contemporary study: brendgen et al (2005)

• aim

  to investigate aggression in MZ and DZ twins to discover the extent to which social and physical aggression can be explained by genetics and environmental influences
• participants

  44 pairs male MZ twins, 50 pairs female MZ twins

  41 pairs male DZ twins, 32 pairs female DZ twins
• procedure- teacher rating

  • 234 pairs of twins selected from the longitudinal quebec newborn twin study (QNTS)
  • study collected data on twins regularly starting from 5 months old till 6yrs old
  • kindergarten teachers rated social and physical aggression of children on a 3 point scale (never, sometimes, often)
• what were the social and physical aggression questions

  social aggression questions - "says mean things and spreads rumours"

  physical aggression questions - "hits, bites and kicks others"
• procedure - peer rating

  • levels of aggression recorded from children's peers
  • each classmate was checked to see if they knew everyone in their class then shown 3 photos of their classmates - had to nominate the 3 children on the page that best fit the description:

  social aggression peer scale - "tells others not to play with a child, tells mean secrets about a child"

  physical aggression scale - "gets into fights, hits, bites and kicks others"
• findings

  54-63% of physical aggression can be explained by genetic factors, whereas social aggression may be better explained by environmental factors so differences in upbringing explains behaviour
• conclusion

  • data suggests that some genetic factors predispose some children to be aggressive
  • environmental factors lead to specific forms of aggression to manifest (copying what they see at home)
  • children may be predisposed to being physically aggressive but learn hitting others is not right so they adopt methods such as being socially aggressive
• strength

  • ratings of aggression utilised from multiple sources
  • teacher and peer ratings - if there was a large difference between 2 ratings, it would highlight biases the teachers/peers had against the children
  • as 2 sets of ratings were similar, provides inter-rater reliability and eliminate biases
• weakness (counterpoint)

  • existence of other potential extraneous variables
  • 409 classrooms being observed extraneous variables such as: gender, culture, life events can affect a child's aggression
  • aggression displayed could be due to factors other than genes and parents
• strength

  • findings have real life application that can lead to effective interventions to reduce aggression
  • children in that age group (5 months - 6yrs) would begin to transition from physical to social aggression
  • therefore, if interventions took place to control a child's physical aggression it may block their development into using social aggression so reducing both aggression types
• weakness

  • cant generalise interventions for aggression as age group is highly specific
  • 5 months - 6yrs sample and aggression would look like bites, hits or spreading rumours
  • therefore, findings of genes impacting aggression may not accurately reflect aggression on adults