Brendgen et al. (2005) The Contemporary Study: Aggression In 6-Year-Old Twins


  • MZ twins share 100% of their geneotype
  • DZ twins share 50% of their genotype
  • Physical aggression is self explantory
  • Social Aggression is:
  • Relational aggression - overt aggression through things like direct insults and faces
  • Indirect aggression - covert aggression through things like passive aggression and spreading rumors etc.
  • Studies show physically aggression occurs at younger ages then when they grow up more social aggression is shown
  • Genotype relates to only genes
  • Phenotype relates to genes and environment
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  • To examine differences in physical and social aggression in 6 year olds by surveying their classmates and teachers.
  • To see if there are any links genetic or environmental to either types.
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  • IV - Looking at aggression scores between DZ and MZ twins, looking at correlation between scores given by teachers and peer given scores, looking at differences between genders and types of twin in relation to the types of aggression
  • DV - Teacher ratings for both social and physical aggression were calculated out of 6, the peer ratings were gathered by getting them to talk about picture of their classmates.
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  • She used 234 twin pairs taken from a longitudinal study already being performed (Qubec Newborn Twin Study), 88 had dropped out from original sample
  • 44 (MZ) male,
  • 50 (MZ) female,
  • 41 (DZ) males,
  • 32 (DZ) females
  • 67 (DZ) mixed.
  • This is a cluster sample
  • Categories - Assigned to DZ or MZ by physical resemblance, except for 123 pairs who were assigned based on DNA tests
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  • Got written consent from parents
  • The teachers were asked to rate students using a 3 point standerdized scale based of 6 different statements about the two different types of aggression
  • The other students were presented pictures and had to circle three that corresponded best to the researchers 4 statements, based of the results each student was given another score 
  • She used the self report method to gather her data
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  • She carried out a Chi Squared inferential test to compare the two scores
  • She found the differences were not significant menaing the two ratings can be combined together
  • She also compared the corelation and found it was moderate (0.25 for PA and 0.33 for SA), the stronger it is the more valid the two views are
  • Teachers rated boys as more physically aggresive and girls as more socially aggressive 
  • Peers tended to say boys were more physically and socially aggressive
  • Also compared correlation between peers and teachers for the MZ and DZ twins. 
  • Found MZ correlations for physical aggression were twice as high as same sex DZ correlations. For teacher ratings this was quite high (0.79) suggesting MZ twins had very similar behaviour whether aggressive or not.
  • Suggests there was litlle disagreement about MZ twins physical aggression
  • A lot more disagreements about DZ twins this could be down to more influence from the environment
  • The MZ and DZ twins correlatiosn for SA were similar suggesting it is less linked to genetics as bothe sets of twins seemed to be equally affected by their environment unlike with PA
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  • 50%-60% of physical aggression can be linked to genes as it was shared by MZ twins but less with DZ twins 
  • 20%  of social aggresion can be accounted for by genes 
  • 40% of physical aggression and 60% of social aggression can be accounted for by non-shared environment
  • 20% of social aggression due to shared environment
  • She says if paretns control children by with holding love and affection, children learn to control peers by doing the same thing
  • Overall physical is mainly genes social is mainly environment
  • The overlap of two was quite moderate, she proposes that genes give a predisposition towards aggression but is only becomes social if there is an encouraged environment.
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Evaluation - Strengths

  • Generaliability
  • Used a large sample averaging out any anomalies and is quite representative
  • May not be representative as many children are slow or fast developers meaning more mature children may have more non-shared environmental influences
  • Reliability
  • Used established questionnaires to measure aggression, these can be easily replicated and so the study is made more reliable
  • Two researchers visited each classroom, suggesting the study has inter-rater reliability 
  • Strong correlation between scores from peers and teachers meaning the scores are more reliable
  • Application
  • If SA is linked to environment then early intervention can stop bullying and trolling
  • If freinds and family are a big impact on the level of social aggression a child will develop then educating parents may be the best way of handling their children and better role models also have to be provided to prevent sa developing in children
  • Validity
  • Twin studys are good way of studying nature vs. nurture sionce MZ share 100% of their genotype but DZ share no more than 50% but both share the same homelife, this means if MZ twins share common behaviours and DZ twins do not then it is due to the genotype
  • Takes a reductionist view of human behaviour, it links with Bandura (learning) and Raine (brain  structure)
  • Ethics
  • She got informed consent from parents 
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Evaluation - Weaknesses

  • Generalisability
  • 88 twin pairs disappered from the study lowering th representativness of the study, called sample attrition
  • Believed it was two small
  • Only used 6 yearolds so may not be very representative
  • Reliability
  • Language differences may have resulted in slightly different meanigs meanig the translations may be unreliable
  • The allocation of twin catrogry was based largely on apperance and so wasen't 100% reliable, it was possible DZ twins were in the MZ condition
  • Validity
  • Correltaions do not neccesatrily prove causation, other factors could affect the MZ twin, for example as MZ twins look alike they maybe sterotyped together leaing to a bad reputaion or high expecations based of the other twin
  • In her study teachers and peers could have been influenced by sterotypes
  • As this is a natural experiment it can not fuilly show cause and effect as one twin may influence the others behaviour leading it astray 
  • Ethics
  • Getting kids to rate each other may lead to bad impacts on freindships and feelings, this goes against the social responsbilitiy of ethica resaerch and may esxpose some children to harm
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