Do Friendships and Sibling Relationships Provide Protection against Peer Victimization in a Similar Way? Véronique Lamarche and Mara Brendgen, Université du Québec à Montréal, Michel Boivin, Laval University, Frank Vitaro and Daniel Pérusse, University of Montreal, and Ginette Dionne, Laval University Abstract Based on the notion that friendship may serve an important protective function against peer victimization, this study examined the moderating effect of reciprocal friends’ prosociality on the link between a child’s reactive aggression and victimization.The study also investigated whether a similar moderating effect could be found with respect to sibling’s prosociality, given that sibling relationships have been found to provide social benefits comparable to friendships. These questions were addressed using a sample of 246 six-year-old twin pairs (246 boys and 246 girls). The results showed that a child’s own reactive aggression uniquely contributed to the risk of victimization for both boys and girls. The link between reactive aggression and victimization was, however, moderated by reciprocal friends’ prosocial behavior and siblings’ prosocial behavior, respectively. The results are discussed in terms of their theoretical and prevention-related implications for children at risk for peer victimization. Keywords: victimization; friends; siblings; prosociality Considerable evidence indicates that approximately 10–15 percent of school-age chil- dren are repeatedly victimized by peers (Kochenderfer & Ladd, 2001; Olweus, 1978; Perry, Kusel & Perry, 1988).Victimization has been found to remain quite stable over time, with the same children enduring these negative peer experiences year after year (Hodges & Perry, 1999; Olweus, 1978). Moreover, a growing corpus of research suggests that children who are maltreated by their peers are at risk for a wide range of psychological, physical, social and academic difficulties (Boivin, Hymel & Hodges, 2001; Rigby, 2001).The recognition that peer victimization may have serious negative consequences for children’s health and well-being has prompted researchers to inves- tigate the factors that place children at risk for peer maltreatment as well as factors that may mitigate this risk. To date, research on this topic has identified several individual and social factors that place children at risk for peer victimization. With regard to individual risk factors, Correspondence should be addressed to Véronique Lamarche, c/o Mara Brendgen, Université du Québec à Montréal, CP 8888, Succ. Centre-ville, Montréal, Québec, H3C 3P8, Canada. Email: Lamarche.Veronique@courrier.uqam.ca © Blackwell Publishing Ltd. 2006. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA.