Brendan et al 2005

• Aim

  Investigate aggression in MZ (monozygotic, identical) and DZ (dizygotic, non-identical) twins to find out

  Extent to which social & physical agg are explained by genetic and environmental influences (shared + non shared)

  Whether the overlap between social & physical agg id explained by direct affect of one type of agg on the other
• Procedure

  234 pairs of twins from QNTS (Quebec Newborn Twin Study) - twin study design. Data collected from twins regularly, starting at 5 months. Data from age of 6 was main focus of this study

  teachers completed questionnaires for the twins in their class, rated social (eg spreading bad rumours) and physical agg (eg kicking) on three point scale (never, sometimes, often). So, scores for both types of agg were developed for each child
• Procedure 2

  Peers (those in class) rated both types of agg of twins

  Had pics of each child and had to place in one of 4 categories based on descriptions of their behaviour (eg gets into fights or tells others to not play w a child)

  Agg scores developed from peer data
• Findings

  Moderate link in those who show social agg to also show physical agg

  Very few gender diffs in findings

  Correlations for physical agg based on teacher and peer ratings were nearly twice as high for same sex MZ twins than same sex DZ twins - suggests strong genetic contribution to physical agg

  MZ and DZ more similar in correlation for social agg - suggests that, as both share same environment, social agg might come from that
• Findings (stats)

  Only 20% (teacher rating) to 23% (peer rating) of social agg was explained by genetic factors

  Social agg is 54% (peer) to 60% (teacher) explained by non shared environment - suggests social agg stems from this

  Physical agg mostly explained by genetic factors as 63% of teacher rating for genetics

  Stat tests showed high physical agg led to high social agg (not vice versa)
• Conclusions 👻

  Data suggests that genetic characteristics predispose some children to agg behaviour in general (like poor self control)

  +stuff in findings already
• Strength

  Application to reducing agg (embed conclusion here in 8 marker)

  Interventions - P agg leads to S agg so prevent P agg in early years - could ultimately reduce both types

  Brendgen et al suggests that future studies can identify the factors that block the path from P to S agg
• Strength 2 ✌️

  Multiple data sources - used agg ratings from 2 sources (teachers and peers)

  2 sets of ratings were essentially the same - reassures that they are valid and reliable measures go agg behaviour

  genetic contribution to social agg 20% for teachers and 23% for peers (similar)
• Weakness 1 🧑‍🧒

  Ratings may not match reality - eg child may remember one act of physical agg from one of their peers more than another act from another child - not valid way of measuring both types of agg

  Also, they are 6
• weakness 2 🧑‍🧑‍🧒‍🧒

  Potential invalid assumptions - assumed that all MZ and DZ twins experience similar treatment to same extent (environmental influences shared equally for both) - may be wrong

  Eg it is likely that many parents of DZ twins behave more aggressively to one child than there as twins are dissimilar but this much less true for MZ twins who are more similar (???)

  Undermines validity of study