Brendgen et al. (2005)



Most of the research investigating the causes of aggression has focused on physical expressions of aggression. This study is interested in the origins of social agression.

Social aggression is aggression characterised by socially manipulative behaviour such as ignoring others, spreading rumours or making threats to withdraw friendship. It can be both overtly and covertly expressed.

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  • To investigate whether there is a genetic cause to social aggression.
  • To determine whether social aggression shares the same underlying cause as physical aggression
  • To find out whether one type of aggression leads to another type of aggression - specifically does physical aggression lead to social aggression?
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  • Twins recruited from the Quebec Newborn Twin Study
  • All from the Montreal area of Canada
  • This was a longitudinal study which followed the twins at 5, 18 and 30 months and then at 4, 5 and 6 years
  • At the start of thestudy, there were 322 pairs of twins (237 were same-sex pairs)
  • They were assigned to identical or non-identical categories based on physical resemblance
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  • Data was gathered in the spring term in order to ensure that the children all knew each other well
  • 409 classrooms were involved as not all of the twins were in the same class
  • Written consent was obtained from parents of all the children in each classroom
  • Teacher ratings and peer ratings were gathered for each twin
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Teacher Ratings

The kindergaten teachers completed questionnaires containing 6 statements. They had to mark their level of agreement to each one using a standardised scale (0=never 1=sometimes 2=often)

Statements measuring social aggression:

  • To what extent does the child try to make orhers dislike a child?
  • Becomes friends with another child for revenge
  • Says bad things or spreads nasty rumours about another child

Statements measuring physical aggression:

  • To what extent does the child get into fights?
  • Physically attacks others
  • Hits, bites or kicks others

Therefore, each child got a physical and social agression score from their teacher's ratings

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Peer Ratings

The children in each class were given photographs of all their classmates. Two researchers ensured that they recognised all the photographs of the children.

The children were asked to circle the pictures of three children who best fitted certain descriptors 

Descriptors used to measure social aggression:

  • Tells others not to play with a child
  • Tells mean secrets about another child

Descriptors used to measure physical aggression:

  • Gets into fights
  • Hits, bites or kicks others

Any peer selection for the physical and social aggression descriptors that were made for each twin were recorded

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What is the Genetic Contribution to Physical Aggre


MZ correlations for physical aggression were nearly twice as high as the same sex DZ correlations on both teacher and peer ratings.


There is a strong genetic component to physical aggression because the MZ twins were more similar to one another and they share 100% of their genes.

Brendgen concluded: 

about 50-60% of physical aggression can be linked to genes (and that the other 40% is due to non-shared environment)

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What is the Genetic Contribution to Social Aggress


The correlations of social aggression scores in MZ and DZ pairs were roughly the same as each other on both teacher and peer ratings.


Social aggression may be better explained by the environment rather than genes.

Brendgen concluded:

 Only 20% of social aggression can be linked to genes (non-shared environment contributes to 60% and the further 20% comes from shared environment). 

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Do Social and Physical Aggression Share the Same C


There was only a moderate overlap between social aggression and physical aggression.


This correlation is mainly due to genes. These might give children a general predisposition towards aggression but this only becomes physical aggression if they have an environment that encourages it.

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Does One Type of Aggression Lead to Another?


Physical aggression may lead to social aggression. Social aggression does not lead to physical aggression.


There is a directional effect. Physical aggression changes to social aggression due to the environment and social factors as the child matures.

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Key Words

  • Brendgen, 2005
  • Quebec, Canada
  • Logitudinal study
  • MZ/DZ twins
  • 234 pairs
  • 5 months - 6 years
  • Teacher/peer ratings
  • Social aggression: 80% environment
  • Physical aggression: 50-60% genetics
  • PA to SA but NOT SA to PA
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  • Large sample (234 twin pairs) - representative sample, male and female
  • Study only looked at 6 year olds and younger - aggression in other ages may have different explanations
  • Twins may not be typical of the population because of their unusual closeness (especially MZ twins)
  • Conclusion: caution must be exercised when generalising findings to the general population
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  • Standardised procedure (e.g all teaches given the same questionnaire). This makes it easy to replicate the study.
  • Strong correlation between teacher and peer ratings, suggesting that scores were reliable (good inter-rater reliability)
  • It is questionable if findings could be replicated - social, environmental factors etc. 
  • Conclusion: Good internal reliability as there was use of standardised scales for measuring aggression. However, cannot be sure about external reliability
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If friends and family are a big influence on social aggression, it suggests that educating parents into better ways of handling their children and being better role models might prevent the children from being socially aggressive with their own firends.

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  • Twin studies are a valid way of studying nature vs nurture
  • Used teacher and peer ratings. This reduces the chance of bias 
  • Longitudinal study - observes changes in an individual over time
  • Something else could be affecting the MZ twins - e.g one twin may be stereotyped based on the behaviuor of the other twin, or the twin may influence the other's behaviour/ one leading the other astray)
  • Extraneous variables could explain the aggressive behaviour
  • Could be demand characteristics from researcher questionning 
  • Identification of MZ twins was based on appearance not DNA so wasn't completely accurate 
  • It is difficult to measure aggression so may not have construct validity
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  • Parents and teachers gave consent
  • The study gets children to look at pictures of their classmates and judge them which could have negative impacts on friendships, especially if they tell who they chose for each statement
  • Socially sensitive research. If aggression is genetic, it could create a self-fulfilling prophecy
  • Conclusion: the understanding of the cause and development of aggression outweighs the ethical cost 
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