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Abstract The current study used functional near‐infrared spectroscopy (fNIRS) to investigate whether and how individual differences in positive social engagement among 5‐month‐old (N= 109;N= 35 final sample) infants relate to variability in functional connectivity in the human brain's Default‐Mode Network (DMN). Neuroimaging results showed that on average infants displayed greater functional connectivity in the right than in the left hemisphere of the DMN, adding to prior work indicating faster connectivity development in the right hemisphere. Results did not show any positive associations between our preregistered measures of positive social engagement and functional connectivity in the DMN. However, an additional analysis revealed that higher levels of infants’ smiling and laughter during daily social interactions with their caregivers positively predicted DMN functional connectivity in the left hemisphere. This suggests that individual differences in connectivity in a long‐range brain network implicated in a host of social and cognitive functions are associated with some aspects of infants’ positive social‐interactive behaviors. 

Abstract Infancy is a sensitive period of development, during which experiences of parental care are particularly important for shaping the developing brain. In a longitudinal study ofN = 95 mothers and infants, we examined links between caregiving behavior (maternal sensitivity observed during a mother–infant free‐play) and infants’ neural response to emotion (happy, angry, and fearful faces) at 5 and 7 months of age. Neural activity was assessed using functional Near‐Infrared Spectroscopy (fNIRS) in the dorsolateral prefrontal cortex (dlPFC), a region involved in cognitive control and emotion regulation. Maternal sensitivity was positively correlated with infants’ neural responses tohappyfaces in the bilateral dlPFC and was associated with relative increases in such responses from 5 to 7 months. Multilevel analyses revealed caregiving‐related individual differences in infants’ neural responses to happy compared to fearful faces in the bilateral dlPFC, as well as other brain regions. We suggest that variability in dlPFC responses to emotion in the developing brain may be one correlate of early experiences of caregiving, with implications for social‐emotional functioning and self‐regulation. Research HighlightsInfancy is a sensitive period of brain development, during which experiences with caregivers are especially important.This study examined links between sensitive maternal care and infants’ neural responses to emotion at 5–7 months of age, using functional near‐infrared spectroscopy (fNIRS).Experiences of sensitive care were associated with infants’ neural responses to emotion—particularly happy faces—in the dorsolateral prefrontal cortex. 

Abstract Dorso-medial prefrontal cortex (dmPFC) plays a vital role in social cognition and behavior among humans. Enhanced responses in dmPFC when viewing social scenes predict increased levels of sociability in adults. The current longitudinal study examined the association between dmPFC response and sociability in early development. Brain responses were measured in response to social smiles and frowns using functional near-infrared spectroscopy (fNIRS) at 11 months. Individual differences in sociability were measured using the Early Childhood Behavior Questionnaire (ECBQ) at 18 months. Our longitudinal results show that greater engagement of the dmPFC when processing social smiles, but not frowns, at 11 months predicts higher levels of sociability at 18 months. This demonstrates that early variability in dmPFC response during positive social interactions is linked to individual differences in overtly displayed social behavior. This supports the view that dmPFC plays an important role in social cognition and behavior from early in human ontogeny. 

Abstract The current longitudinal study (n = 98) utilized a developmental cognitive neuroscience approach to examine whether and how variability in social perception is linked to social behavior in early human development. Cortical responses to processing dynamic faces were investigated using functional near-infrared spectroscopy at 7 months. Individual differences in sociability were measured using the Early Childhood Behavior Questionnaire at 18 months. Confirming previous work with infants and adults, functional near-infrared spectroscopy results show that viewing changing faces recruited superior temporal cortices in 7-month-old infants, adding to the view that this brain system is specialized in social perception from early in ontogeny. Our longitudinal results show that greater engagement of the right superior temporal cortex at 7 months predicts higher levels of sociability at 18 months. This suggests that early variability in social perception is linked to later differences in overtly displayed social behavior, providing novel longitudinal evidence for a social brain–behavior association. 

Recent comparative findings by Lewiset al.(Lewis LS, Ritov O, Herrmann E, Reddy RB, Sanchez-Amaro A, Gopnik A, Engelmann JM. 2025 Chimpanzees and children are curious about social interactions.Proc. R. Soc. B.292, 20242242.) reveal a shared drive in chimpanzees and children to seek information about third-party social interactions, even at a material cost. This commentary situates these results within a neurodevelopmental and evolutionary framework, proposing that social curiosity is a manifestation of a deeply conserved, motivationally driven system for social valuation centred on the medial prefrontal cortex (mPFC). We synthesize neuroimaging evidence showing selective engagement of mPFC while infants attend to third-party social interactions. We propose that the mPFC, as a primary valuation hub, guides the specialization of social perceptual regions over development. This framework interprets social curiosity as a high-level cognitive function driven by the mPFC, assigning intrinsic reward value to social information, consistent with the social information hypothesis. The willingness of older children to preferentially orient to and forgo a reward for social information can thus be understood as the behavioural output of a brain system calibrated from infancy to tag such information as intrinsically valuable. 

The medial prefrontal cortex (mPFC) is thought to play a central role in human social perception, cognition, and behavior. In adults, the mPFC is involved in representing and interpreting the mental states in self and others. Developmental research using neuroimaging techniques like functional near-infrared spectroscopy and functional magnetic resonance imaging has begun to extend these findings into infancy. Novel evidence reviewed in this opinion demonstrates that infant mPFC (1) plays a specialized, proactive, and evaluative role in social perception, (2) is involved in connecting with other minds while interacting and when watching other minds interact, and (3) predicts overt social behavior beyond infancy. These findings suggest that, from early in human ontogeny, the mPFC plays a multifaceted role in social perception, cognition, and behavior. 

The notion that the self is fundamentally social in nature and develops through social interactions has a long tradition in philosophy, sociology, and psychology. However, to date, the early development of the social self and its brain bases in infancy has received relatively little attention. This presents a review and synthesis of existing neuroimaging research, showing that infants recruit brain systems, involved in self-processing and social cognition in adults, when responding to self-relevant cues during social interactions. Moreover, this review draws on recent research, demonstrating the early developmental emergence and social embeddedness/dependency of the default-mode network in infancy, a brain network considered of critical importance to the sense of self and social cognition. This stands in contrast to research pointing to the relatively late ontogenetic emergence of the conceptual self, by about 18-24 months of age, as seen in the mirror-self recognition test. Based on this review and synthesis, the social self first hypothesis (SSFH) is formulated, presenting an integrated view, arguing for the early ontogenetic emergence of the social self and its brain basis. This developmental account informs and extends existing evolutionary thinking, emphasizing the primary role that social interdependence has played in the evolution of the human mind.