Attachment theory proposes that children's representations of interactions with caregivers guide informationprocessing about others, bridging interpersonal domains. In a longitudinal study (N = 165), preschoolers (M age = 5.19 years) completed the MacArthur Story Stem Battery to assess parent representations. At schoolage (M age = 8.42 years), children played a virtual ballgame with peers who eventually excluded them to track event-related cardiac slowing, a physiological correlate of rejection, especially when unexpected. At both ages, parents and teachers reported on peer and emotional problems. During exclusion versus inclusion-related events, cardiac slowing was associated with greater positive parent representations and fewer emerging peer problems. Cardiac slowing served as a mediator between positive parent representations and peer problems, supporting a potential psychophysiological mechanism underlying the generalization of attachment-related representations to peer relationships."An unwanted child is likely not only to feel unwanted by his parents, but to believe that he is essentially unwantable, namely unwanted by anyone" (Bowlby, 1973, p. 204).How caregivers relate to children affects how children relate to others. Ample work confirms a robust effect of child-parent attachment on peer relationships (Groh, Fearon, van Ijzendoorn, Bakermans-Kranenburg, & Roisman, 2017). Yet, the issue of why these social domains inter-relate is less often subjected to empirical scrutiny (Thompson, 2016). Many theories assume that children internalize repeated interactions with caregivers, forming representations or internal working models helping them to make sense and predict the future course of interpersonal exchanges (e.g., Bowlby, 1973;Stern, 1985). Looking time studies indeed suggest that Annette M. Klein and Kai von Klitzing shared senior authorship. We are grateful to Robert N. Emde for helpful comments on an earlier draft. The authors would like to thank all families for taking part and the Public Health Department of the City of Leipzig for cooperating in this study. Grants of the German Research Foundation (DFG) to Kai von Klitzing and Annette M. Klein (Grants KL 2315/1-1, KL2315/1-2 and KL 2338/1-2) and the Federal Ministry of Education and Research (BMBF) to Kai von Klitzing, Lars O. White, and Annette M. Klein (01KR1201A and 01KR1802A) supported this research.