Discussion

The aim of the current study was to investigate the effect of ostracism on theta power, inter‐brain synchrony and affective and behavioural responses in infant‐caregiver dyads using the ball‐tossing game paradigm. To our knowledge, the current study is the first study to investigate infants’ responses to cues of ostracism experience at such an early age and to map how caregivers and infants synchronize at both neutral and behavioural levels in response to ostracism. Results showed that infants’ (but not caregivers’) theta power is higher in the exclusion block. Moreover, although both infants and caregivers showed affective and behavioural responses to ostracism, neural synchrony was not generally higher in ostracism compared to control conditions and it was highly dependent on the context. This may reflect a flexible modulation of neural synchrony in line with emotional co‐regulation processes.

Theta Power and Alpha Power

In accordance with our hypotheses, infants did show higher theta power in exclusion compared to not my turn and inclusion trials, and also higher alpha power in exclusion compared to not my turn trials. This is in line with previous research in populations of older children. For example, Tang et al. (2019) and Van Noordt et al. (2015b) also found that children showed heightened theta power in response to ostracism. While previous research has shown that thirteen‐ to fourteen‐month‐old infants exhibit behavioural responses to ostracism (Quadrelli et al. 2025), our findings are the first to demonstrate that even younger infants show early neural responses to ostracism, particularly in left/medial frontal regions.

Interestingly, in contrast to our hypothesis, there were no differences in alpha and theta power for caregivers in exclusion trials. Previous research findings were mixed: Cristofori et al. (2013) showed that adults showed higher theta power during exclusion, whereas Tang et al. (2019) observed higher theta power in response to not my turn events compared to exclusion events. At the behavioural level, Svetieva et al. (2019) also found that adults exhibit Duchenne smiling during exclusion, reflecting up‐regulation of positive emotion. Therefore, our findings confirm age‐related differences in neural responses to ostracism and further suggest that the presence of infants may activate caregivers’ regulatory responses, as discussed below.

To investigate which behavioural and affective responses map onto theta and alpha power, correlation analyses between neural responses and affective and behavioural responses were conducted. No significant results were found. This may suggest that alpha and theta power reflect a wide range of cognitive processes including a holistic cognitive evaluation of exclusion or monitoring and adjusting behaviours in response to conflict and implicit self‐control (Adamczyk and Wyczesany 2023; Cohen 2011; Cohen and Donner 2013), as all these processes can be evoked during exclusion rather than reflecting explicit affective responses in participants.

Inter‐brain Synchrony

Although at the individual level, findings showed that ostracism increased infants’ theta power, our hypothesis that ostracism would enhance inter‐brain synchrony between infants and caregivers was not confirmed. We found no significant differences in neural synchrony during ostracism compared to control and permuted conditions. However, PLV was higher in the main condition compared to caregiver and video‐control conditions only during not my turn trials. In contrast, behavioural analyses indicated that caregivers were sensitive to their infants’ ostracism, as reflected in synchrony in emotional expressions, social referencing, and visual attention. This dissociation between behavioural and neural synchrony may indicate a co‐regulation process—how caregivers adjust their internal states to support infants’ emotional balance and emerging self‐regulation (Feldman 2003; Butler and Randall 2012). Consistent with this, caregivers may downregulate negative emotions to buffer infants’ distress, which could reduce neural synchrony despite behavioural alignment. Prior work also suggests that synchrony may vary across levels (e.g., neural vs. physiological) and that flexible shifts between synchronized and desynchronized states may be adaptive (Wass et al., 2019). Importantly, this study is the first to examine infant–caregiver neural synchrony during ostracism, and further research is needed to replicate and clarify these findings.

Regarding the association between parental bonding and neural synchrony, the results showed that neural synchrony can be influenced by traits that dyads can bring to their interactions, with parental bonding being particularly influential in the “not my turn” trials. Although there was no significant association between PLV and parental bonding in exclusion and inclusion conditions, they showed the same trend, suggesting that when the parental bonding is better, the neural synchrony is higher (Ibáñez et al., 2017; Nguyen et al. 2020).

