Procedure
The Institutional Review Board at [details omitted for double-anonymized peer review] approved all study procedures and all parents provided written consent for their children to participate. Table 1 provides demographic and measure characteristics, respectively. Prior to study visits, families were provided with a social story and habituation protocol to familiarize children with the physiological recording equipment. Caregivers and their children completed two visits to the lab separated by 4-6 weeks. During both visits children completed the MacArthur Reactivity protocol (MRP; Bush et al. 2011). If children were comfortable, ECG was recorded during the MRP. Parents completed the Behavior Assessment System for Children (BASC) Parent Rating Scale – Preschool or Child (PRS-P or PRS-C, respectively).