A Monozygotic Twin Difference Study of Friends’ Aggression and Children’s Adjustment Problems


  • All authors are also affiliated with the Research Unit on Children’s Psycho-Social Maladjustment and with the Ste-Justine Hospital Research Centre.

    This research was made possible by grants from the Social Sciences and Humanities Research Council of Canada (SSHRC), the Fonds Québécois de Recherche sur la Société et la Culture (FQRSC), the Fonds de la Recherche en Santé du Québec (FRSQ), and the Canadian Institutes of Health Research (CIHR). We thank the children, parents, and teachers who participated in this study.

concerning this article should be addressed to Frank Vitaro, Research Unit on Children’s Psycho-Social Maladjustment, University of Montreal, 3050 Edouard-Montpetit Blvd., Montreal, QC, Canada H3T 1J7. Electronic mail may be sent to frank.vitaro@umontreal.ca.


This study used the monozygotic (MZ) twin difference method to examine whether differences in friends’ aggression increased the differences in MZ twins’ aggression and depressive symptoms from kindergarten to Grade 1 and whether perceived victimization by the friend played a mediating role in this context. Participants were 223 MZ twin pairs. Results showed that differences in kindergarten friends’ aggression significantly predicted an increased difference in MZ twins’ aggression from kindergarten (mean age = 6.7 years) to Grade 1 (mean age = 7.5 years) for both boys and girls. Differences in perceived victimization by the friend mediated this association, albeit only in boys. Differences in perceived victimization by the friend also predicted an increase in MZ twins’ differences in depressive symptoms. These results support the importance of friendship experiences during early childhood.