We investigated the role of socially contingent interactions between infants and their parents in shaping infants’ language development. This hypothesis is motivated by a broader theory that socially contingent interactions lead to a qualitative shift in infants’ understanding of the nature of words. We predicted that infants of ‘more contingent’ parents would experience this shift sooner and will have larger vocabularies (a proxy measure) than infants of ‘less contingent’ parents. In contrast to previous investigations, we evaluated the unique contribution of social contingency, separate from the effects of language input and attentional spotlighting. We annotated a longitudinal video-corpus of naturalistic parent-infant interactions when infants were 9 and 12 months to predict their vocabulary at 12 and 18 months. We showed that infants’ experience with social continency is significantly associated with receptive and productive vocabulary size and that these associations are separate from the effects of language input and attentional spotlighting.

How and when do infants gain the ability to learn entirely new information from language input alone? We propose that this ability requires that infants form a referential link between words, real-world objects or events, and mental representations of such objects or events. To investigate when this link emerges, we designed a novel paradigm that takes advantage of semantic priming and ostensive labeling. We showed that by 15 months, infants possess this referential link: they form representations of objects they have never seen and learn their names. Fourteen-moth-olds, however, failed on this task. This failure is surprising because prior research showed that infants at this age comprehend adults’ speech about hidden objects. New data from a follow-up study shows that bolstering younger infants’ vocabulary does not help them succeed on this task, suggesting that perhaps the referential link between words, referents and mental representations is fragile before 15 months.

Human language permits us to call to mind objects, events, and ideas that we cannot witness directly. This capacity requires that one links words not only to their referents, but to mental representations of those referents. Together with the recognition that words are used intentionally for communication, this link constitutes ‘verbal reference.’ Although the development of verbal reference is a pivotal achievement, questions concerning its origins remain. To address this gap, we investigate infants’ ability to establish a representation of an object that is hidden from view based on language input and to learn its name.

A proto-understanding of absent reference (reference to absent entities) emerges around 12 months provided with richcontextual support, infants look and point to the location of a displaced object. When can infants understand absent ref-erence without contextual support? We modified the procedure from Hendrickson and Sundara (2017) who showed thatwith very minimal pre-exposure and no demonstration of referent displacement, 14-month-olds identify absent referents offamiliar words. Fourteen- and 16-month-olds first listened to passages containing target words, while viewing a checker-board. Then, two objects the referent and a distractor appeared on the screen. We analyzed infants’ looking to the targetduring 3 seconds from the onset of image display. Only 16-month-olds looked significantly above chance, suggestingthat listening to the passages activated their representations of the referents. These results are the first to show that by 16months, infants can retrieve mental representations of objects upon just hearing their labels.

A nascent understanding of absent reference emerges around 12months: provided with rich contextual support, infants look and pointto the location of a displaced object. When can infants understandabsent reference without contextual support? Using a proceduremodified from Hendrickson and Sundara (2017), 13- and 16-month-olds first listened to utterances containing familiar target words, whileviewing a checkerboard. Then, two objects – a referent and a distractor(e.g., a cup and a shoe) – appeared on the screen. Only 16-month-oldsdemonstrated a reliable looking preference for the referents, suggestingthat listening to the utterances activated their mental images of thereferents. These results establish that at 16 months, infants comprehendreference to absent entities without any contextual support.