Brendgen et al (2005)

Question 1

Why was the research carried out?

Answer 1

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

Question 2

What were the three aims?

Answer 2

• 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

Question 3

The study looked for a correlation between

Answer 3

the aggression scores of MZ twins and DZ twins, the teacher ratings and the peer ratings

Question 4

The study also looked at differences between

Answer 4

the genders and the different types of twins

Question 5

How were peer ratings obtained?

Answer 5

Via asking children to match 3 photographs of their peers to each of the 4 descriptions (2 for social, 2 for physical)

Question 6

What is a key advantage of the study in terms of how the data was collected?

Answer 6

Not only was behaviour assessed by teachers, but also peers, who are rarely employed as a reporting source in twin studies

Question 7

Where were the twins recruited from and when?

Answer 7

Montreal, Canada between 1995-1998

Question 8

How many children were used?

Answer 8

234 pairs of twins from the Quebec Newborn Study

Question 9

How many of each twin group were used?

Answer 9

>MZ MALES = 44 
>MZ FEMALES = 50
>DZ MALES = 41
>DZ FEMALES =32
>DZ MIXED = 67

Question 10

How did they control for a language barrier and why did they need to?

Answer 10

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.

Question 11

The teachers’ questionnaires for social aggression where they rated students on a 3 point scale: 0 = never 2 = sometimes 3 = often included statements like…

Answer 11

> tries to make others dislike a child
says bad things or spreads nasty rumors about another child
becomes friends with another child for revenge

Question 12

The questionnaires for physical aggression for the teachers included statements like…

Answer 12

> gets into fights
physically attacks others
hits, bites or kicks others

Question 13

How were overall scores obtained?

Answer 13

The scores for social aggression and physical aggression were added together to produce two overall scores.

Question 14

The 4 descriptions for the children were…

Answer 14

> tells others not to play with a child
tells mean secrets about another child
gets into fights
hits, bites or kicks others

Question 15

What were the results for physical aggression?

Answer 15

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

Question 16

The MZ twins’ correlations for social aggression were similar to the DZ twins’ correlations. This suggests social aggression is less linked to genetics, because

Answer 16

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. )

Question 17

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,

Answer 17

genes only seem to account for about 20%.

Question 18

So physical aggression is mostly nature (genes), but social aggression

Answer 18

is mostly nurture (environment).

Question 19

The overlap between social and physical aggression was only moderate. Brendgen proposes that genes might give children a general predisposition towards aggression, but

Answer 19

this only becomes social aggression if they have an environment that encourages it.

Question 20

Other findings included:

Answer 20

• High level of physical aggression = high level social
• Boys more physically aggressive
• Girls more socially aggressive

Question 21

Brendgen uses a large sample (234 twin pairs), so unusual children (anomalies) with very high/low levels of aggression are to be

Answer 21

“averaged out” by the size of the data. This makes the sample representative.

Question 22

Brendgen uses established questionnaires to measure aggression. What were these questionnaires and why were they beneficial?

Answer 22

Preschool Social Behaviour Sale and Direct and Indirect Aggression Scale - reliable as other researchers can replicate using the same scales

Question 23

Two researchers visited each classroom,

Answer 23

suggesting the study has inter-rater reliability.

Question 24

There was a strong correlation between teacher- and peer- ratings,

Answer 24

suggesting the scores are reliable

Question 25

The allocation of zygocity (MZ and DZ categories) was based largely on their appearance and wasn’t 100% reliable. In particular

Answer 25

it is possible there were DZ twins in the MZ condition.

Question 26

The study avoids taking a reductionist view of human behaviour: it looks at genetics but also at environmental factors as

Answer 26

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).

Question 27

Caution needs to be taken when generalising the results beyond the assessed age as

Answer 27

physical aggression is already diminished at school entry whereas social aggression may not be fully developed until age 8

Question 28

It has application as if high physical aggression is commonly found with high social aggression

Answer 28

parents should ensure their children are not in an environment where they can learn social aggression, e.g TV

Question 29

What was the correlation for teacher and peer ratings of physical aggression?

Answer 29

r=0.25 - moderate agreement

Question 30

What was the correlation for teacher and peer ratings of social aggression?

Answer 30

r=0.33 - moderate agreement

Question 31

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?

Answer 31

r= 0.79 - suggesting that mz were displaying very similar behaviour

Question 32

What were the attrition rates?

Answer 32

2% p/a meaning it is unlikely that this number of leaving twin pairs would have significantly altered results

Question 33

What was the heritability for teacher ratings of physical aggression compared to non shared environment?

Answer 33

63% for heritability vs 37% for non shared environment suggesting physical is due to genes

Question 34

What was the heritabilty for social aggression compared to shared environment and non shared?

Answer 34

20% heritability, 20% shared environment vs 60% non shared environment