koledin, myriahPhysiological reactivity and regulation, via measures of vagal tone (a physiological measure), and heart period (HP), and their relation to social behavior as witnessed during lunchtime were examined in preschoolers. Participants included 100 children, 3-6 years of age from a day care center. Vagal tone and heart period were collected during a 25-minute protocol including four conditions. Parent ratings of their child's behavior were collected using the Child Behavior Checklist. Results indicated that for a group of withdrawn children (classified from lunchtime observations), significant correlations were found for HP recovery from a challenge task with total problems, and stability of HP was correlated with internalizing, externalizing and total problems. The stability measure for vagal tone in the HWD group was significantly correlated with externalizing and total problems. It is concluded that children high in withdrawn behavior exhibit a different physiology than those exhibiting low withdrawn behaviors as witnessed during lunchtime observations.en-USThesisPsychology, Developmentalvagal toneinternalizingexternalizinglunchtimephysiological regulation