Summary Brendgen et al 2005

Contemporary Study: Brendgen et al 2005
Aims:
To investigate aggression in MZ (monozygotic/identical) and DZ (dizygotic/non-identical) twins
to discover:
The extent of social and physical aggression explained through genetic and
environmental influences
Overlap between social and physical aggression

Findings:
Procedure:
Teacher Ratings:
234 pairs of twins were selected from
Quebec New born Twin Study (QNTS). non-
shared
Collected data regularly, beginning at aggressio shared
genetic environm
5 months. n environm
ent
ent
At 6 years, the data was collected and
there was:
44 pairs of male MZ twins social 20% 20% 60%
50 pairs of MZ twins
41 pairs of male DZ twins
phsyical 63% 0% 37%
32 pairs of female DZ twins
67 mixed sex DZ twins - not used
in the final analysis Peer Ratings:
Teacher ratings: kindergarden
non-
teachers rated the social and physical shared
aggressio shared
genetic environm
aggression of each child using a 3 n
ent
environm
point scale. ent

Peer ratings: rated by classmates,
research assistant made sure the child social 23% 23% 54%
could recognise their class. Pictures of
child shown and the children told to
phsyical 54% 0% 46%
match the photo to the behaviour. 2
definitions for social and physical
aggression used.
Physical aggression was mostly explained by
genetic factors and partly by shared
environmental influences.
Moderate significant link between physical and
social - linked to genetic factors.
Statistical testing showed high physical
aggression led to high social aggression.
Boys more physically aggressive, girls more
socially aggressive.