Social buffering by siblings in childhood and adolescence.

Although positive sibling relationships are associated with better developmental outcomes for children and adolescents, it is unknown whether siblings may be effective social buffers of physiological stress. In addition, there is little research on whether depressive symptoms may moderate the effectiveness of social buffering. The current study examined (1) whether siblings, compared to strangers, buffered the cortisol response to a social evaluative stressor in children and adolescents, (2) whether depressive symptoms moderated this effect, and (3) whether self-reported sibling relationship quality influenced sibling buffering effectiveness. A total of 72 children (9-11 years) and 65 adolescents (15-17 years) were randomly assigned to complete the Trier Social Stress Test-Online with a same-age or older sibling versus a stranger. Participants provided saliva samples to measure cortisol reactivity and recovery across the session and reported on their depressive symptoms and sibling relationship quality. The sibling versus stranger buffering effect varied based on participants' developmental stage (child vs. adolescent) and level of depressive symptoms. Specifically, adolescents with high depressive symptoms who prepared with a sibling showed lower cortisol reactivity than those who prepared with a stranger (b = 0.70, SE = 0.32, p < .05). In contrast, there were no significant differences by condition for children or adolescents with low depressive symptoms. For individuals who prepared with a sibling (n = 69), higher alienation predicted impaired cortisol recovery post-stressor (b = 0.32, SE = 0.18, p < .05). These findings are in line with research on normative development of sibling relationships and cortisol activity in the context of depression from childhood to adolescence.