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This study examined salivary cortisol, a stress-sensitive hypothalamic-pituitary-adrenocortical (HPA) axis hormone in 20 infants (12 females; M age = 10.8 months) and 35 toddlers (20 females; M age = 29.7 months) in full-day, center-based child care. Samples were taken at approximately 10:00 a.m. and 4:00 p.m. at child care and at home. At child care, 35% of infants and 71% of toddlers showed a rise in cortisol across the day; at home, 71% of infants and 64% of toddlers showed decreases. Toddlers who played more with peers exhibited lower cortisol. Controlling age, teacher-reported social fearfulness predicted higher afternoon cortisol and larger cortisol increases across the day at child care. This phenomenon may indicate context-specific activation of the HPA axis early in life.

In this study, we examined a model that describes both direct and indirect pathways between children's temperament and activity of the hypothalamic-pituitary-adrenal (HPA) axis when children are in peer-group settings. We hypothesize that in peer-group settings both shy, inhibited and exuberant, undercontrolled children would exhibit higher cortisol levels, but these associations would operate through different pathways. Sociometric measures of peer rejection, salivary cortisol, and teacher reports of temperament were collected on 82 preschoolers. Children who were rejected by classmates had higher cortisol levels than the other children. The combination of Surgency and Poor Effortful Control (Effortful Control, reverse scored) was associated with elevated cortisol through a pathway mediated by aggressive interactions with peers and peer rejection. With the indirect path explained, the combination of Surgency and Poor Effortful Control also was directly and negatively associated with classroom cortisol levels. These results help explain why temperament associations with HPA activity have been variable and difficult to discern when children are assessed in peer-group contexts. In these contexts, both direct and indirect pathways between temperament and cortisol need to be examined.