All subjects · Psychology · WPS02 Biological Psychology, Learning Theories and Development

Contemporary study: Brendgen et al. (2005)

Edexcel IAL Psychology · WPS02 Biological Psychology, Learning Theories and Development

Brendgen et al. (2005), 'Examining genetic and environmental effects on social aggression: a study of 6-year-old twins', is the compulsory contemporary study for Topic C (spec 3.3.2).

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Aims

1. To find how far social aggression (harming relationships, such as spreading rumours) and physical aggression are explained by genes, shared environment and non-shared environment.

2. To find whether the overlap between social and physical aggression is due to shared genes, shared environment, or one type leading to the other.

Sample and procedure

Twin pairs from a longitudinal study in Montreal, Canada, mostly of European descent, assessed at 6 years old in kindergarten.
Zygosity was judged by physical resemblance at 18 months, checked with DNA for some same-sex pairs.
Written parental consent was obtained.
Peer ratings: classmates were shown photographs and circled the three children who best fitted descriptions such as 'tells mean secrets about another child' (social) or 'hits, bites or kicks others' (physical).
Teacher ratings: teachers rated each twin using rating scales of social and physical aggression.

Results

Physical aggression: the MZ correlation was almost twice the DZ correlation, suggesting it is mainly genetic.
Social aggression: MZ and DZ correlations were similar, suggesting shared environment matters.

Teacher ratings	Genetic	Shared environment	Non-shared environment
Physical aggression	63%	(not reported)	37%
Social aggression	20%	20%	60%

Variance explained, teacher ratings, as given in Pearson's summary of studies

Conclusions and evaluation

Conclusions: physical aggression is strongly influenced by genes and non-shared environment; social aggression is influenced more by environment, especially non-shared environment. Children who are physically aggressive may also use social aggression, depending on their environment.

Evaluation:

Strengths: twin design separates genes and environment; two sources of ratings (peers and teachers) increase reliability; parental consent.
Weaknesses: zygosity was mostly judged by appearance; a mainly European sample from one city limits generalisation; ratings are subjective; the equal environments assumption may not hold.

Worked example

Using the teacher ratings, compare the genetic influence on physical and social aggression.

Physical aggression: 63% of variance genetic.
Social aggression: 20% genetic.
63 - 20 = 43 percentage points more genetic influence for physical aggression.

Answer: Genes explain far more of physical aggression (63%) than social aggression (20%).

Key idea: Brendgen et al. (2005): 6-year-old Montreal twins rated by peers and teachers. Physical aggression mainly genetic (MZ correlation nearly double DZ); social aggression mainly environmental.

Check yourself

How old were the twins when data were collected?
Show answer
6 years. The twins were 6, in kindergarten.
Where did the twins live?
- Los Angeles, USA
- Montreal, Canada
- London, UK
- Sydney, Australia
Show answer
Montreal, Canada. They were from a longitudinal study in Montreal.
Which is an example of social aggression?
- Hitting another child
- Kicking a ball hard
- Biting another child
- Telling others not to play with a child
Show answer
Telling others not to play with a child. Social aggression damages relationships.
What did the twin correlations suggest about physical aggression?
- It is mainly influenced by genes
- It is entirely due to shared environment
- It has no genetic component
- It only occurs in DZ twins
Show answer
It is mainly influenced by genes. MZ correlations were nearly double DZ correlations.
In the teacher ratings, social aggression was 20% genetic and 20% shared environment. What percentage was non-shared environment?
Show answer
60 %. 100 - 20 - 20 = 60%.
Why does using both peer and teacher ratings strengthen the study?
- The two sources can be compared, so consistent findings are more reliable
- It removes the need for consent
- It makes the sample larger
- It turns the data into qualitative data
Show answer
The two sources can be compared, so consistent findings are more reliable. Agreement between two sources supports reliability.
Explain one weakness of Brendgen et al. (2005).
Show answer
Zygosity was mostly judged by physical resemblance rather than DNA, so some twins may have been wrongly classified as MZ or DZ, which would distort the genetic estimates.. Name it, then explain the consequence.

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Sources

Last updated 8 November 2026. Spotted a mistake? Tell us.