Introduction

Ostracism

Social interaction plays an important role in our daily life. Therefore, social exclusion or ostracism—being ignored and excluded from social groups—can threaten primary needs such as belonging, self‐esteem, and a sense of meaningful existence across the life span (Williams 2007; Williams et al. 2000). This phenomenon commonly occurs across the lifespan from infancy to adulthood and in different contexts (e.g., school, romantic relationships, and workplace) (Abrams et al. 2011; Masclet 2003; Testa et al. 2025; Wölfer and Scheithauer 2013; Zadro and Gonsalkorale 2014). In research, the effects of ostracism have been examined using a ball‐tossing game and its online version (Cyberball paradigm), where two experimenters play a ball game with participants and manipulate inclusionary states of participants by either tossing a ball towards them or ostracizing them from the game (Williams et al. 2000).

Summary

This study demonstrates that infants as young as five months old exhibit neural sensitivity (higher theta power) to ostracism.Caregivers showed no changes in theta power in response to ostracism, suggesting effective emotion regulation abilities, possibly in place to downregulate infants’ affective responses.Infants and caregivers showed behavioural synchrony during ostracism, but neural synchrony depended strongly on the specific context.These findings suggest a co‐regulation process during ostracism, highlighting flexible modulation of synchrony rather than consistently higher synchrony as key for effective interaction.

Behavioural Responses to Ostracism in Infancy

Most research has focused on the effect of ostracism on children, adolescents, and adults. Evidence suggests that infants also experience different forms of ostracism early in life, not only from their caregivers (e.g., through separation) but also in interactions with their peers (Hart and Legerstee 2010). Prendergast (2019) investigated how infants between 7 and 9 months react to seeing others being ostracised: They found that infants gazed differently at animations depicting inclusion compared to exclusion. But how do infants react when they are ostracised themselves? To our knowledge, this has never been investigated in infants under 1 year of age. According to Williams’ Temporal Need‐Threat Model (Williams 2007, 2011), individuals experiencing ostracism first reflexively respond with negative emotions (Step 1). Following this, they evaluate the situation (Step 2) and may show either prosocial behaviours, such as attention‐seeking or antisocial behaviours, such as aggression in the reflective stage. Quadrelli et al. (2023) confirmed that 13–14‐month‐old toddlers display negative emotionality in response to ostracism. They also showed increased visual attention and attention‐seeking behaviours, suggesting that they may attempt to reconnect with other players to compensate for their unmet needs. But how early are infants sensitive to ostracism? In the present study, we adapt Quadrelli et al.’s approach to test whether younger infants exhibit similar responses to ostracism.

Neural Responses to Ostracism in Infancy

Responses to ostracism are evidenced not only at the behavioural level but also at the neural level. Several studies highlighted the important role of the medial frontal cortex in regulating negative affect in response to ostracism (Gunther Moor et al. 2012). Quadrelli et al. (2025) showed that 13‐month‐old infants showed increased Nc (indicator of attention allocation) event‐related potentials (ERPs) to emotional pictures after experience of exclusion, suggesting an effect of ostracism on social information processing and the early emergence of a social monitoring system that may facilitate the detection of opportunities for re‐affiliation or potential social threats. Another important neural indicator of ostracism is theta power, which is increased during ostracism (Tang et al. 2021; Van Noordt et al. 2015a). Theta power appears to reflect distinct cognitive‐affective processes over time, with early increases associated with the monitoring of social expectations and the detection of expectancy violations, and later increases related to the appraisal and regulation of the emotional impact of ostracism (Bekkedal et al. 2011; Cristofori et al. 2013). Theta activity is also related to processing error/conflict monitoring (Trujillo and Allen 2007), self‐regulation (Ertl et al. 2013; Knyazev 2007), stress, and anxiety (Sreekrishnan et al. 2014). Given that many of these processes are involved is ostracism, theta power can serve as a neural indicator of ostracism (Van Noordt et al. 2015b). However, to date, no study has examined whether ostracism modulates theta power in early infancy. As infants are pre‐verbal, they cannot explicitly be asked about their ostracism experience. Therefore, non‐verbal measures such as EEG (Kulke et al. 2016) can provide relevant information in infants. The current study aims to examine how infants process ostracism and to investigate whether theta power is modulated by early ostracism experiences.

Vicarious Ostracism and Neural Synchrony

Ostracism affects not only victims but also observers (Giesen and Echterhoff 2018; Wesselmann et al. 2009). Previous research has shown the mere observation of ostracism induces negative emotions, activation in the same brain areas as victims, and increased heart rate and skin conductance (i.e., signs of stress) (Coyne et al. 2011; Eisenberger and Lieberman 2004). Since infants and caregivers share a special emotional connection (Bowlby 1958), we predict that ostracism may affect both infants and their caregivers. This is particularly relevant, as caregivers play a crucial role in infants’ emotion regulation as an external regulator (Feldman 2012) and contingent maternal responsiveness helps infants regulate their negative emotions (Bozicevic et al. 2025; Lowe et al. 2012). In a co‐regulation process, caregivers help their infants to downregulate strong emotions, such as potential negative feelings during exclusion (Feldman et al. 1999; Somers et al. 2022). Although parent–child behavioural synchrony and its association with children's emotion regulation have been well documented (Feldman 2015), little is known about the neural mechanisms underlying these processes, especially during ostracism. According to Konvalinka and Roepstorff (2012), such a neural synchronization mechanism may not solely arise from behavioural coupling but may also reflect inter‐individual top‐down modulations. Electroencephalography (EEG) Hyper scanning allows us to measure responses of multiple brains simultaneously (Montague 2002; Turk, Vroomen et al. 2022) and therefore can uncover this co‐regulation process by mapping contingencies between parents’ and infants’ neural regulation. For example, Liu et al. (2024) showed that neural synchrony between parents and children is associated with shared positive affect and parental emotional warmth. This synchronous coordination supports interpersonal emotional co‐regulation, which in turn promotes the child's ability to self‐regulate emotions and adapt physiologically to social stress (Ambrose and Menna 2013; McDonald and Perdue 2018). In the current study, we investigate the inter‐brain synchrony during ostracism. As brain synchrony is affected not only by intrinsic neural processes, but also by dyad‐specific factors such as attachment quality and parental bonding (Turk, Endevelt‐Shapira et al. 2022), we also investigate how variations in parental bonding affect the neural synchrony.

Current Study

In the present study, we applied EEG hyperscanning to measure neural synchrony of caregivers and infants in frontal/mid‐frontal regions during the ball‐tossing game and its association with parental bonding. Caregiver‐infant dyads played a ball‐tossing game which consisted of three conditions; the inclusion condition (dyad and experimenters have equal opportunity to toss the ball), the exclusion condition (two experimenters ignore the dyad and toss the ball to each other), and the reinclusion condition (two experimenters again toss the ball to the dyad to reinclude them). Moreover, behavioural responses of caregivers and infants were recorded, including emotional expressions, attention‐seeking behaviours, visual attention, and social referencing. We predicted (https://osf.io/dzh9s/overview) that: (1) Infants and caregivers show higher theta power during the exclusion condition compared to the inclusion condition, respectively; (2) Caregiver–infant dyads exhibit enhanced theta‐band neural synchronization compared to permuted dyads and control conditions; and (3) The higher the caregiver's parental bonding score, the more strongly the synchronization between caregiver‐infant dyads. We furthermore expected that (4) negative emotionality increases and positive emotionality decreases in the exclusion condition compared to the inclusion condition in both caregivers and infants, indicating a sensitivity to ostracism; and (5) There is a positive correlation between infants’ and caregivers’ positive and negative emotionality. Regarding attention‐seeking behaviour, we expected that (6) Infants’ and caregivers’ visual attention and attention‐seeking behaviours differ between the inclusion and exclusion conditions, indicating a sensitivity to ostracism; and (7) There is a positive correlation between Infants’ and caregivers’ visual attention and attention‐seeking. Finally, we expected social referencing to occur, with (8) infants and caregivers showing more social referencing in the exclusion condition compared to the inclusion condition.