Early-life trajectories of dyadic synchrony among infants developing autism or concerns for ADHD.

This study followed infants at elevated and average likelihood for autism or ADHD (n = 163) from 6-36 months, classifying them into 36-month outcome groups: Autism (n = 28), ADHD Concerns (n = 20), or Comparison (n = 115). The sample was predominantly male, White, and non-Hispanic. Between 2013 and 2019, caregiver-child dyads played up to five times, with gaze, positive affect, and vocalizations coded from video. Developmental trajectories of dyadic synchrony (the degree of state matching and turn-taking relative to chance) were estimated via mixed-effects models. From 6 to 36 months, overall synchrony was lower in the Autism vs. Comparison group. Within behavior modalities, there were unique patterns of actor responsiveness, with mixed evidence for outcome group differences. Future research should explore links between synchrony, developmental milestones, services, and longer-term outcomes.