Affective and Behavioural Outcomes

As expected, infants showed more negative emotionality in the exclusion compared to the inclusion conditions, showing that infants as early as 4 months of age are sensitive to cues of ostracism experience and show negative emotionality during ostracism. Although alternative explanations for these findings could be proposed, they appear unlikely. For example, higher negative emotionality in ostracized infants could reflect boredom (Quadrelli et al. 2025). However, our results on attention‐seeking behaviours, supported by Bayes factors, show a trend for increased attention‐seeking during exclusion, suggesting infants’ responses reflect social sensitivity to ostracism rather than mere boredom. In terms of visual attention, infants showed higher attention during inclusion than exclusion, contrary to Quadrelli et al. (2025). Humans respond differently to ostracism depending on perceived opportunities for reaffiliation (Lyyra et al. 2017). In Quadrelli et al. (2025), experimenters smiled during exclusion, which may have signalled reengagement and increased infants’ attention. In contrast, experimenters in our study maintained neutral facial expressions. Consistent with this, infants showed increased attention during reinclusion, reflecting attempts to reconnect when reaffiliation is possible. Infants also did not exhibit increased social referencing during the exclusion. This null effect may be related to the study setup, as infants were seated on their caregivers’ laps, which may have limited their physical ability to look back toward the caregiver. This result for social referencing persists despite our efforts to code infants’ attempts to look toward the caregiver. Furthermore, the reinclusion phase partially recovered affective responses, with lower negative emotionality and reduced attention‐seeking compared to exclusion. These findings align with previous research in school‐aged children and adults (Tang and Richardson 2013; Zheng et al. 2024).

In line with our hypotheses, we found that caregivers showed less positive affect and more negative affect in the exclusion condition, supporting sharing of emotional states between caregivers and infants (Bowlby 1958). Moreover, caregivers showed more social referencing in the exclusion condition, showing that caregivers refer to their infants in ambiguous situations to respond to them accordingly and regulate their emotions. In terms of visual attention, caregivers, similar to infants, showed more visual attention in the inclusion condition, possibly because they did not receive any opportunity to reconnect and therefore withdrew from the interaction. However, they try to reconnect with others as soon as they see some affiliation cues in the reinclusion condition. Moreover, attention‐seeking behaviours suggest the same pattern; caregivers did not show higher attention‐seeking behaviour during the exclusion than the inclusion conditions, so they might withdraw from the interaction. Furthermore, the reinclusion phase appeared to help caregivers recover from negative emotional states and showed reduced social referencing compared to the exclusion condition. However, reinclusion did not restore positive emotionality or visual attention, nor caregivers’ positive emotionality. Previous research suggests that affective recovery occurs only when reinclusion matches or exceeds pre‐exclusion levels and may depend on individual differences such as self‐esteem (Kuang et al. 2024).

This study was the first to examine behavioural and neural responses to ostracism in 5‐ to 7‐month‐old infants and their caregivers. Combining EEG hyperscanning and behavioural measures allowed us to distinguish explicit and implicit synchrony during negative social experiences. Moreover, control conditions helped ensure that synchrony reflected shared emotional experience rather than joint movement. Some caveats should be considered in the interpretation of our findings. First, due to the live nature of the ball‐tossing game, caregivers supported infants differently during the ball game, which may have influenced infants’ sense of agency and experience of exclusion. In other words, there have been variabilities in how engaged caregivers were in the ball game compared to infants due to the naturalistic nature of the task, which may lead to variability in how intensely such effects are evoked and less neural modulation in response to exclusion among caregivers. Second, although during the debriefing, most of the caregivers reported that they understood that they were excluded during the game, we did not explicitly check the manipulation using a questionnaire. Additionally, not all dyads completed all control conditions due to time constraints. Furthermore, lack of longitudinal data limits our interpretation regarding developmental trajectories. Neural and behavioural responses to ostracism may become more pronounced or become more differentiated later in development. The present study is the first to demonstrate sensitivity to ostracism in infants aged 5 to 7 months, as well as caregiver‐infant coregulation during ostracism, and paves the way for future research to replicate these findings and longitudinally investigate the developmental trajectories. Particularly, we cannot directly conclude that infants can generate subjective feelings or evaluation of ostracism similarly to adults. Instead, our findings provide evidence of behavioural and neural sensitivity to cues associated with ostracism—a longitudinal study could provide further evidence. Given that overt emotional and behavioural responses did not directly map into neural responses during ostracism, neural responses should be interpreted cautiously, and it may show early cognitive evaluation of the situation, which can be a promising direction for future studies. Finally, the medial frontal cluster in our study was based on a 32‐electrode EEG montage, which included a relatively small subset of electrodes compared with previous studies. Therefore, future studies using a higher‐density EEG montage could further improve spatial specificity.

In summary, the present study is the first to demonstrate that infants as young as 5 months are sensitive to cues of ostracism at both neural and behavioural levels. This early sensitivity highlights the importance of emerging regulatory abilities and underscores why infants should not be excluded from social interactions, as such experiences may impact the development of social attention, emotional understanding, and later social, emotional, and cognitive skills. The current findings provide important insights into the early development of infants’ social cognition and into patterns of caregiver–infant coregulation during ostracism.