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Brendgen et al. (2005) - Examining genetic and environmental effects on…
Brendgen et al. (2005) - Examining genetic and environmental effects on social aggression: A study of 6-year-old twins (BIOLOGICAL CONTEMPORARY STUDY)
Aim
To investigate the extent to which social and physical aggression can be explained by genetic and environmental factors.
To investigate whether social and physical aggression are caused by the same genetic and environmental (shared and non-shared) factors.
To investigate whether one type of aggression directly influences the other type of aggression (phenotype to phenotype).
Conclusion
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 a social convention environemnt that encourages it.
There is a genetic component (nature) to physical aggression but not social aggression, which is more likely to be due to environmental factors (nurture).
Sample
234 pairs at the final assessment at 6 years old were recruited from birth from Montreal, Canada and were part of an ongoing longitudinal study - Quebec Newborn Twin Study. There were 44 sets of identical MZ male twins, 50 sets of identical MZ female twins, 32 non-identical DZ female twins and 67 sets of non-identical DZ mixed-sex twins.
Results
There was a twice as high chi-squared correlation between the ratings of MZ twins on physical aggression than between same-sex DZ twins in both peer and teacher ratings. For example, for physical aggression ratings of teachers, MZ twins had a correlation of 0.61 wheres same-sex DZ twins had 0.25. - AIM 1
Chi-square scores for social aggression were roughly equally correlated in MZ and same-sex DZ twins for both peer and teacher ratings. For example, for social aggression ratings of teachers, MZ twins had a correlation of 0.35 and same-sex DZ twins had 0.34, making them modest. - AIM 1
Brendgen also considered a non-shared environment such as friends or school as an influence on aggression. This accounts for about 40% of physical aggression and 60% of social aggression. Brendgen links about 20% of social aggression to a shared environment, which is due to parenting strategies. - AIM 2
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%. - AIM 2
The data suggested that physical aggression may lead to social aggression but not the other way round. - AIM 3
Evaluation
✓ Reliability - This study is highly reliable. Brendgen uses established questionnaires to measure aggression, making them easily replicable to check for consistency. Two researchers visited each classroom, suggesting the study has inter-rater reliability. Data was gathered from more than one source in the twin study and a strong correlation between teacher and peer ratings suggests the scores were reliable.
✗ Generalisability - The researchers complained that their sample size was small which preluded the examination of sex differences concerning genetic and environmental effects on social and physical aggression. They looked at 409 classrooms but said further studies need to be replicated with larger samples before definite conclusions can be made. Furthermore, caution needs to be taken in generalising the results beyond the assessed age of 6 years old. Physical aggression is already diminished at school entry whereas social aggression may not be fully developed till 8 years old.
✓ - Application - If social aggression is strongly linked to the environment, it's possible to reduce the worst effects of verbal bullying, gossiping and “trolling” in social media by children, by using early intervention. Classes and workshops might help children learn less aggressive ways of interacting. Friends and family are a big influence on social aggression, so educating parents about better ways of handling their children and being better role models may prevent children from being socially aggressive with their friends.
✓ Validity - Twin studies are a valid way of studying nature versus nurture. Since MZ twins share 100% of their genotype, but DZ twins no more than 50%, but both share the same home life, if MZ twins have similar behaviours where DZ twins do not, this is likely to be due to the genotype. Furthermore, the translation of measures used was checked by bilingual researchers and was translated twice to ensure it was done correctly. This increases the validity of the measures used.
✓/✗ Ethics - The teachers and parents of the twins agreed for them to be in the study, so presumptive consent was given by responsible adults. However, the study asks the children to look at photos of their classmates and judge them. This might have a bad impact on friendships and have led to animosity. This goes against the social responsibility of ethical research and might create risks for the children who participated.
Method
This was a longitudinal study followed from 5, 18, 30, 48, to 60 months and then again at 6 years (Quebec Newborn Twin Study).
Procedure
Consent was gained from parents and the study took place from birth till 6 years old. Peer and teacher ratings were done in a classroom with two researchers present. The peer ratings were done by children circling 3 students that best fitted 4 behavioural descriptors in a booklet of photos of their classmates. For each behavioural descriptor, the total nominations were calculated and turned into a total social and physical aggression score. The teacher ratings were done by teachers rating the children's social and physical aggression using the Preschool Social Behaviour Scale and Direct and Indirect Aggression Scales. They rated them on a 3 point scale on 6 statements and these were used to calculate a total social and physical aggression score.