Brendgen et al (2005) Flashcards
Why was the research carried out?
Most aggression studies focus on physical aggression whereas children tend to show aggression socially which suggest there is a need to understand the factors affecting social and physical aggression
What were the three aims?
- To investigate the extent to which social and physical aggression can be explained by genes and environment
- To investigate whether social and phys aggression are caused by the same genetic and environmental factors
- To investigate whether on type of agg directly influences the second
The study looked for a correlation between
the aggression scores of MZ twins and DZ twins, the teacher ratings and the peer ratings
The study also looked at differences between
the genders and the different types of twins
How were peer ratings obtained?
Via asking children to match 3 photographs of their peers to each of the 4 descriptions (2 for social, 2 for physical)
What is a key advantage of the study in terms of how the data was collected?
Not only was behaviour assessed by teachers, but also peers, who are rarely employed as a reporting source in twin studies
Where were the twins recruited from and when?
Montreal, Canada between 1995-1998
How many children were used?
234 pairs of twins from the Quebec Newborn Study
How many of each twin group were used?
>MZ MALES = 44 >MZ FEMALES = 50 >DZ MALES = 41 >DZ FEMALES =32 >DZ MIXED = 67
How did they control for a language barrier and why did they need to?
Because this was Quebec, some of the schools were English-speaking and some were French-speaking, so they surveys had to be translated and the researchers had to speak both languages.
The teachers’ questionnaires for social aggression where they rated students on a 3 point scale: 0 = never 2 = sometimes 3 = often included statements like…
> tries to make others dislike a child
says bad things or spreads nasty rumors about another child
becomes friends with another child for revenge
The questionnaires for physical aggression for the teachers included statements like…
> gets into fights
physically attacks others
hits, bites or kicks others
How were overall scores obtained?
The scores for social aggression and physical aggression were added together to produce two overall scores.
The 4 descriptions for the children were…
> tells others not to play with a child
tells mean secrets about another child
gets into fights
hits, bites or kicks others
What were the results for physical aggression?
MZ correlations 2X DZ as they were 0.61 for teacher ratings of MZ twins and 0.25 for teacher ratings of DZ twins. Peers showed 0.49 for MZ and 0.28 for DZ. = Genetic contribution
The MZ twins’ correlations for social aggression were similar to the DZ twins’ correlations. This suggests social aggression is less linked to genetics, because
both types of twins seemed to be equally affected by their surroundings, unlike with physical aggression. (Teacher ratings for MZ 0.35 and 0.34 for DZ. )
Brendgen concludes that about 50-60% of physical aggression can be linked to genes, since it was shared by MZ twins but not so much by DZ twins. For social aggression,
genes only seem to account for about 20%.
So physical aggression is mostly nature (genes), but social aggression
is mostly nurture (environment).
The overlap between social and physical aggression was only moderate. Brendgen proposes that genes might give children a general predisposition towards aggression, but
this only becomes social aggression if they have an environment that encourages it.
Other findings included:
- High level of physical aggression = high level social
- Boys more physically aggressive
- Girls more socially aggressive
Brendgen uses a large sample (234 twin pairs), so unusual children (anomalies) with very high/low levels of aggression are to be
“averaged out” by the size of the data. This makes the sample representative.
Brendgen uses established questionnaires to measure aggression. What were these questionnaires and why were they beneficial?
Preschool Social Behaviour Sale and Direct and Indirect Aggression Scale - reliable as other researchers can replicate using the same scales
Two researchers visited each classroom,
suggesting the study has inter-rater reliability.
There was a strong correlation between teacher- and peer- ratings,
suggesting the scores are reliable
The allocation of zygocity (MZ and DZ categories) was based largely on their appearance and wasn’t 100% reliable. In particular
it is possible there were DZ twins in the MZ condition.
The study avoids taking a reductionist view of human behaviour: it looks at genetics but also at environmental factors as
It links in with the findings about aggression by Bandura (who claims it is learned) and Raine et al. (who suggest it is due to brain structure).
Caution needs to be taken when generalising the results beyond the assessed age as
physical aggression is already diminished at school entry whereas social aggression may not be fully developed until age 8
It has application as if high physical aggression is commonly found with high social aggression
parents should ensure their children are not in an environment where they can learn social aggression, e.g TV
What was the correlation for teacher and peer ratings of physical aggression?
r=0.25 - moderate agreement
What was the correlation for teacher and peer ratings of social aggression?
r=0.33 - moderate agreement
For teacher ratings, the correlation for MZ twins for physical aggression was twice as high as that for same sex DZ physical aggression, what was it?
r= 0.79 - suggesting that mz were displaying very similar behaviour
What were the attrition rates?
2% p/a meaning it is unlikely that this number of leaving twin pairs would have significantly altered results
What was the heritability for teacher ratings of physical aggression compared to non shared environment?
63% for heritability vs 37% for non shared environment suggesting physical is due to genes
What was the heritabilty for social aggression compared to shared environment and non shared?
20% heritability, 20% shared environment vs 60% non shared environment
