Brendgen et al.

• Aim: Investigate origins/causes of social aggression, and the contributions of genes compared with physical aggression; said no study has looked at if social aggression comes from genes, shared environment or non-shared environment
• Studies show up to 50% of physical aggression is down to genes (varying between 40 and 80%, with an average of 50%) and the other 50% is down to non-shared environments outside the family
• Social aggression is characterised by socially manipulative behaviour, such as ignoring others, spreading rumours or making threats to withdraw friendships; it can be expressed overtly or covertly
• 3 key aims; to see: if social aggression is caused by genes or the environment, if social aggression shared the same cause as physical aggression, and if one type of aggression leads to another kind - social to physical or vice versa
• Sample: 234 twin pairs from Quebec Newborn Twin Study in Canada, all born between November 1995 and July 1998, MZ (Identical): 44 males, 50 females; DZ (non-identical): 41 males, 32 females and 67 mixed
• Data was gathered longitudinally on the sample at 5, 18, 30, 48, 60 months and finally at 6 years old, the researchers used the final data set in this study
• Assumed MZ and DZ twins share equal environments; only difference is their genes (but is this really true? School? Favouritism?), parental income, education, age and marital status (also when twins were born) were all recorded
• Data consisted of 2 ratings of each twin’s behaviour by their teacher and classmates, gathered in the Spring term so the twins were known to those providing the ratings
• Teacher ratings based on agreement with items (on a 3-point likert scale) from the Preschool Social Behaviour Scale and Direct and Indirect Aggression Scale, e.g. ‘To what extent does the child try to make others dislike a child’ (social aggression) or ‘To what extent does the child get into fights’ (physical aggression)
• Peer ratings were done by classmates circling 3 photos of students in the class, that best matched 4 different behaviour descriptions, e.g. ‘Tells others not to play with a child’ (social aggression) or ‘Gets into fights’ (physical aggression)
• Each twin was given a physical and social aggression score from the teachers’ ratings, and any peer selections for each twin were also recorded
• Brendgen et al. 


  2005
• Brendgen et al. (Background)

  • Claim, no study looked at social aggression origin (genes, shared environment or non-shared environment)
  • Aim
  • Is social aggression is caused by genes or environment
  • Is social aggression shared same cause as physical
  • Does one type lead to other
  • Sample
  • 234 twins from Quebec
  • Born November 1995 - July 1998
  • MZ (Identical): 44 M, 50 F
  • DZ (non-identical): 41 M, 32 F + 67 mix
  • Data gathered longitudinally at age 5, 18, 30, 48, 60 mths + 6 yrs
  • Used final data set
• Brendgen et al. (2005)

  • Assumed MZ + DZ twins equal environments; only diff is genes
  • Parental income, education, age + marital status recorded
  • Data was 2 ratings o twin’s behaviour by teacher + classmates
  • Gathered in Spring so twins known
  • Teacher ratings based on agreement (3-point likert scale); Preschool Social Behaviour Scale + Direct and Indirect Aggression Scale
  • Peer ratings done by classmates circling 3 photos of peers, that best matched 4 behaviour description
  • Each twin got physical + social aggression score from teacher ratings
  • Any peer selections recorded