ARTICLES: SPECIAL SECTION: MULTIMODAL TREATMENT STUDY OF ADHD-MTA
Kindergarten Children's Genetic Vulnerabilities Interact With Friends' Aggression to Promote Children's Own Aggression

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ABSTRACT

Objective:

To examine whether kindergarten children's genetic liability to physically aggress moderates the contribution of friends' aggression to their aggressive behaviors.

Method:

Teacher and peer reports of aggression were available for 359 6-year-old twin pairs (145 MZ, 212 DZ) as well as teacher and peer reports of aggression of the two best friends of each twin. Children's genetic risk for aggression was based on their cotwin's aggression status and the pair's zygosity.

Results:

Children's aggression was highly heritable. Unique environment accounted for most of the variance in friends' aggression, although there was also a small genetic contribution (15%). Both genetic liability to aggression and having aggressive friends predicted twins' aggression. However, the contribution of aggressive friends to children's aggression was strongest among genetically vulnerable children. This result was similar for boys and girls, despite sex differences in both aggression and the level of aggression of friends.

Conclusions:

Affiliation with aggressive friends at school entry is a significant environmental risk factor for aggression, especially for children genetically at risk for aggressive behaviors. Developmental models of aggression need to take into account both genetic liability and environmental factors in multiple settings, such as the peer context, to more precisely describe and understand the various developmental pathways to aggression. The implications for early prevention programs are discussed.

Section snippets

Subjects

Participants were part of an ongoing longitudinal study of a population-based sample of twins from greater Montreal, Canada (Quebec Newborn Twin Study [QNTS]) who were recruited at birth between November 1995 and July 1998 (N = 648 twin pairs). Zygosity of same-sex twin pairs was assessed through detailed ratings at 18 months (Goldsmith, 1991) and confirmed in 94% by genotyping (Forget-Dubois et al., 2003). Most families (84%) were of European descent. The sample was followed longitudinally at

Descriptive Statistics

Mean (SD) aggression scores were 0.43 (1.84) for males and −0.66 (1.07) for females (F1,713 = 93.47; p <.001). Mean friends' aggression scores were 0.19 (0.77) for males and −0.39 (0.49) for females (F1,689 = 141.02; p <.001). Forty-eight MZ twins (42% males) and 95 DZ twins (40% males) were considered at high risk, whereas 242 MZ (49% males) and 329 DZ twins (53% males) were at low risk. Logistic regression analyses revealed that MZ and DZ twins were equally likely to score at or above the

DISCUSSION

High levels of aggression in early elementary school predict chronically high aggression through adolescence and related maladjustment problems such as delinquency (Broidy et al., 2003, Nagin and Tremblay, 1999). The goal of this study was to examine whether the genetic liability to aggression moderated the association between having aggressive friends and displaying aggression among kindergarten children. We found that both genetic liability and having aggressive friends were generally

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    This research was supported by grants from the National Health Research Development Program, the Social Sciences and Humanities Research Council of Canada, the Canadian Institutes of Health Research, the Canada Research Chair program, the Fonds Québécois de la Recherche sur la Société et la Culture, and the Fonds de la Recherche en Santé du Québec. The authors are grateful to the participating families as well as to the authorities, directors, and teachers of the participating schools. They also thank Jocelyn Malo for coordinating the data collection, and Hélène Paradis, Bei Feng, Bernadette Simoneau, and Jacqueline Langlois for their assistance in data management and preparation.

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