The Stress of Being Left Out: Physiological and Behavioral Responses to Ostracism in Preverbal Infants.

Being ostracized is a stressful experience that affects behavioral and physiological reactivity in adults. However, evidence regarding its impact early in life remains limited. This study investigated whether inclusion or exclusion during a ball-tossing game influenced physiological and behavioral reactivity in 8-month-old infants (N = 59; 24 girls and 35 boys). It also explored whether parent-infant interaction dynamics and parental mind-attuned comments modulate infants' physiological responses, fostering greater adaptation to the social context. Results highlighted that excluded infants showed reduced heart rate variability (indexed via the Root Mean Square of Successive Differences, RMSSD), reflecting a heightened stress response, compared to included infants. They also displayed lower positive emotionality, higher negative emotionality, and greater active engagement, suggesting that sensitivity to ostracism emerges early in life. Exploratory analyses also revealed a modulatory role of parental behavior in shaping infants' physiological responses. Overall, these findings highlight that, since early life, infants exhibit physiological and behavioral reactions to ostracism, in line with the hypotheses that a system to detect and respond to social exclusion is present since the early stages of development.