker-specific language use. In four experiments using looking time measures, we asked whether monolingual and bilingual infants in the U.S. could learn pairings between speakers and languages. Infants were first familiarized with two speakers, each using a different language. Then, after infants habituated, the two speakers switched languages, and we measured whether infants showed increased interest in hearing the speakers use a different language. Across all four studies, infants did not show reliable evidence that they detected a change in the language used by individual speakers, suggesting that speaker-language associations may not be a salient source of information for infants.

  PublisherOA PDFDOI

• A Systems Framework of Bilingual Language Acquisition: How Development, Experience, and Contexts Interact to Shape Outcomes

  2025-02-20

  preprintOpen accessSenior author

  Millions of children grow up learning multiple languages, yet their outcomes vary dramatically: some have high proficiency across languages while others have more limited abilities in some languages. This review presents a systems framework for understanding diverse trajectories in bilingual language acquisition. Drawing on systems theories, we examine how multiple levels of influence interact, from individual factors such as maturational processes that lay the foundation, to immediate language experiences with family and educational contexts that provide learning opportunities. These experiences unfold both dynamically over time, and within broader societal contexts that determine language status and community support. The framework reveals how successful bilingual development depends on alignment across system levels: children, equipped with powerful learning abilities, must meet rich and sustained language experiences, as well as supportive social conditions. This approach illuminates systematic patterns in bilingual development, and emphasizes coordinated, multi-level approaches for supporting bilingual development.

  PublisherDOI

Labs

• Princeton Baby LabPI

Awards & honors

• Phi Beta Kappa