آیا هنجارهای گروه همسالان بیان خطر ژنتیکی برای خشونت را تعدیل می کند؟

Purpose Using a genetically informed design based on 192 Monozygotic and Dizygotic twin pairs assessed in kindergarten, this study examined whether the expression of genetic risk for physical aggression or for relational aggression varies depending on the peer group’s injunctive behaviour norms. Methods Physical aggression and relational aggression, as well as injunctive peer group norms in regard to these behaviours, were measured via peer nominations in the twins’ kindergarten classes. Results Peer groups varied considerably in terms of the level of acceptability of both physical and relational aggression. Bivariate Cholesky modeling revealed a significant gene-environment interaction, indicating that a strong genetic disposition for physical aggression was much more likely to be expressed when peer group injunctive norms were favourable to such behaviour. In contrast, genetic factors essentially played no role in explaining inter-individual differences when peer group norms discouraged physical aggression. Relational aggression was generally less explained by genetic influences and more by environmental influences regardless of peer group norms, but environmental influences became even more important when peer group norms favoured such behaviour. Conclusions These findings speak to the importance of the peer group in shaping aggression already in young children by either condoning or penalizing such behaviour.

Aggressive behaviour during childhood is a major risk factor for subsequent developmental maladjustment. For a long time, attempts to understand and prevent childhood aggression have focused on physical aggression. However, already preschool aged children can also seriously hurt their peers through more subtle forms of aggression, for example through spreading vicious gossip and rumors, writing or saying hurtful things, exploiting personal secrets, and ignoring, alienating, and isolating others. While different terms have been used to describe these more subtle forms of aggression (e.g., relational, social, or indirect aggression) all refer to attacks on the self-esteem or on the social relationships and reputation of the victim ( Archer & Coyne, 2005). Whereas physical aggression is more prevalent among boys, the opposite pattern has been found – albeit not always consistently - for girls ( Card, Stucky, Sawalani, & Little, 2008). Many aggressive children seem to use both forms of aggression, however, with correlations ranging between r = .4 and .8. Despite this overlap, physical and relational aggression have been found to represent two factorially distinct forms of aggression, whose measurement structure remains stable from the pre-school years through early adolescence ( Vaillancourt, Brendgen, Boivin, & Tremblay, 2003). Notably, relational aggression is as harmful as physical aggression with victims suffering a host of deleterious effects, including anxiety, depression, and even suicide ideation ( Crick and Grotpeter, 1996, Owens et al., 2000 and Paquette and Underwood, 1999). Emphasized by several highly mediatized suicides of young victims of relational aggression, the devastating consequences of relational aggression have led to the inclusion of relational aggression in the legal definition of bullying and the implementation of strict anti-bullying legislations that hold the perpetrators legally responsible for their actions ( U.S. Department of Education, 2011 and Saint-Cyr, 2012).

Using the Mplus software package (Muthén & Muthén, 1998-2010), preliminary analyses revealed that girls were significantly more relationally aggressive (χ2 (2) = 8.65, p = .01) but less physically aggressive (χ2 (2) = 44.01, p = .00) than boys. No sex mean differences emerged in regard to peer group norms (χ2 (2) = 1.39, p = .50 for the physical aggression norm and χ2 (2) = 1.27, p = .53 for the relational aggression norm). Moreover, no mean differences in regard to the study variables were found between MZ twins and DZ twins (χ2 (1) = .03, p = .87, for physical aggression, χ2 (1) = .05, p = .83, for relational aggression; χ2 (3) = .00, p = .95, for the physical aggression norm; χ2 (1) = .35, p = .55, for the relational aggression norm). To account for the sex differences in physical aggression and relational aggression, these two variables were z-standardized within sex groups for subsequent analyses. Additional analyses were performed to test potential sex differences in the variance-covariance structure of physical (or relational) aggression and the corresponding peer group norms. These analyses, which were run as a four-group model with equality constraints across sex groups but with freely estimated parameters across MZ and DZ pairs, revealed no significant difference between boys and girls (χ2(8) = 4.424, p = .37 for the covariance structure of physical aggression and physical aggression norms and χ2(8) = 3.30, p = .51 for the covariance structure of relational aggression and relational aggression norms). Data were therefore pooled combining male and female MZ pairs and combining male and female DZ pairs, respectively, to maximize statistical power.