Brendgen et al (2005)

• Aim

  To find out if there is a difference between physical and social aggression in 6 year old school students, whether physically aggressive students can also be socially aggressive, and whether it is linked down to genetics or situation
• Sample

  • 234 pairs of twins from the Quebec newborn twin study
  • 44 identical male (MZ) twin pairs
  • 50 identical female (MZ) twin pairs
  • 41 non identical (DZ) male twin pairs
  • 32 non identical (DZ) female twin pairs
  • 67 non identical (DZ) mixed gender twin pairs
• Procedure

  • Longitudinal study
  • Teachers were asked to rate students on a scale on 6 statements with 0 = never, 1 = sometimes, 2 = often
• social question for teachers

  • Try to make others dislike a child
  • Says bad things or spread rumours about a child
  • Becomes friends with a child for revenge
• physical questions for teachers

  • Gets into fights
  • Physically attacks others
  • Hits, bites, or kicks others
• social questions for peera

  • Tell others not to play with a child
  • Tell mean secrets about another child
• physical questions for peers

  • Gets into fights
  • Hits, bites, or kicks others
• Results

  • strong correlation between teacher and student ratings
  • teachers rated boys aggression as more physical, and girls more social
  • peers rated boys as both more physically and socially agressive
  • MZ twins had very similar behaviour, r = 0.79
  • similar correlation between MZ and DZ twins in social aggression
• Conclusion

  physical aggression is mostly genetics, social aggression is mostly environmental influence

  • children who are physically aggressive are more likely to be socially aggressive as well
• Grave

  large sample, representative of investigated age group, but not the whole world
• gRave

  • standardised questionnaire, easily replicable, but the original language was French, translation may be unreliable
  • strong correlation between teacher and peer ratings
  • zygocity allocation was based on appearance, not reliable
• grAve

  if social aggression is linked to nurture, then early intervention should be possible
• graVe

  • twin studies are a valid way of studying nature vs nurture, MZ twins are 100% genetically similar, DZ only 50%
  • natural experiment does not explain cause and effect
• gravE

  • presumptuous consent from parents
  • children looked at their classmates photos and pointed to the most 'aggressive looking' ones, could impact friendships
  • this research maximises benefits so it was worth it