brendgen's study

• Aim: to see if there’s a difference in social and physical aggression in 6-year-olds, specifically to see if aggression is down to genetics or environmental factors.
• o   234 pairs of twins were selected from the longitudinal twin study.

  o   Used MZ and DZ twins to see if there’s any difference between them.
• o   Kindergarten teachers rated the social and physical aggression of the children on a 3-point scale, ranging from ever, sometimes, and often. Social aggression questions were “says mean things and spreads rumours” and physical aggression questions were “hits, bites and kicks others”.
• o   Levels of aggression were also recorded from the children’s peers. The children were shown 3 photos of their classmates. They were asked to nominate the 3 children on the page to the description that best fits them. Social aggression was “tells others not to play with a child” and physical aggression was “gets into fights”.
• Findings: Brendgen found that teachers rated the boys more physically and girls more socially aggressive, there was a moderate correlation between the teachers and the students’ responses. 50%-60% of physical and 20% of social aggression can be linked to genes.
• Conclusion: the data suggests some genetic factors predispose some children to be aggressive, particularly physical aggression whereas social aggression is more likely due to environmental effects.
• A strength of this study is ratings from multiple sources were utilised, that being the teacher and the peer ratings, therefore if there were a large difference between the two ratings, it would highlight potential biases the teachers/peers had against the child. As the two sets of ratings were very similar, it provided inter-rater reliability and eliminated the possibility of biases.
• A weakness is that there were 409 classrooms being observed as part of this study, alongside many different children in different circumstances, extraneous variables such as gender and culture could also affect the child’s aggression, which have not been accounted for in this study. Future studies need to use larger samples from more diverse backgrounds before definite conclusions can be drawn regarding the relative contribution of genetic verses environmental effects on social aggression ad potential sex differences in this context.