Mother-infant behavioral synchrony and maternal cortisol accumulation predict later infant hair cortisol levels

Early-life exposure to sustained stress is associated with several adverse developmental outcomes. Research on acute cortisol measures has identified maternal cortisol regulation and behavioral synchrony - defined as the coordination of mother–infant behaviors during interactions - as important contributors to infant stress regulation. However, their interplay with chronic stress, as indexed by hair cortisol concentration (HCC), remains unexplored. This study investigated how maternal HCC and mother-infant behavioral synchrony relate to infants’ HCC accumulation from 9 to 12 months. Behavioral synchrony was assessed at 6 months of age (N=76 dyads). Video-recorded 3-minute interactions were coded using the Coding Interactive Behavior system, yielding composites of maternal sensitivity, intrusiveness, and dyadic reciprocity. At 12 months, 3-cm hair samples were collected from mothers and infants to assess cortisol accumulation over the previous three months. A General Linear Model showed that lower maternal HCC and higher maternal sensitivity predicted lower infant HCC, while the effect of maternal intrusiveness on infant HCC was fully accounted for by socioeconomic status. The effect of dyadic reciprocity was marginally significant. These findings highlight maternal chronic cortisol and behavioral synchrony as independent predictors of infants’ chronic cortisol accumulation, providing novel evidence linking relational processes and chronic stress biomarkers in infancy.